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  Human Body  
  The physical structure of the human being with all its complexities may, for convenience, be considered as a number of systems, such as the circulatory system and the digestive system, all of which are interdependent and interact to maintain life at both a cellular level and a bodily level.  
Human Body: Composition
chemical element or substance body weight
pure elements  
  magnesium, iron, manganese, copper, iodine, cobalt, zinc  
water and solid matter  
  total solid material  
organic molecules  
  small organic particles  


  Skeletal System  
  The skeleton is the rigid framework of bone and cartilage that supports and gives form to the body, protects its internal organs, and provides anchorage points for its muscles. With its flexible (joints acting as fulcrums, the skeleton also forms a system of levers upon which muscles can act to produce movement. The human skeleton is composed of 206 bones, ranging in size from the femur (thigh bone) to the tiny ossicles of the middle ear.  
  Human Anatomy Online
  Fun, interactive, and educational site on the human body. The site is divided into many informative sections, including hundreds of images, and animations for Java compatible browsers.  
  It may be considered in two parts: the axial skeleton and the appendicular skeleton. The axial skeleton comprises 80 bones, and consists of the skull, vertebral column (spine), rib cage, and sternum. It supplies the central structure onto which the bones of the appendicular skeleton are joined, and encloses and protects the c0016-01.gifcentral nervous system (brain and spinal cord). The appendicular skeleton consists of the limb bones and the pectoral (shoulder) and pelvic (hip) girdles. It comprises 126 bones—64 in the shoulders and upper limbs, and 62 in the pelvis and lower limbs.  
  Virtual Body
  If ever something was worth taking the time to download the "shockwave" plug-in for, this is it. Authoritative and interactive anatomical animations, complete with voice-overs, guide you round the whole body, with sections on the brain, digestive system, heart, and skeleton.  
  special bones  
  skull The skull is a collection of flat and irregularly shaped bones that enclose the brain and the organs of sight, hearing, and smell, and provide support for the  




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  human body The adult human body has approximately 650 muscles, 100 joints,
100,000 km/60,000 mi of blood vessels and 13,000 nerve cells. There are 206 bones
in the adult body, nearly half of them in the hands and feet.
  To remember the bones of the skull:  
  Old people from Texas eat spiders  
  (occipital, parietal, frontal, temporal, ephnoid, sphenoid)  


  jaws. It consists of 22 bones joined by fibrous immobile joints called sutures. The floor of the skull is pierced by a large hole (foramen magnum) for the spinal cord and a number of smaller apertures through which other nerves and blood vessels pass.  
  The skull comprises the dome-shaped cranium (brain case) and the bones of the face, which include the upper jaw, enclose the sinuses, and form the framework for the nose, eyes, and the roof of the mouth cavity. The lower jaw is hinged to the middle of the skull at its lower edge. The plate at the back of the head is jointed at its lower edge with the upper section of the spine. Inside, the skull has various shallow cavities into which fit different parts of the brain.  
  Visible Human Project
  Sample images from a long-term U.S. project to collect a complete set of anatomically detailed, three-dimensional representations of the human body.  


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Bones of the Human Body
Cranium (Skull)    
  Parietal: 1 pair  
  Inferior nasal conchae  
  Frontal: 1 pair, fused  
  Nasal: 1 pair  
  Lacrimal: 1 pair  
  Temporal: 1 pair  
  Maxilla: 1 pair, fused  
  Zygomatic: 1 pair  
  Palatine: 1 pair  
  Mandible (jawbone): 1 pair, fused  
  Malleus (hammer)  
  Incus (anvil)  
  Stapes (stirrups)  
  Vertebral Column (Spine)  
  Cervical vertebrae  
  Thoracic vertebrae  
  Lumbar vertebrae  
  Sacral vertebrae: 5, fused to form the sacrum
Coccygeal vertebrae: between 3 and 5,
fused to form the coccyx
  Ribs, ''true:" 7 pairs  
  Ribs, "false:" 5 pairs, of which 2 pairs are floating  
Sternum (Breastbone)    
Pectoral Girdle  
  Clavicle: 1 pair (collar-bone)  
  Scapula (including coracoid): 1 pair (shoulder blade)  
Upper Extremity (Each Arm)  
  Carpus (wrist)  
  Phalanges (fingers)  
First digit
  Second digit
  Third digit
  Fourth digit
  Fifth digit
Pelvic Girdle    
  Ilium, ischium, and pubis (combined):  
  1 pair of hip bones, innominate  
Lower Extremity (Each Leg)  
Femur (thighbone)
  Tibia (shinbone)
  Patella (kneecap)
  Tarsus (ankle)  
  Cuneiform, medial
  Cuneiform, intermediate
  Cuneiform, lateral
  Metatarsals (foot bones)
  Phalanges (toes)  
First digit
  Second digit
  Third digit
  Fourth digit
  Fifth digit





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  skull The skull is a protective casing for the brain, eyes, and hearing organs.
It is also a framework for the teeth and flesh of the face. The cranium has
eight bones: occipital, two temporal, two parietal, frontal, sphenoid, and
ethmoid. The face has 14 bones, the main ones being two maxillae,
two nasal, two zygoma, two lacrimal, and the mandible.
  vertebral column The vertebral column, otherwise known as the backbone or spine, is the body's central supporting structure and also serves to protect the spinal cord. It is made up of a series of bones, or vertebrae, running from the skull to the coccyx (vestigial tail), with a central canal containing the nerve fibers of the spinal cord. The shape of the vertebrae varies according to position. For example, in the chest region the upper or thoracic vertebrae are shaped to form connections to the ribs. The vertebral column is only slightly flexible to give adequate rigidity to the body.  
  Each vertebra has a body and an arch. The body is a cylinder connected to the adjacent vertebrae by a disc of fibro-cartilage, which absorbs shock. The arch consists of two halves which join together behind, forming a ring. The whole succession of these rings forms the vertebral canal, or channel, containing the main nerve trunk known as the spinal cord. Viewed from the side, the vertebral column has a series of curves. This curvature maintains the strength of the structure and adapts itself to the various movements of the body. The spinal cord occupies the upper two-thirds of the vertebral canal, and 31 pairs of spinal nerves arising from the spinal cord leave the vertebral canal through spaces at the sides between successive vertebrae.  
vertebral column
  The vertebral column, or spine, extends every night during sleep. During the day, the cartilage discs between the vertebra are squeezed when the body is in a vertical position, standing or sitting, but at night, with pressure released, the discs swell and the spine lengthens by about 8 mm/0.3 in.  


  long bone The long bones, such as those of the arms and legs, consist of a shaft, called a diaphysis, with an expanded, rounded portion called an epiphysis at each end. The whole bone is covered with a tough, fibrous membrane, called the periosteum, to which muscles and ligaments are attached.  
  The diaphysis is a tube of compact bone, resistant to bending, with a core—the medullary cavity—filled with a soft matrix called bone marrow.  
  The epiphyses, which are covered in a smooth layer of cartilage, are the regions where the bones meet other bones to form a c0016-01.gifjoint. They are largely composed of light spongy bone.  
  To remember the bones of the upper limb:  
  Some criminals have underestimated Royal Canadian Mounted Police.  
  (scapula, clavicle, humerus, ulna, radius, carpals, metacarpals, phalanges)  


  skeletal tissues  
  bone Bone is a living connective tissue, composed of a network of collagen fibers impregnated with mineral salts (largely calcium phosphate and calcium carbonate)—a combination that gives it great density and strength, comparable in some cases with that of reinforced concrete. Enclosed within this solid matrix are bone cells (osteoblasts), blood vessels, and nerves.  
  The osteoblasts, which are housed in small spaces called lacunae, are responsible for secreting the matrix.  




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  bone Bone is a network of fibrous material impregnated with mineral salts
and as strong as reinforced concrete. The upper end of the thighbone or
femur is made up of spongy bone, which has a fine lacework structure
designed to transmit the weight of the body. The shaft of the femur consists
of hard compact bone designed to resist bending. Fine channels carrying
 blood vessels, nerves, and lymphatics interweave even the densest bone.
  Tiny, fluid-filled channels, called canaliculi, penetrate the matrix to supply the cells with oxygen and nutrients and to transfer calcium salts to the exterior.  
  Compact bone, the dense tissue that forms the outer layer of most bones, is composed of many Haversian systems: tightly packed layers (lamellae) of matrix, formed concentrically around a Haversian canal. This canal acts as a duct for blood and lymph vessels and nerves. Spongy bone is a lighter tissue that makes up the rounded ends (epiphyses) of long bones and the inner layer of many other bones. It is composed of a loose, branching network of bone, with many irregularly shaped interconnected spaces that may be filled with bone marrow.  
  Bone may also be classified according to its origin: whether it developed by replacing c0016-01.gifcartilage or was formed directly from connective tissue. The latter, which includes the bones of the cranium, are usually platelike in shape and form in the skin of the developing embryo.  
  cartilage Cartilage is a flexible bluish-white connective tissue made up of the protein collagen. It forms the greater part of the embryonic skeleton, and is replaced by c0016-01.gifbone in the course of development, except in areas of wear such as bone endings, and the discs between the backbones. It also forms structural tissue in the larynx, nose, and external ear.  
  A joint is a region where two bones meet. Some joints allow no motion (the sutures of the skull), others allow a very small motion (the sacroiliac joints in the lower back), but most allow a relatively free motion. Of these, some allow a gliding motion (one vertebra of the spine on another), some have a hinge action (elbow and knee), and others allow motion in all directions (hip and shoulder joints) by means of a ball-and-socket arrangement.  
  The ends of the bones at a moving joint are covered with cartilage for greater elasticity and smoothness, and enclosed in an envelope (capsule) of tough white fibrous tissue lined with a membrane which secretes a lubricating and cushioning synovial fluid. The joint is further strengthened by ligaments—strong, flexible bands of connective tissue, that prevent bone dislocation but allow joint flexion.  
  Muscle System  
  Muscle is a contractile tissue that produces locomotion and power, and maintains the movement of body substances. It is made of long cells that can contract to between one-half and one-third of their relaxed length.  
  types of muscle The elongated cells or fibers that constitute muscle are classified into three types: striated, cardiac, and smooth.  
  striated muscle Striated muscle cells are so called because of their striped appearance under the microscope. They comprise the bulk of the body musculature (about 40% of the total body weight). Striated muscle fibers contract as a result of nervous stimulation and they are mostly under voluntary control. The  




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  antagonistic muscle Even simple movements such as bending and straightening
the arm require muscle pairs to contract and relax synchronously.
  muscles of the arms and legs are examples. Striated muscle is also known as striped, skeletal, or voluntary muscle.  
  cardiac muscle Cardiac muscle occurs only in the heart. It also has cross-striations, but, unlike striated muscle fibers, its fibers branch so that the mass of muscle tends to function as one unit. It is controlled by the autonomic nervous system, but will continue to contract rhythmically even when its nerve supply is cut.  
  smooth muscle Smooth muscle lacks the visible cross-striations of striated and cardiac muscle; it is found in the intestinal walls, blood vessels, the iris of the eye, and various ducts. Like cardiac muscle, smooth muscle is normally under the dual control of hormones and the autonomic nervous system, but it also has intrinsic contractility. It is not under voluntary control.  
  muscle structure  
  Striated muscles are composed of a large number of parallel cylindrical fibers supported by non-contractile connective tissue. The connective tissue extends at both ends of the muscle to be continuous with the c0016-01.giftendons that attach the muscle to c0016-01.gifbone.  
  muscle Muscles make up 35–45% of our body weight; there are over 650
skeletal muscles. Muscle cells may be up to 20 cm/0.8 in long. They are
arranged in bundles, fibers, fibrils, and myofilaments.




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  The functional element of the muscle is the muscle fiber, which has many fine threads, or fibrils, running throughout its length. Although the fibrils themselves can only be observed under an electron microscope, their regular banded structure gives rise to the striped appearance of the muscle fiber. The light bands are called isotropic bands (I-bands), and the dark ones anisotropic bands (A-bands).  
  A fibril contains two different types of still finer myofilaments. The thicker myofilaments consist of the contractile protein myosin and each is surrounded by a hexagonal arrangement of thinner contractile protein called actin.  
  muscular contraction  
  The myosin molecules of the muscle fibrils each have a projecting head that forms a minute cross bridge with an actin myofilament. After nervous stimulation, electrical changes in the membrane surrounding each fibril cause the release of calcium ions, which are normally stored in sacs along the fibril. It is thought that the free calcium ions stimulate the heads of the myosin molecule to swivel, pulling the actin filaments past the myosin filaments. The cross bridges then break temporarily, as the myosin heads swing back to their original angle and attach themselves to the actin filaments again, before repeating the operation. Each cycle of attachment, swivel, and detachment, shortens the muscle by about 1%, so it must be repeated many times in order to produce a significant contraction.  
  muscles and movement  
  Usually a striated muscle is linked to bones at both its ends, and spans one or two joints in between. When the muscle fibers contract as a result of nervous stimulation, they tend to bring the bones closer together creating a rotatory movement at the intervening joint(s).  
  For coordinated movement around a joint, a pair of antagonistic muscles is required. The extension of the arm, for example, requires one set of muscles to relax, while another set contracts. The individual components of antagonistic pairs can be classified into extensors (muscles that straighten a limb) and flexors (muscles that bend a limb).  
  Circulatory System  
  The circulatory system is the system that transports blood to and from the different parts of the body. It consists principally of a pumping organ—the c0016-01.gifheart—and a network of blood vessels. In humans and other mammals, there is a double circulation system—the pulmonary (or lung) circulation and the systemic (or body) circulation—whereby blood passes to the lungs and back to the heart before circulating around the remainder of the body. In the systemic circulation, blood rich in oxygen is pumped to the tissues of the body, returning to the heart as blood rich in carbon dioxide; in the pulmonary circulation, blood rich in carbon dioxide is pumped to the c0016-01.gifalveoli of the lungs, where carbon dioxide is exchanged for oxygen, so that oxygenated blood returns to the heart. Valves in the heart, large arteries, and veins ensure that blood flows in one direction only.  
  The circulatory system performs a number of functions: it supplies the cells of the body with the food and oxygen they need to survive; it carries carbon dioxide and other wastes away from the cells; it helps to regulate the temperature of the body and conveys substances that protect the body from disease. In addition, the system transports hormones, which help to regulate the activities of various parts of the body.  
  There are three main parts to the human circulatory system. These are the heart, the blood vessels, and the blood itself.  
  The human heart is more or less conical in shape and is positioned within the chest, behind the breast bone, above the diaphragm, and between the two lungs. It has flattened back and front surfaces and is, in health, the size of an adult's closed fist. It has four chambers: two upper chambers—the left and right atria (singular "atrium")—and two lower chambers—the left and right ventricles.  
  atria and ventricles The atria are thin-walled chambers that act as reservoirs, receiving blood from the veins. The two venae cavae, the major veins bringing back deoxygenated blood from the head, body, and limbs, join the right atrium. This chamber is separated from its respective ventricle by a valve with three flaps, the tricuspid valve. The right ventricle is a pyramidal chamber with thicker walls than the atria. The opening of the pulmonary artery, which leaves the right ventricle, has a valve that prevents the ejected blood from flowing back into the ventricle when it relaxes. The left atrium receives blood from the lungs via the four pulmonary veins, and transfers it into the left ventricle. This chamber has the stoutest walls of all, as its contraction should generate sufficient blood pressure to propel the blood into all the arteries of the body. The valve between the left atrium and ventricle has only two flaps and looks somewhat like a bishop's miter, hence the name bicuspid or mitral valve.  
  cardiac cycle The cardiac cycle is the sequence of events during one complete cycle of a heart beat. This  




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  circulatory system Blood flows through 96,500 km/60,000 mi of arteries and veins,
supplying oxygen and nutrients to organs and limbs. Oxygen-poor blood (black)
circulates from the heart to the lungs where oxygen is absorbed. Oxygen-rich blood
(gray) flows back to the heart and is then pumped round the body through the aorta,
the largest artery, to smaller arteries and capillaries. Here oxygen and nutrients are
exchanged with carbon dioxide and waste products and the blood returns to the
heart via the veins. Waste products are filtered by the liver, spleen, and kidneys,
and nutrients are absorbed from the stomach and small intestine.
  consists of the simultaneous contraction of the two atria, a short pause, then the simultaneous contraction of the two ventricles, followed by a longer pause while the entire heart relaxes. The contraction phase is called systole and the relaxation phase that follows is called diastole. The whole cycle is repeated 70–80 times a minute under resting conditions.  
  When the atria contract, the blood in them enters the two relaxing ventricles, completely filling them. The mitral and tricuspid valves, which were open, now begin to shut and as they do so, they create vibrations in the heart walls and tendons, causing the first heart sound. The ventricles on contraction push open the pulmonary and aortic valves and eject blood into the  




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Blood: The Discovery of Circulation
  By Julian Rowe  
After completing a preliminary medical course at the University of Cambridge, England, where would an ambitious young man in the 17th century go to get a really good medical training? To the University of Padua in Italy, where the great Italian anatomist Hieronymous Fabricius (1537–1619) taught. So this is where English physician William Harvey (1578–1657) naturally went.
William Harvey had a consuming interest in the movement of the blood in the body. In 1579, Fabricius had publicly demonstrated the valves, which he termed "sluice gates," in the veins: his principal anatomical work was an accurate and detailed description of them.
Galen's theory
Galen, a Greek physician (c. 130–c. 200), had 1,500 years previously written a monumental treatise covering every aspect of medicine. In this work, he asserted that food turned to blood in the liver, ebbed and flowed in vessels and, on reaching the heart, flowed through pores in the septum (the dividing wall) from the right to left side, and was sent on its way by heart spasm. The blood did not circulate. This doctrine was still accepted and taught well into the 16th century.
one-way flow
Harvey was unconvinced. He had done a simple calculation. He worked out that for each human heart beat, about 60 cm3 of blood left the heart, which meant that the heart pumped out 259 liters every hour. This is more than three times the weight of the average man.
Harvey examined the heart and blood vessels of 128 mammals and found that the valve which separated the left side of the heart from the right ventricle is a one-way structure, as were the valves in the veins discovered by his tutor Fabricius. For this reason he decided that the blood in the veins must flow only toward the heart.
Harvey's experiment
Harvey was now in a position to do his famous experiment. He tied a tourniquet round the upper part of his arm. It was just tight enough to prevent the blood from flowing through the veins back into his heart—but not so tight that arterial blood could not enter the arms. Below the tourniquet, the veins swelled up; above it, they remained empty. This showed that the blood could be entering the arm only through the arteries. Further, by carefully stroking the blood out of a short length of vein, Harvey showed that it could fill up only when blood was allowed to enter it from the end that was furthest away from the heart. He had proved that blood in the veins must flow only toward the heart.
a new theory of circulation
Galen's pores in the septum of the heart had never been found. Belgian physician Andreas Versalius (1514–1564) was another alumnus of Padua University. Although brought up in the Galen tradition, he had carried out secret dissections to discover the pores, and had failed. He did, however, show that men and women had the same number of ribs! Harvey clinched his researches into the movement of the blood when he demonstrated that no blood seeps through the septum of the heart. He reasoned that blood must pass from the right side of the heart to the left through the lungs. He had discovered the circulation of the blood, and thus, some 20 years after he left Padua, became the father of modern physiology. In 1628 Harvey published his proof of the circulation of the blood in his classic book On the Motion of the Heart and Blood in Animals. A new age in medicine and biology had begun.


  respective vessels. The closed mitral and tricuspid valves prevent the return of blood into the atria during this phase. As the ventricles start to relax, the aortic and pulmonary valves close to prevent backward flow of blood, and their closure causes the second heart sound. By now, the atria have filled once again and are ready to start contracting to begin the next cardiac cycle.  
  sinoatrial node The heart has a natural pacemaker, called a sinoatrial node, which is a group of muscle cells in the heart wall that contracts spontaneously and rhythmically, setting the pace for the contractions of the rest of the heart. The pacemaker's intrinsic rate of contraction is increased or decreased, according to the needs of the body, by stimulation from the autonomic nervous system.  
  Heart: An Online Exploration
  Explore the heart: discover its complexities, development, and structure; follow the blood on its journey through the blood vessels; learn how to maintain a healthy heart; and look back on the history of cardiology.  
  blood vessels There are three major types of blood vessels: arteries, which carry blood away from the heart; veins, which return blood to the heart; and capillaries, which are extremely tiny vessels connecting the arteries and the veins.  




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  heart The structure of the human heart. During an average lifetime, the
human heart beats more than 2,000 million times and pumps 500 million
1/110 million gal of blood. The average pulse rate is 70–72 beats per minute
at rest for adult males, and 78–82 beats per minute for adult females.
  artery When blood is pumped out of the heart into the arteries, it is forced out at high pressure by contractions of the muscular ventricles. Arteries therefore have a layer of strong, elastic muscle tissue in their walls that can stretch and recoil with the force of the blood; the resulting pulse or pressure wave can be felt at the wrist as the pulse. Not all arteries carry oxygenated (oxygen-rich) blood; the pulmonary arteries convey deoxygenated (oxygen-poor) blood from the heart to the lungs.  
  capillary As the blood travels through the body, the arteries divide into smaller and smaller vessels, finally forming capillaries—the narrowest blood vessels of all, 0.008–0.02 mm in diameter, barely wider than a red blood cell. Capillaries are distributed as beds, complex networks connecting arteries and veins. Their walls are extremely thin, consisting of a single layer of cells, and so nutrients, dissolved gases, and waste products can easily pass through them. This makes the capillaries the main area of exchange between the fluid (lymph) bathing body tissues and the blood.  
  vein As the blood continues its passage around the body, capillaries merge again to form veins through which the blood is returned to the heart. By the time blood reaches the veins its pressure has been greatly reduced and its flow is much slower. The walls of veins therefore do not need to be as thick or as elastic as those of arteries. Veins contain valves that prevent the blood from running back when moving against gravity. They always carry deoxygenated blood, with the exception of the pulmonary veins leading from the lungs to the heart, which carry newly oxygenated blood.  
  In humans, blood makes up 5% of the body weight, occupying a volume of 5.5 1/10 pt in the average adult. It is composed of a colorless, transparent liquid called plasma, which is  
  In 1996 U.S. companies began testing different varieties of artificial blood. All are able to transport oxygen from the lungs round the body and carry carbon dioxide back.  





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  made up mostly of water, but also contains proteins, glucose, amino acids, salts, hormones, and antibodies. Suspended in the plasma are three kinds of microscopic cell: red blood cells, white blood cells, and platelets.  
  red blood cell The red blood cell, or erythrocyte, is the most common type of blood cell, responsible for transporting oxygen around the body. Each is disk-shaped with a depression in the center and no nucleus. It contains hemoglobin, a complex red protein that gives blood its color and combines with oxygen from the lungs to form oxyhemoglobin. When transported to the tissues, the red blood cells are able to release the oxygen because the oxyhemoglobin splits into its original constituents.  
  Red blood cells are manufactured in the bone marrow—mainly in the ribs, vertebrae, and limbs—at an average rate of 9,000 million per hour. However, they have a relatively short life of only about four months before being destroyed in the liver and spleen.  
  white blood cell The white blood cell, or leukocyte, is a relatively large, nucleated cell found in the blood, c0016-01.giflymph, and elsewhere in the body's tissues. It is colorless, with clear or granulated cytoplasm, and is capable of independent ameboid movement.  
  There are a number of different types, each of which plays a part in the body's defenses and gives immunity against disease. Some (neutrophils and macrophages) engulf invading microorganisms and infected cells, while lymphocytes produce more specific immune responses.  
  Human blood contains about 11,000 leukocytes to the cubic millimeter—about one to every 500 red cells.  
  platelet Platelets are small fragments of cells with no nucleus. They too are produced in the bone marrow and their function is to release substances which enable blood to clot (see blood clotting). Thus they help to prevent the loss of blood from damaged vessels.  
  blood group A blood group is one of the types into which blood is classified according to the presence or absence of certain proteins on the surface of its red cells. These proteins act as antigens—that is, they would induce the production of antibodies if they were introduced into the blood of an individual whose red blood cells lack these proteins. Correct typing of blood groups is vital in transfusion, since incompatible types of donor and recipient blood will result in the production of antibodies and the clumping and rupturing of blood cells, with possible death of the recipient.  
  In the ABO system, the two main antigens are designated A and B. These give rise to four blood groups: having A only (A), having B only (B), having both (AB), and having neither (O). People of blood group A, for example, have antibodies against antigen B in their blood plasma, and therefore cannot receive blood transfusions from people possessing this antigen on their red blood cells—that is, from people who are blood group B or AB. People who are blood group AB, on the other hand, have no anti-A or anti-B antibodies in their plasma and so can receive blood of any group.  
  A further complication is the presence or absence of yet another antigen, called the rhesus factor (Rh factor). Most individuals possess the rhesus factor on their red blood cells—that is, they are rhesus positive (Rh+). However, those without this factor are rhesus negative (Rh–) and produce antibodies if they come into contact with it.  
  blood clotting Blood clotting prevents excessive bleeding after injury. It involves a complex series of events (known as the blood clotting cascade). The result is the formation of a meshwork of protein fibers (fibrin) and trapped blood cells over the cut blood vessels.  
  When platelets (cell fragments) in the bloodstream come into contact with a damaged blood vessel, they and the vessel wall itself release the enzyme thrombokinase, which brings about the conversion of the inactive enzyme prothrombin into the active thrombin. Thrombin in turn catalyzes the conversion of the soluble protein fibrinogen, present in blood plasma, to the insoluble fibrin. This fibrous protein forms a net  
Compatibility of Blood Groups
Blood group        
  Antigen on red blood cell Antibody in plasma Blood groups that can be received by this individual Blood groups that can receive donations from this individual
A A anti-B A, 0 A, AB
B B anti-A B, 0 B, AB
AB A and B none any AB
0 neither A nor B anti-A and    
    anti-B 0 any





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  over the wound that traps red blood cells and seals the wound; the resulting jellylike clot hardens on exposure to air to form a scab. Calcium, vitamin K, and a variety of enzymes called factors are also necessary for efficient blood clotting.  
  Lymph is the fluid that exudes from the capillaries into the tissue spaces between the cells and is similar in composition to blood plasma. It carries some nutrients, and white blood cells to the tissues, and waste matter away from them. It may drain back from the tissues into the blood capillaries and from thence into the veins, or it may drain into a system of lymphatic vessels. These lead to lymph nodes (small, round bodies chiefly situated in the neck, armpit, groin, thorax, and abdomen), which process the lymphocytes produced by the bone marrow, and filter out harmful substances and bacteria. From the lymph nodes, vessels carry the lymph to the thoracic duct and the right lymphatic duct, which drain into the large veins in the neck.  
  functions of the circulatory system  
  The circulatory system plays an important role in many of the body's processes including respiration, nutrition, and the removal of wastes and poisons.  
  In respiration, the circulating blood delivers oxygen to the body's cells and removes carbon dioxide from them.  
  In nutrition, it carries digested food substances to the cells. Nutrients from food enter the bloodstream  
  To remember the functions of the blood:  
  Old Charlie Foster hates women having dull clothes.  
  (oxygen (transport), carbon dioxide (transport), food, heat, waste, hormones, disease, clotting)  


  by passing through the walls of the small intestine into the capillaries. The blood then carries most of the nutrients to the liver, where some of these are extracted and stored for release back into the blood as and when the body needs them. Other nutrients are converted by the liver into substances which are required in the production of energy, enzymes, and new building materials for the body.  
  Hormones, which affect or control the activities of various organs and tissues, are produced by the endocrine glands—including the thyroid, pituitary, adrenal, and sex glands—and they too are transported by the blood through the body.  
  The circulatory system helps to dispose of waste products which would prove harmful if allowed to accumulate. In processing food, the liver removes ammonia and other wastes, together with various poisons that enter the body through the digestive system. These are converted into water-soluble substances, which are carried by the blood to the kidneys. The kidneys then filter out these wastes and expel them from the body in urine.  
  lymph Lymph is the fluid that carries nutrients and white blood cells to the tissues.
Lymph enters the tissue from the capillaries (right) and is drained from the tissues
by lymph vessels. The lymph vessels form a network (left) called the lymphatic system.
At various points in the lymphatic system, lymph nodes (center) filter and clean the lymph.




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  The circulation also plays a role in temperature regulation. Some parts of the body, such as the liver and muscles, produce heat in the course of their activities. This heat is transported by the blood to warm other parts of the body. As the temperature of the body rises, the flow of blood into vessels in the skin increases, and excess heat is conveyed to the surface where it is dispersed. When the temperature of the body drops the flow of blood to the skin is restricted. Thus, the circulatory system acts as a natural thermostat allowing the body to maintain an optimum and stable temperature.  
  Respiratory System  
  Respiration is a biochemical process by which food substances are broken down to release energy, which the body's cells can use to carry out work. This process requires the presence of oxygen, and humans—like other aerobic organisms—possess special features that enable us both to take in oxygen from the atmosphere and transport it to the respiring cells, and also to eliminate the carbon dioxide that is a waste product of respiration.  
  The lungs are a pair of large cavities in the chest that are used to exchange oxygen and carbon dioxide between the blood and the atmosphere. The lung tissue, consisting of multitudes of air sacs (alveoli; singular alveolus) and blood vessels, is very light and spongy, and functions to create a huge surface area (of about 80 m2/95 yd2) where inhaled air comes into close contact with the blood so that oxygen can pass into the body and waste carbon dioxide can be passed out. The efficiency of lungs is enhanced by c0016-01.gifbreathing movements, by the thinness and moistness of their surfaces, and by a constant supply of circulating blood.  
  lung The human lungs contain 300,000 million tiny blood vessels which would
stretch for 2,400 km/1,500 mi if laid end to end. A healthy adult at rest breathes 12
times a minute; a baby breathes at twice this rate. Each breath brings 350 milliliters of
fresh air into the lungs, and expels 150 milliliters of stale air from the nose and throat.




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  The lung may be regarded as a many-chambered elastic bag placed in the air-tight chest cavity and having communication with the exterior only by means of the trachea (windpipe)—a flexible tube reinforced by rings of cartilage. Atmospheric pressure acting down the trachea keeps the lungs so far stretched that, together with the heart and great blood vessels, they completely fill the chest cavity. The trachea divides and subdivides into bronchi, bronchioles, and bronchial tubes, which, diverging in all directions, never join up, but terminate separately.  
  gas exchange at the alveoli  
  After a certain stage of subdivision, when their diameter is about 1 mm/0.04 in, the bronchial tubes develop into blind, grape-like pouches called alveoli, the walls of which consist of a delicate, moist membrane made up of transparent, flattened cells. The diffusion distance between the surrounding mesh of capillaries and the air inside the alveoli is therefore very small.  
  Oxygen in the alveolar air dissolves into the thin film of moisture that lines the alveolus and diffuses from this region of high concentration across the alveolar and capillary walls into the blood, where it is at a lower concentration. There it combines with hemoglobin in the red blood cells for transport around the body. Carbon dioxide follows a reverse path, diffusing from the blood plasma, where it is at a high concentration, through the capillary and alveolar walls into the alveolar air, where it is at a low concentration and can be exhaled.  
  Although lungs are specialized for gas exchange, they are not themselves muscular, consisting of spongy material. In order for fresh supplies of air to be drawn into the alveoli and for stale air to be eliminated, the lungs must be expanded and compressed, respectively, by muscular movement. Air is drawn into the lungs (inhaled) by the contraction of the diaphragm and intercostal muscles (the muscles between the ribs); relaxation of these muscles enables air to be breathed out (exhaled). The rate of breathing is controlled by the brain. High levels of activity lead to a greater demand for oxygen and an increased rate of breathing.  
  Nervous System  
  The nervous system coordinates the body's activities: it receives and processes information about them and about changes in the external environment, and initiates appropriate responses. It is composed of millions of interconnecting nerve cells, which are organized into a central nervous system, comprising c0016-01.gifbrain and spinal cord, and a peripheral nervous system, which connects with sensory organs, muscles, and glands.  
  Neuroscience for Kids
  Explore the nervous system—your brain, spinal cord, nerve cells, and senses—by means of this impressive site, designed for primary and secondary school students and teachers.  
  nerve cell  
  A nerve cell, or neuron, is an elongated, branched cell that forms the basic functional unit of the nervous system. It transmits information rapidly—at up to 160 m/525 ft per second—between different parts of the body. Each nerve cell has a cell body, containing the nucleus, from which trail processes called dendrites, responsible for receiving incoming signals. The cell's  
  nerve cell The anatomy and action of a nerve cell.
The nerve cell or neuron consists of a cell body with the
nucleus and projections called dendrites which pick up
messages. An extension of the cell, the axon, connects
one cell to the dendrites of the next. When a nerve cell is
stimulated, waves of sodium (Na+) and potassium (K+)
ions carry an electrical impulse down the axon.




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  longest process, called an axon, transmits signals away from the cell body.  
  The unit of information is the nerve impulse, a traveling wave of chemical and electrical changes involving the membrane of the nerve cell. The impulse involves the passage of sodium and potassium ions across the nerve-cell membrane. Sequential changes in the permeability of the membrane to positive sodium (Na+) ions and potassium (K+) ions produce electrical signals called action potentials. Impulses are received by the cell body and passed, as a pulse of electric charge, along the axon.  
  The axon terminates at the synapse, a specialized area closely linked to the next cell (which may be another nerve cell or a specialized effector cell such as a muscle). On reaching the synapse, the impulse releases a chemical neurotransmitter, which diffuses across to the neighboring cell and there stimulates another impulse or the action of the effector cell.  
  A nerve is a bundle of nerve cells enclosed in a sheath of connective tissue (myelin sheath) and transmitting nerve impulses to and from the brain and spinal cord. A single nerve may contain both motor and sensory nerve cells, but they function independently.  
  The myelin sheath serves to speed up the passage of nerve impulses. It is made up of fats and proteins and is formed from up to a hundred layers, laid down by special cells, the Schwann cells.  
  central nervous system  
  The central nervous system (CNS), which consists of the brain and spinal cord, integrates all nervous function.  
  brain The brain is a mass of interconnected c0016-01.gifnerve cells, forming the anterior part of the central nervous system, the activities of which it coordinates and controls. It is enclosed within three membranes, called the meninges. The thickest is the outermost layer, the dura mater; the middle layer is the arachnoid mater and the innermost layer is the pia mater. Cerebrospinal fluid circulates between the two innermost layers. The brain is further contained and protected by the skull.  
  The brain may be divided into three parts: forebrain, midbrain, and hindbrain.  
  The forebrain is mainly composed of the cerebrum—a pair of outgrowths (cerebral hemispheres) separated by a central fissure. This is the largest and most developed part of the brain, taking up about 70% of its volume. The cerebrum is covered with a deeply infolded layer of gray matter, the cerebral cortex, which is responsible for all higher functions, such as speech, emotions, and memory, and for initiating voluntary movement.  
  Many of the nerve fibers from the two sides of the body cross over as they enter the brain, so that the left cerebral hemisphere is associated with the right side of the body and vice versa. In right-handed people, the left hemisphere seems to play a greater role in controlling verbal and some mathematical skills, whereas the right hemisphere is more involved in spatial perception.  
  Other structures in the forebrain, lying between the hemispheres, are the thalamus, hypothalamus, and pituitary gland.  
  The midbrain and hindbrain together make up the brainstem—the oldest part of the brain in evolutionary terms—with the midbrain acting as a link with the structures of the forebrain. The hindbrain, which connects with the spinal cord, consists of the medulla oblongata, a region that contains centers for the control of respiration, heartbeat rate and strength, and blood pressure. Overlying this is the cerebellum, which is concerned with coordinating complex muscular processes such as maintaining posture and moving limbs.  
  spinal cord The spinal cord is the body's main nerve trunk, consisting of bundles of nerve cells enveloped in three layers of membrane (the meninges) and bathed in cerebrospinal fluid. It is encased and protected by the c0016-01.gifvertebral column, lying within the vertebral canal formed by the posterior arches of successive vertebrae. In adults, the spinal cord is about 45 cm/18 in long, extending from the bottom of the skull, where it is continuous with the medulla oblongata, to about waist level.  
  The spinal cord consists of nerve cell bodies (gray matter) and their myelinated processes or nerve fibers (white matter). In cross-section, the gray matter is arranged in an H-shape around the central canal of the spinal cord, and it is surrounded in turn by the white matter.  
  Paired spinal nerves arise from the cord at each vertebra. Each is a mixed nerve, consisting of both sensory and motor nerve fibers. The sensory fibers enter the spinal cord at a dorsal root and the motor fibers enter at a ventral root. This arrangement enables the spinal cord to relay impulses coming in and out at the same level, to relay impulses going up and down the cord to other levels, and relay impulses to and from the brain. The first of these involves a reflex arc, by which a sensory impulse can create a very rapid, involuntary response to a particular stimulus.  
  peripheral nervous system  
  The peripheral nervous system can be classified in many ways. It can be classified anatomically into  




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  cranial and spinal nerves—those that arise from the brain and spinal cord, respectively. It can be classified into sensory nerves—those that carry information from receptor cells or sensory organs to the central nervous system—and motor nerves—those that carry information from the central nervous system to the effector organs (muscles and glands). Mixed nerves contain both sensory and motor nerve fibers.  
  Cranial nerves consist of three sensory nerves, two motor nerves, and seven mixed nerves. The 31 pairs of spinal nerves are all mixed.  
  The peripheral nervous system can also be classified functionally into the voluntary nervous system, which controls voluntary activities, and the autonomic nervous system, which controls involuntary functions, such as heart rate, activity of the intestines, and the production of sweat. The autonomic nervous system can be further subdivided into the sympathetic and the parasympathetic systems. The sympathetic system responds to stress, when it speeds the heart rate, increases blood pressure, and generally prepares the body for action. The parasympathetic system is more important when the body is at rest, since it slows the heart rate, decreases blood pressure, and stimulates the digestive system.  
  sense organ  
  Sense organs are used to gain information about our surroundings. They all have specialized receptors (such as light receptors in the eye) and some means of translating their response into a nerve impulse that travels to the brain. The main human sense organs are the eye, which detects light and color (different wavelengths of light); the ear, which detects sound (vibrations of the air) and gravity; the nose, which detects some of the chemical molecules in the air; and the tongue, which detects some of the chemicals in food, giving a sense of taste. There are also many small sense organs in the skin, including pain, temperature, and pressure sensors, contributing to our sense of touch.  
  It's All in the Brain
  As part of a much larger site called ''Seeing, Hearing, and Smelling the World," here is a set of pages introducing the way in which we perceive the world through our senses. It is divided into five sections called "illusions reveal the brain's assumptions," "sensing change in the environment," "vision, hearing, and smell: the best-known senses," "a language the brain can understand," and "more than the sum of its parts."  
  ear The ear is primarily the organ of hearing, translating the vibrations that constitute sound into nerve signals that are passed to the brain.  
  It consists of three parts: outer ear, middle ear, and inner ear. The outer ear is a funnel that collects sound, directing it down a tube to the ear drum (tympanic membrane), which separates the outer and middle ears. Sounds vibrate this membrane, the mechanical movement of which is transferred to a smaller membrane leading to the inner ear by three small bones, the ossicles. Vibrations of the inner ear membrane move fluid contained in the snail-shaped cochlea, which vibrates hair cells that stimulate the auditory nerve connected  
  ear The structure of the ear. The three bones (ossicles) of the middle ear—hammer, anvil,
and stirrup—vibrate in unison and magnify sounds about 20 times. The  spiral- shaped
cochlea is the organ of hearing. As sound waves pass down the spiral tube, they vibrate
fine hairs lining the tube, which activate the auditory nerve connected to the brain.
The semicircular canals are the organs of balance, detecting movements of the head.




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  to the brain. There are approximately 30,000 sensory hair cells (stereocilia). Three fluid-filled canals of the inner ear detect changes of position; this mechanism, with other sensory inputs, is responsible for the sense of balance.  
  Ear Anatomy
  Concise medical information on the anatomy and function of the ear. It includes separate sections on perforated eardrums and their treatment, ear tubes and their use, tinnitus, and a series of different hearing tests.  
  eye The eye is the organ of vision. It is a roughly spherical structure contained in a bony socket called the orbit. Light enters the eye through the cornea, and passes through the circular opening (pupil) in the iris (the colored part of the eye). The ciliary muscles act on the lens (the rounded transparent structure behind the iris) to change its shape, so that images of objects at different distances can be focused on the retina. This is at the back of the eye, and is packed with light-sensitive receptor cells (rods and cones), connected to the brain by the optic nerve.  
  A U.S. team of developmental biologists established for the first time in 1997 that the vertebrate eye begins in the embryo as a single structure that later splits in two.  


  Breaking the Code of Color
  As part of a much larger site called "Seeing, Hearing, and Smelling the World," here is a set of pages examining the way we perceive the world through the sense of sight. It is divided into five sections called "how do we see colors?," "red, green, and blue cones," "color blindness: more prevalent among males," and ''judging a color." This site makes good use of images and animations to help with the explanations, so it is best viewed with an up-to-date browser.  
  Endocrine System  
  Like the nervous system, the endocrine system plays a role in controlling the body's activities. However, its actions are much slower and are mediated via the bloodstream, and its effects—such as growth and development—tend to be more long lasting.  
  The key players in the endocrine system are the hormones.  
  Hormones are chemical secretions, produced mainly by the c0016-01.gifendocrine glands, that bring about changes in the functions of distant tissues according to the body's requirements. They maintain homeostasis (a stable  
  eye The human eye. The retina of the eye contains about 137 million lightsensitive cells
in an area of about 650 sq mm/1 sq in. There are 130 million rod cells for black and white
vision and 7 million cone cells for color vision. The optic nerve contains about 1 million
nerve fibers. The focusing muscles of the eye adjust about 100,000 times a day. To
exercise the leg muscles to the same extent would need an 80 km/50 mi walk.




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  internal state) and also control tissue development, morphogenesis (the development of the body's form and structure), and reproduction. Many human diseases that are caused by hormone deficiency can be treated with hormone preparations.  
  Hormones usually act on specific targets. By binding themselves to special receptor sites on cell membranes, they can stimulate or suppress the actions of these cells—either by altering the permeability of the membrane to other chemicals, by activating enzymes, or by activating or inhibiting genes.  
  hormone interaction The relationships between hormones can be very complex in that some endocrine glands produce hormones that in turn stimulate other glands to produce hormones.  
  In this way, the anterior pituitary gland controls the thyroid gland, the adrenal cortex, the testis, and the ovary. When stimulated by the relevant pituitary hormone (known as a trophic hormone), each one of these glands produces its own hormones which have their own effects. But these hormones do something else that is highly ingenious; they act on the pituitary gland itself. They actually decrease (inhibit) the pituitary's secretion of the original trophic hormone, so that the level of the circulating hormone can never get too high and is kept at an optimum level. This control mechanism is known as negative feedback.  
  Feedback systems are used by the body to keep many of its constituents, such as body temperature, at a constant level; they are analogous to the thermostat system used for keeping the temperature of a house within narrow limits.  
  neurohormone Hormone secretion is not confined to the endocrine glands. For example, the small intestine secretes a hormone called secretin (the first hormone ever discovered). Other hormones, called neurohormones, are secreted by specialized nerve cells, either into the bloodstream or directly into the target tissue. For example, a neurohormone produced by nerve cells in the brain's hypothalamus acts on the pituitary gland, stimulating it to produce ADH (antidiuretic hormone), a hormone that, in turn, acts on the kidney to promote the reabsorption of water. Thus, chemical messages can be relayed to the appropriate part, or parts, of the body in response to stimulants either through nervous impulses via the nerves or through hormones secreted in the blood, or by both together.  
  endocrine gland  
  The endocrine glands are ductless glands that have capillaries running through them which provide direct access to the bloodstream. So as a hormone is produced by a gland, it is released directly into the bloodstream and carried to the tissues or organs that it affects. Normal functioning of the endocrine glands enables normal functioning of the body cells and results in general well-being.  
  The major glands are the pituitary, thyroid, parathyroid, adrenal, pancreas, ovary, and testis. There are also hormone-secreting cells in the kidney, liver, gastrointestinal tract, thymus (in the neck), pineal (in the brain), and placenta.  
  The anterior pituitary gland and the brain's hypothalamus act as control centers for overall coordination of hormone secretion.  
  major hormones  
  adrenaline Adrenaline, or epinephrine, is secreted by the medulla of the adrenal glands. It is synthesized from a closely related substance, noradrenaline, and the two hormones are released into the bloodstream in situations of fear or stress. Adrenaline's action on the liver raises blood-sugar levels by stimulating glucose production and its action on adipose tissue raises blood fatty-acid levels; it also increases the heart rate, increases blood flow to muscles, reduces blood flow to the skin with the production of sweat, widens the smaller breathing tubes (bronchioles) in the lungs, and dilates the pupils of the eyes.  
  thyroxine The secretion of thyroxine by the thyroid gland for the regulation of metabolism and growth is an almost continuous process, although the hormone is secreted in quite small amounts. Thyroxine helps to control metabolism mainly by regulating the speed at which mitochondria in the cells break down glucose in respiration. A deficiency of thyroxine in children will result in abnormal physical growth and retarded mental growth—a condition known as cretinism. This can be cured by injections of thyroxine. In adults, thyroxine deficiency leads to sluggishness and overweight; an overactive thyroid has the opposite effect, resulting in hyperactivity and underweight. Thyroxine contains iodine, and a lack of this in the diet limits production of the hormone, resulting in enlargement of the gland in the form of a swelling, or goiter. This can be treated by the addition of iodine to the diet.  
  insulin Insulin is a protein hormone, produced by specialized cells in the islets of Langerhans in the pancreas, that regulates the metabolism (rate of activity) of glucose, fats, and proteins. Normally, insulin is secreted in response to rising blood sugar levels (after a meal, for example), stimulating the body's cells to store the excess as glycogen. Failure of this  




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Hormones and their Functions
Gland Hormone Functions
posterior pituitary gland antidiuretic hormone (ADH) water reabsorption from kidney tubules
  oxytocin contraction of the uterus during birth
anterior pituitary gland growth hormone (GH) growth
  prolactin milk production and secretion
  follice-stimulating hormone (FSH) in females, maturation of the Graafian follicle; in males, sperm production
  luteininzing hormone (LH) in females, ovulation, formation of the corpus luteum; in males, testosterone synthesis
  thyroid stimulating hormone (TSH) stimulates the thyroid to release thyroid hormones
  adrenocorticotrophic hormone (ACTH) stimulates the adrenal cortext to produce corticosteroid hormones
ovary estrogen female secondary sexual characteristics
ovary and placenta progesterone prepares uterus for pregnancy; maintains it during pregnancy
testis testosterone male secondary sexual characteristics
adrenal gland (cortex) corticosteroid hormones controls salt and water
    metabolism; regulates use of carbohydrates, proteins, and fats
adrenal gland (medulla) adrenaline "fright, flight, or fight": increases heart activity, rate and depth of breathing, blood flow to muscles; inhibits digestion and excretion
thyroid thyroxine regulates metabolism and growth
  calcitonin regulates blood calcium levels by reducing release of calcium from bones
parathyroid parathormone regulates blood calcium levels by stimulating release of calcium from bones
pancreas (islets of Langerhans) insulin regulates blood glucose levels by stimulating conversion of glucose to glycogen
  glucagon regulates blood glucose levels by stimulating conversion of glycogen to glucose


  regulatory mechanism in diabetes mellitus requires treatment with insulin injections or capsules taken by mouth.  
  sex hormones The reproductive organs contain endocrine glands: ovaries in females and testes in males. Male sex hormones are called androgens, the most important of which is testosterone. The androgens regulate the development of the male sex organs and, at puberty, secondary sexual characteristics such as the growth of facial and pubic hair, breaking of the voice, and muscular development. The female sex hormones, estrogens, regulate the development of female sex organs and, at puberty, breast development and the growth of pubic hair. They also regulate menstruation and pregnancy.  
  endocrine gland The main human endrocrine glands. These
glands produce hormones—chemical messengers—which
travel in the bloodstream to stimulate certain cells.




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  Digestive System  
  The digestive system consists of all the organs and tissues involved in the digestion of food. The process of digestion breaks down the food by physical and chemical means into the different elements that are needed by the body for energy and tissue building and repair. Digestion begins in the mouth and is completed in the stomach and small intestine; most nutrients are absorbed into the small intestine from where they pass through the intestinal wall into the bloodstream; what remains is stored and concentrated into feces in the large intestine.  
  The digestive system of humans consists primarily of the alimentary canal, a tube that starts at the mouth, continues with the pharynx, esophagus (or gullet), stomach, large and small intestines, and rectum, and ends at the anus. The food moves through this canal by peristalsis whereby waves of involuntary muscular contraction and relaxation produced by the muscles in the wall of the gut cause the food to be ground and mixed with various digestive juices. Most of these juices contain digestive enzymes, specialized proteins that speed up reactions involved in the breakdown of food. Other digestive juices empty into the alimentary canal from the salivary glands, gall bladder, and pancreas, which are also part of the digestive system.  
  digestion and absorption  
  The fats, proteins, and carbohydrates (starches and sugars) in foods contain very complex molecules that are broken down by the processes of digestion for absorption into the bloodstream: starches and complex sugars are converted to simple sugars; fats are converted to fatty acids and glycerol; and proteins are converted to amino acids and peptides. Foods such as vitamins, minerals, and water do not need to undergo digestion prior to absorption into the bloodstream. The lining of the small intestine, which is the main site of absorption, is covered with small prominences called villi which increase the surface area for absorption and allow the digested nutrients to diffuse into small blood vessels lying immediately under the epithelium.  
  mouth, pharynx, and esophagus  
  The digestive process begins in the mouth with mastication (chewing) and salivation. The purpose of mastication is to crush and grind the food into small  
  digestive system When food is swallowed, it is moved down the esophagus
by the action of muscles (peristalsis) into the stomach. Digestion starts in the
stomach as the food is mixed with enzymes and strong acid. After several hours,
the food passes to the small intestine. Here more enzymes are added and digestion
is completed. After all nutrients have been absorbed, the indigestible parts pass
into the large intestine and thence to the rectum. The liver has many functions,
such as storing minerals and vitamins and making bile, which is stored in the gall
bladder until needed for the digestion of fats. The pancreas supplies enzymes.
The appendix appears to have no function in human beings.




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Digestive System: Pioneering Experiments on the Digestive System
  By Julian Rowe  
an army marches on its stomach
On June 6, 1822 at Fort Mackinac, Michigan, an 18-year-old French Canadian was accidentally wounded in the abdomen by the discharge of a musket. He was brought to the army surgeon, U.S. physician William Beaumont (1785–1853), who noted several serious wounds and, in particular, a hole in the abdominal wall and stomach. The surgeon observed that through this hole in the patient "was pouring out the food he had taken for breakfast."
The patient, Alexis St. Martin, a trapper by profession, was serving with the army as a porter and general servant. Not surprisingly, St. Martin was at first unable to keep food in his stomach. As the wound gradually healed, firm dressings were needed to retain the stomach contents. Beaumont tended his patient assiduously and tried during the ensuing months to close the hole in his stomach, without success. After 18 months, a small, protruding fleshy fold had grown to fill the aperture (fistula). This "valve" could be opened simply by pressing it with a finger.
digestion . . . inside and outside
At this point, it occurred to Beaumont that here was an ideal opportunity to study the process of digestion. His patient must have been an extremely tough character to have survived the accident at all. For the next nine years he was the subject of a remarkable series of pioneering experiments, in which Beaumont was able to vary systematically the conditions under which digestion took place and discover the chemical principles involved.
Beaumont attacked the problem of digestion in two ways. He studied how various substances were actually digested in the stomach, and also how they were "digested" outside the stomach in the digestive juices he extracted from St. Martin. He found it was easy enough to drain out the digestive juices from his fortuitously wounded patient "by placing the subject on his left side, depressing the valve within the aperture, introducing a gum elastic tube and then turning him . . . on introducing the tube the fluid soon began to run."
a typical experiment
Beaumont was basically interested in the rate and temperature of digestion, and also the chemical conditions that favoured different stages of the process of digestion. He describes a typical experiment (he performed hundreds), where (a) digestion in the stomach is contrasted (b) with artificial digestion in glass containers kept at suitable temperatures, like this: (a) "At 9 o'clock he breakfasted on bread, sausage, and coffee, and kept exercising. 11 o'clock, 30 minutes, stomach two-thirds empty, aspects of weather similar, thermometer 298°F, temperature of stomach 1011/28 and 1003/48. The appearance of contraction and dilation and alternative piston motions were distinctly observed at this examination. 12 o'clock, 20 minutes stomach empty." (b) "February 7. At 8 o'clock, 3 minutes a.m. I put twenty grains of boiled codfish into three drachms of gastric juice and placed them on the bath." "At 1 o'clock, 30 minutes, p.m., the fish in the gastric juice on the bath was almost dissolved, four grains only remaining: fluid opaque, white, nearly the color of milk. 2 o'clock, the fish in the vial all completely dissolved."
all a matter of chemistry
Beaumont's research showed clearly for the first time just what happens during digestion and that digestion, as a process, can take place independently outside the body. He wrote that gastric juice: "so far from being inert as water as some authors assert, is the most general solvent in nature of alimentary matter—even the hardest bone cannot withstand its action. It is capable, even out of the stomach, of effecting perfect digestion, with the aid of due and uniform degree of heat (100°Fahrenheit) and gentle agitation . . . I am impelled by the weight of evidence . . . to conclude that the change effected by it on the aliment, is purely chemical." Our modern understanding of the physiology of digestion as a process whereby foods are gradually broken down into their basic components follows logically from his work. An explanation of how the digestive juices flowed in the first place came in 1889, when Russian physiologist Ivan Pavlov (1849–1936) showed that their secretion in the stomach was controlled by the nervous system. By preventing the food eaten by a dog from actually entering the stomach, he found that the secretions of gastric juices began the moment the dog started eating, and continued as long as it did so. Since no food has entered the stomach, the secretions must be mediated by the nervous system.
Later, it was found that the further digestion that takes place beyond the stomach was hormonally controlled. But it was Beaumont's careful scientific work, which was published in 1833 with the title Experiments and Observations on the Gastric Juice and Physiology of Digestion, that triggered subsequent research in this field.


  fragments that can pass safely into the gut and that will have an increased surface area for the action of digestive enzymes. The teeth play an important role in this process.  
  tooth A tooth is a hard, bonelike structure in the mouth, used for biting and chewing food. The first teeth—a set of 20 milk teeth—appear from age six months to two and a half years. The permanent denti-  




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  In human mouths there are over 200 different species of bacteria, fungi, and protozoa. Billions of bacteria still cling to a tooth just brushed. Most species are harmless and some may be even be beneficial.  


  tion replaces these from the sixth year onward, the wisdom teeth (third molars) sometimes not appearing until the age of 25 or 30. Adults have 32 teeth.  
  Each tooth consists of an enamel coat (hardened calcium deposits), dentine (a thick, bonelike layer), and an inner pulp cavity, housing nerves and blood vessels. The roots are surrounded by cementum, which fuses them into their sockets in the jawbones. The neck of the tooth is covered by the gum, while the enamel-covered crown protrudes above the gum line.  
  saliva Alkaline saliva is poured into the mouth from three pairs of glands, named parotid, submandibular, and sublingual. The parotid gland secretes a clear saliva, the other two a sticky saliva containing mucin. Saliva has an important chemical action which, by means of an enzyme called ptyalin, converts cooked starch in the food into maltose, a kind of sugar. The saliva also moistens the food so that it can be rolled by the tongue and palate into a soft bolus.  
  swallowing When the food is sufficiently masticated, the bolus is pushed backward by the tongue into the pharynx. Here the swallowing reflex causes a series of muscular movements which propel the food into the gullet, or esophagus, and from there into the stomach.  
  About one in every 2,000 babies is born with a tooth. Louis XIV of France was born with two teeth, which may explain why he had had eight wet-nurses by the time he moved on to solid foods.  


  A small flap, the epiglottis, located behind the root of the tongue, closes off the end of the windpipe during swallowing to prevent food from passing into it and causing choking. The action of the epiglottis is a highly complex reflex process involving two phases. During the first stage a mouthful of chewed food is lifted by the tongue toward the top and back of the mouth. This is accompanied by the cessation of breathing and by the blocking of the nasal areas from the mouth. The second phase involves the epiglottis moving over the larynx while the food passes down into the esophagus.  
  The stomach is a bag of muscle situated just below the diaphragm. It is entered by the cardiac sphincter, which relaxes to admit the food and then closes. The mucous membrane of the stomach is lined with columnar epithelium, in which are embedded little pits called the gastric glands. Gastric juice, containing digestive enzymes and hydrochloric acid, pours from these glands when they are stimulated by the approach of food. The muscular coat of the stomach produces movements that churn the food into a semi-fluid called chyme and tend to urge it towards the intestine, but the pyloric sphincter (opening from the stomach) opens only in response to an acid stimulus. The food received from the mouth is alkaline, owing to the presence of saliva, and so the chyme stays in the stomach until thorough mixing with the hydrochloric acid in the gastric juice has rendered it acid.  
  Gastric juice contains an enzyme called pepsin, which is released as a precursor called pepsinogen; this is a protease (enzyme that acts on protein) that converts the proteins in the food into polypeptides. In babies, there is an enzyme called rennin that coagulates milk.  
  small intestine  
  The small intestine is 6 m/20 ft long, 4 cm/1.5 in in diameter, and consists of the duodenum, jejunum, and  
  tooth Adults have 32 teeth: two incisors, one canine, two premolars,
and three molars on each side of each jaw. Each tooth has three parts:
crown, neck, and root. The crown consists of a dense layer of mineral,
the enamel, surrounding hard dentine with a soft center, the pulp.




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  stomach The human stomach can hold about 1.5 1/2.6 pt
of liquid. The digestive juices are acidic enough to dissolve
metal. To avoid damage, the cells of the stomach lining are
replaced quickly–500,000 cells are replaced every minute,
and the whole stomach lining every three days.
  ileum. It is a muscular tube comprising an inner lining that secretes alkaline digestive juice, a submucous coat containing fine blood vessels and nerves, a muscular coat, and a serous coat covering all, supported by a strong peritoneum, which carries the blood and lymph vessels, and the nerves. The contents are passed along slowly by peristalsis (waves of involuntary muscular action).  
  The stomach rumbles when empty because of muscle contractions necessary to move the food along. The contractions are controlled by small rhythmic electrical currents generated by cells in the intestine, known as the cells of Cajal.  


  duodenum The duodenum is responsible for the digestion of carbohydrates, fats, and proteins, producing smaller molecules that can then be absorbed either by the duodenum itself or the ileum.  
  Once food has passed into the duodenum from the stomach, it is mixed with intestinal secretions (succus entericus), bile released from the gall bladder, and with a range of enzymes secreted from the pancreas, a digestive gland near the top of the intestine.  
  Bile consists of bile salts, bile pigments, cholesterol, and lecithin. Bile salts assist in the emulsification of fats—breaking them down into droplets small enough for attack by the enzyme lipase. The bile pigments are the breakdown products of old red blood cells that are passed into the gut to be eliminated with the feces.  
  Pancreatic juice contains three enzymes: trypsin (released as a precursor, trypsinogen), which attacks proteins more completely than gastric juice, converting them into amino acids; amylase, which converts starch into maltose, thus taking over the function of saliva whose activity is stopped in the stomach; and lipase, which splits the fats into glycerin and fatty acids.  
  The succus entericus contains the enzymes enterokinase, which is concerned in the production of trypsin, and aminopeptidases, which aid trypsin in the breaking up of polypeptides; it also contains enzymes that convert maltose and other sugars into glucose.  
  jejunum and ileum The jejunum connects the duodenum to the ileurn—the part of the intestine that absorbs digested food. The ileum wall is muscular so that waves of contraction (peristalsis) can mix the food and push it forward. Numerous fingerlike projections, or villi, point inward from the wall, increasing the surface area available for absorption. Each villus contains a network of blood vessels, which receives the food molecules diffusing through the wall and transports them to the liver via the hepatic portal vein. It also possess a small lymph vessel, called a lacteal.  
  Glycerin and fatty acids are carried into the lacteal and are again united into small globules of fat (giving the lacteal its characteristic milky appearance). The globules pass into the thoracic duct; from there they pass into the bloodstream and ultimately into the tissues, where they produce energy by oxidation or are stored in the form of adipose tissue.  
  The amino acids produced by protein digestion are carried in the bloodstream and used to repair and build up the tissues; any excess is converted by the liver into urea, which then passes to the kidneys and excreted via the bladder as urine.  
  Glucose sugar produced by the digestion of carbohydrates is temporarily stored in the liver as glycogen and converted back to glucose as and when required.  
  large intestine  
  The large intestine is 1.5 m/5 ft long, 6 cm/2.5 in in diameter, and includes the cecum, colon, and rectum. Materials travel through it slowly, taking from  




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  12 to 18 hours to reach the rectum. During this time, water and mineral salts are absorbed in the colon, and the waste residue is gradually compressed as a compact mass in the rectum, eventually to be egested, or expelled, as feces at the anus.  
  Urinary System  
  The urinary system is responsible for removing nitrogenous waste products such as urea and excess salts and water from the body. It consists of a pair of kidneys, which produce urine; ureters, which drain the kidneys; and a bladder that stores the urine before its discharge through the urethra.  
  The kidneys are a pair of organs situated on the rear wall of the abdomen, which are responsible for osmoregulation (water regulation), excretion of waste products, and maintaining the ionic composition of the blood. Each kidney receives copious quantities of blood via the renal artery, which are processed by more than a million filtering units called nephrons, or kidney tubules.  
  The outer cortex of the kidney contains the outermost parts of the nephrons, into which some of the blood's fluid component—plasma—is forced under pressure. The inner medulla contains the inner, looping part of the nephron, where about 85% of the plasma—less the waste products and the excess salts and water—is reabsorbed. The remaining fluid in the nephrons drain into the ureters and bladder as urine.  
  The action of the kidneys is vital, although if one is removed, the other enlarges to take over its function.  
  nephron Each nephron consists of a knot of blood capillaries called a glomerulus, enclosed in a cup-shaped structure called a Bowman's capsule, and a long, looping tubule enmeshed with yet more capillaries.  
  Blood enters the capillaries of the glomerulus at high pressure, forcing about 20% of the blood plasma into the Bowman's capsule. A process of ultrafiltration takes place—small molecules in the plasma with a molecular mass of less than 68,000 (such as those of water, salts, urea, and glucose) pass through into the Bowman's capsule, while larger molecules, such as proteins, and blood cells remain in the blood.  
  The Bowman's capsule leads into the nephron's tubule, which forms a long, U-shaped loop (loop of  
  urine Urine consists of excess water and waste products
that have been filtered from the blood by the kidneys; it is
stored in the bladder until it can be expelled from the body
via the urethra. Analyzing the composition of an individual's
urine can reveal a number of medical conditions, such as
poorly functioning kidneys, kidney stones, and diabetes.




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  nephron The kidney (left) contains more than a million filtering units, or nephrons
(right), consisting of the glomerulus, Bowman's capsule, and the loop of Henle. Blood
flows through the glomerulus—a tight knot of fine blood vessels from which water and
metabolic wastes filter into the tubule. This filtrate flows through the convoluted tubule
and loop of Henle where most of the water and useful molecules are reabsorbed into the
blood capillaries. The waste materials are passed to the collecting tubule as urine.
  Henle) in the kidney's medulla. This is where the selective reabsorption of salts, glucose, and water into the capillaries surrounding the tubule takes place. Additional waste products are also actively secreted into the tubule from the blood.  
  The antidiuretic hormone (ADH), produced by the pituitary gland, helps to regulate the amount of water reabsorbed from the tubule, depending on whether the overall concentration of the blood is high or low.  
  The fluid remaining in the tubule—a slightly acidic solution of salts, urea, and other wastes, tinted yellow by a pigment derived from bile—is called urine. It passes into the collecting tubules and ureter to be stored in the bladder, before being discharged to the exterior via the urethra.  
  kidney Blood enters the kidney through the renal artery. The blood is filtered
through the glomeruli to extract the nitrogenous waste products and excess
water that make up urine. The urine flows through the ureter to the bladder;
the cleaned blood then leaves the kidney via the renal vein.




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  Human Body Chronology  
Human Body Chronology
c. 1600 B.C. The Edwin Smith papyrus is written. The first medical book, it reveals an accurate understanding of the workings of the heart, stomach, bowels, and larger blood vessels. The papyrus is named for U.S. scientist Edwin Smith, a pioneer in the study of Egyptian science who acquired it in Luxor, Egypt, in 1862.
c. 1550 B.C. The Ebers papyrus is written. One of the oldest known medical works, it accurately describes the circulatory system, and, for the first time, recognizes the brain's central control function. The papyrus is named for German Egyptologist and novelist George Maurice Ebers, who acquired it in 1873.
c. 550 B.C. A document from Mohenjo Daro, in the Indus valley, includes information on anatomy, physiology, pathology, and obstetrics.
c. 285 B.C. Herophilus, an anatomist working at Alexandria, dissects human bodies and compares them with large mammals. He distinguishes the cerebrum and cerebellum, establishes the brain as the seat of thought, writes treatises on the human eye and on general anatomy, and writes a handbook for midwives.
c. 250 B.C. Greek anatomist Erasistratus of Ceos notes the difference between sensory and motor nerves, and correctly describes the functions of the valves of the heart.
157 Greek physician Galen becomes physician to the gladiators in his native Pergamum, offering him a unique insight into anatomy and the treatment of wounds.
180 Galen, practising at Rome, writes Methodus medendi/Method of Physicians, a medical textbook that will become the ultimate authority for medieval medicine.
1110 The text Anatomia porci/The Anatomy of the Pig describes the dissection of a pig in the medical school at Salerno, Italy. With human dissection still forbidden by the church, it becomes a valuable reference.
1163 At the council of Tours in France, the Catholic Church issues an edict against the mutilation of dead bodies. Although primarily aimed at the stripping of crusaders' bones for transport back to Europe, it also affects anotomical research.
1214 The Italian physician Marus of the medical school at Salerno, Italy, writes Anatomia Mauri/Anatomy of a Moor, one of the earliest Latin texts on anatomy.
1316 The Italian physician Mondino de Liuzzi conducts the first properly recorded dissection of a human corpse at Bologna University, Italy. His book Anatomia will become the standard work on anatomy for two centuries.
1527 Swiss physician Paracelsus lectures at Basel University, inviting the public to attend his lectures, and burning the books of Avicenna and Galen—the standard medical works of the day.
1530 Paracelsus writes Paragranum, arguing that the body is based on chemical processes, and suggesting specific chemical treatments for different diseases.
1540 Flemish anatomist Andreas Vesalius performs dissections on human cadavers at the University of Bologna. His discoveries contradict the writings of the ancient Greek physician Galen, until now the highest authority.
1542 Vesalius writes De humani corporis fabrica/On the Fabric of the Human Body, a highly illustrated, clearly written study of the human body, and effectively the beginning of the science of anatomy.
1552 Italian anatomist Bartolomeo Eustachio, in his Tabulae anatomicae/Anatomical Writings, details his discovery of the Eustachian tube between the ear and pharynx, and the Eustachian valve of the heart.
1553 Spanish theologian Michael Servetus, relates his discovery of the pulmonary circulation of the blood.
1561 Italian anatomist Gabriello Falloppio describes the inner ear and female reproductive organs for the first time.
1565 The Royal College of Physicians, London, England, is empowered by Queen Elizabeth I to carry out human dissections.
1568 Italian anatomist Contanzo Varolio publishes his research into the structure of the human brain.
1603 Italian anatomist Hieronymus Fabricius of Acquapendente discovers that the veins contain valves.
1604 Fabricius publishes De formata foetu/On the Formation of the Fetus, the first important study of embryology, n which the placenta is identified for the first time.
1614 Italian physician Santorio Santorio (Sanctorius) publishes De medicina statica/On Medical Statics, a pioneering study of perspiration and the metabolism.
1619 English physician William Harvey first announces his discovery of the circulation of the blood.
1641 Belgian anatomist Franciscus Sylvius discovers the cerebral or Sylvian fissure that separates the temporal, frontal, and parietal lobes of the brain.
1647 French medical student Jean Pecquet discovers the thoracic duct of the human body, root of the lymphatic system.
1652 Danish physician Thomas Bartholin publishes the first full description of the human lymphatic system.
1667 Italian anatomist Marcello Malpighi identifies the lower layer of the skin known today as the Malpighian layer.





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1679 Swiss physician Théophile Bonet collates the results of over 3,000 postmortems in his Sepulcretum/Cemetery, founding the study of morbid anatomy.
1684 Dutch microscopist Anton van Leeuwenhoek first describes red blood cells accurately.
1706 Italian anatomist Giovanni Morgagni publishes Adversaria anatomica/Anatomical Arguments, establishing him as an important anatomist.
1709 English philosopher and scientist George Berkeley publishes his New Theory of vision. Berkeley maintains that the eye one conveys sensations of color and that perceptions of form are gathered by touch.
1717 Leeuwenhoek discovers the structure of the nerves.
1745 French naturalist Pierre de Maupertuis attacks the currently favoured theory of reproduction, that the sperm contain a miniature version of the adult. He argues that characteristic of both parents influence the offspring.
1757 Swiss physiologist Albrecht von Haller publishes the first volume of his vast anatomy encyclopedia Elementa physiologiae corporis humanae/Elements of the Physiology of the Human Body.
1761 Italian physician Giovanni Morgagni writes De Sedibus et Causis Morborum per Anatoen Indagatis/On the Seats and Causes of Diseases Investigated by Anatomey. Based on almost 700 postmortem examinations, it is one of the first books on pathological anatomy.
1766 Albrecht von Haller shows that nerves stimulate muscles to contract, and that all nerves lead to the spinal column and brain. His work lays the foundation of modern neurology.
c. 1780 Italian anatomist Luigi Galvani discovers the electric nature of the nervous impulse.
1787 The German writer and polymath Johann Wolfgang von Goethe discovers the intermaxillary bone.
1802 English physician and physicist Thomas Young postulates that the eye requires only three receptors to the full spectrum, instead of receptors for each color as is generally believed.
1811 Scottish anatomist Charles Bell distinguishes between sensory and motor nerves.
1824 French chemists Jean-Baptiste-André Dumas and C. Prevost show that sperm is essential to fertilization.
1827 Estonian embryologist Karl von Baer reports the discovery of eggs in mammals and humans. It dispels the idea of the preformation of the embryo.
c. 1833 German physician Gustav Henle describes in details the structures of the eye and brain.
1834 The English microscopist Joseph Lister discovers the true shape of red blood cells.
1836 German physiologist Theodor Schwann discovers pepsin, the first known animal enzyme to be isolated.
1837 Bohemian physiologist Jan Purkinje discovers large nerve cells in the cerebellum with branching extensions; they are now called Purkinje cells.
1838 German chemist Justus von Liebig demonstrates that animal heat is due to respiration.
1840 Swiss embryologist Rudolf Albert von Kölliker identifies spermatozoa as cells.
1841 German physician Gustav Henle discovers that ''ductless glands" secrete their products directly into the bloodstream.
1842 Polish embryologist Robert Remak discovers that the early embryo consists of three layers: ectoderm, mesoderm, and endoderm.
1846 French physiologist Claude Bernard discovers that pancreatic secretions break down fat molecules into fatty acids and glycerin.
1852 Remak discovers that the growth of tissues, involves both the multiplication and division of cells.
1852 Swiss embryologist and histologist Rudolf Albert von Kölliker publishes Handbuch der Gewebelehre des Menschen/Handbook of Human Histology, the first textbook in histology, and the first to discuss tissues in terms of cell theory.
1855 Claude Bernard discovers that ductless glands produce hormones, which he calls "internal secretions."
1858 British physician Henry Gray publishes Anatomy of the Human Body, Descriptive and Surgical (Gray's Anatomy). It remains the standard text in anatomy for over 100 years.
1875 German embryologist Oskar Hertwig discovers that fertilization occurs with the fusion of the nuclei of the sperm and ovum.
1880 The parathyroid gland, which secretes parathormone that regulates calcium levels in the blood, is first described by Swedish physiologist Ivar Sandström.
1889 German physiologists Oskar Minkowski and Joseph von Mering remove the pancreas from a dog, which then develops diabetic symptoms. It leads them to conclude that the pancreas secretes an antidiabetic substance which is now known as insulin.
1895 Belgian bacteriologist Jules Bordet discovers antibodies.
1896 German biochemist E. Baumann discovers iodine in the thyroid gland; it is absent in all other tissues.
1898 Belgian bacteriologist Jules Bordet discovers hemolysis, the rupture of foreign red blood cells in blood serum. It soon leads to the discovery of blood groups.
1900 Austrian immunologist Karl Landsteiner discovers the ABO blood group system.
1901 Japanese-born U.S. biochemist Jokichi Takamine first synthesizes the heart stimulant adrenaline (epinephrine) from the suprarenal gland. It is the first pure hormone to be synthesized from natural sources.
1902 U.S. surgeon Harvey Williams Cushing publishes The Pituitary Body and its Disorders, which investigates the pituitary gland and its relationship to various disease.





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1903 Dutch physiologist Willem Einthoven invents the string galvanometer (electrocardiograph), which measures and records the tiny electrical impulses produced by contractions of the heart muscle.
1903 Russian physiologist Ivan Pavlov describes learning by conditioning. He trains dogs to expect food when they hear a bell and eventually they salivate every time the bell rings.
1914 U.S. biochemist Edward Kendall isolates the hormone thyroxine from the thyroid gland. It regulates metabolism by stimulating all cells to consume oxygen.
1921 French physician Jean Athanese Sicard uses a radiopaque iodine substance to X-ray the internal structure of the spinal column. He studies the bronchial tube the next year.
1921 Scottish bacteriologist Alexander Fleming discovers the antibacterial enzyme lysozyme, which is found in tears and saliva.
1922 English biochemist Frederick Gowland Hopkins isolates glutathione and demonstrates its vital role in the cell's utilization of oxygen.
1922 French surgeon Alexis Carrel discovers white blood cells (leukocytes).
1925 U.S. pathologist George whipple demonstrates that iron is the most important factor involved in the formation of red blood cells.
1929 German biochemist Adolf Butenandt and, simultaneously and independently, U.S. biochemist Edward Doisy isolate an estrogen hormone, which is involved in the growth abd development of females.
1931 Adolf Butenandt isolates the male sex hormone androgen.
1932 German-born British biochemist Hans Krebs discovers the urea cycle, in which ammonia is turned into urea in mammals.
1934 Adolph Butenandt isolates the female sex hormone progesterone.
1935 U.S. biochemist Edward Calvin Kendall isolates the steroid hormone cortisone from the adrenal cortex.
1937 German-born British biochemist Hans Krebs describes the citric acid cycle in cells, which converts sugars, fats, and proteins into carbon dioxide, water, and energy—the "Krebs cycle."
1940 Karl Landsteiner and U.S. physician and immunohematologist Alexander Wiener discover the rhesus (Rh) factor in blood, in the United States
1940 U.S. physiologist Herbert M. Evans uses radioactive iodine to prove that iodine is used by the thyroid gland.
1948 Swiss physiologist Walter Hess describes using fine electrodes to stimulate or destroy specific regions of the brain in cats and dogs; it allows him to discover the role played by various parts of the brain.
1949 U.S. researchers synthesize adrenocorticotropic hormone (ACTH) which the pituitary gland secretes to stimulate the adrenal glands.
1951 U.S. biochemist Robert Woodward synthesizes cortisone.
1953 English biochemist Frederick Sanger determines the structure of the insulin molecule. The largest protein molecule to have its chemical structure determined to date, it is essential in the laboratory synthesis.
1957 Interferon, a natural protein that fights viruses, is discovered by Scottish virologist Alick Isaacs and Swiss virologist Jean Lindemann.
1959 Austrian-born British biochemist Max Perutz determines the structure of hemoglobin.
1960 English biochemist John Kendrew, using X-ray diffraction techniques, elucidates the three-dimensional structure of the muscle protein myoglobin.
1971 Li Choh Hao and associates at the University of California Medical Center announce the synthesis of the human growth hormone somatotrophin.
1971 Polish-born U.S. endocrinologist Andrew Schally isolates the luteinizing hormone-releasing hormone (LH-RH), essential to human ovulation.
1971 Surgeons develop the fiberoptic endoscope, making it possible to view inside the human body by inserting catheters into the arms or legs and manipulating them into organs, such as the heart.
1972 Venezuelan-born U.S. immunologist Baruj Benacerraf and U.S. microbiologist Hugh O'Neill McDevitt show immune response to be genetically determined.
1974 British-born Danish immunologist Niels Jerne proposes a network theory of the immune system.
1975 Swiss scientists publish details of the first chemically directed synthesis of insulin.
1975 U.S. physiologist John Hughes discovers endorphins (morphine-like chemicals) in the brain.
1979 H. Goodman and J. Baxter of the University of California, Berkeley, together with D. V. Goeddel of Genentech, announce the biosynthetic production of a human growth hormone.
1988 The Human Genome Organization (HUGO) is established in Washington, D.C., United States; scientists announce a project to compile a complete "map" of human genes.
1992 Sperm cells are discovered by U.S. physician David Garbers of the University of Texas to have odour receptors and may therefore reach eggs by detecting scent.
1996 Two U.S. dentists discover a new muscle running from the jaw to just behind the eye socket. About 3 cm/1 in long, it helps to support and raise the jaw.
1998 Researchers at the University of Texas in conjunction with the British company SmithKline Beecham identify a hormone that triggers hunger in humans.





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  Abel, John Jacob (1857–1938) U.S. biochemist, discoverer of adrenaline. He studied the chemical composition of body tissues, and this led, in 1898, to the discovery of adrenaline, the first hormone to be identified, which Abel called epinephrine. He later became the first to isolate amino acids from blood.  
  Axelrod, Julius (1912– ) U.S. neuropharmacologist who shared the 1970 Nobel Prize for Physiology or Medicine with the biophysicists Bernard Katz and Ulf von Euler (1905–1983) for his work on neurotransmitters (the chemical messengers of the brain). Axelrod wanted to know why the messengers, once transmitted, should stop operating. Through his studies he found a number of specific enzymes that rapidly degraded the neurotransmitters.  
  Bayliss, William Maddock (1860–1924) English physiologist who discovered the digestive hormone secretin, the first hormone to be found, with Ernest Starling 1902. During World War I, Bayliss introduced the use of saline (salt water) injections to help the injured recover from shock.  
  By experimenting with the inner lining of the duodenum (first part of the intestines), Bayliss concluded that as hydrochloric acid from the stomach's digestive juices passes into the duodenum during the normal digestive process, the duodenal lining releases a chemical into the bloodstream which, in turn, makes the pancreas secrete its juices. This is the hormone secretin.  
  Bayliss went on to study the activation of enzymes, particularly the pancreatic enzyme trypsin. Bayliss and Starling also investigated the peristaltic movements of the intestines and their nerve supply, and pressures within the venous and arterial systems.  
  Cohen, Stanley (1922– ) U.S. biochemist who was awarded the Nobel Prize for Physiology or Medicine 1986 jointly with Rita Levi-Montalcini for their work to isolate and characterize growth factors, small proteins that regulate the growth of specific types of cells.  
  Cohen helped to purify and characterize nerve growth factor, a small protein produced in the male salivary gland that regulates the growth of small nerves and affects the development of the sensory and sympathetic nerve cells. He went on to discover another growth factor, called epidermal growth factor, that affects epithelial cell growth, tooth eruption, and eyelid opening. He then labored to link epidermal growth factor to the regulation of embryonic growth. Subsequent studies by other scientists have shown that this growth factor also plays a crucial part in the exaggerated growth rate of some cancer cells.  
  Cori, Carl Ferdinand (1896–1984) and Gerty Theresa (1896–1957) U.S. biochemists born in Austro-Hungary who, together with Argentine physiologist Bernardo Houssay (1887–1971), received a Nobel prize 1947 for their discovery of how glycogen (animal starch)—a derivative of glucose—is broken down and resynthesized in the body, for use as a store and source of energy.  
  Glycogen is broken down in the muscles into lactic acid, which, when the muscles rest, is reconverted to glycogen. In the 1930s the Coris set out to determine exactly how these changes take place. Gerty Cori found a new substance in muscle tissue, glucose-1-phosphate, now known as Cori ester. Its formation from glycogen involves only a small amount of energy change, so that the balance between the two substances can easily be shifted in either direction. The second step in the reaction chain involves the conversion of glucose1-phosphate into glucose-6-phosphate. Finally this second phosphate is changed to fructose-1,6-diphosphate, which is eventually converted to lactic acid. The first set of reactions from glycogen to glucose-6-phosphate is now termed glycogenolysis; the second set, from glucose-6-phosphate to lactic acid, is referred to as glycolysis.  
  Du Bois-Reymond, Emil Heinrich (1818–1896) German physiologist. He showed the existence of electrical currents in nerves, correctly arguing that it would be possible to transmit nerve impulses chemically. His experimental techniques proved the basis for almost all future work in electrophysiology.  
  Investigating the physiology of muscles and nerves, Du Bois-Reymond demonstrated the presence of electricity in animals, especially researching electric fishes. By 1849 he had evolved a delicate multiplier for measuring nerve currents, enabling him to detect an electric current in ordinary localized muscle tissues, notably contracting muscles. He observantly traced it to individual fibers, finding their interior was negative with regard to the surface.  
  Erasistratus (c. 304–c. 250 B.C.) Greek physician and anatomist. Regarded as the founder of physiology, he came close to discovering the true function of several important systems of the body, which were not fully understood until nearly 1,000 years later. For example, the principle of blood circulation, though he had it circulating in the wrong direction. Tracing the network of veins, arteries, and nerves, he postulated that the nerves carry the "animal" spirit, the arteries the "vital" spirit, and the veins blood. He did, however, grasp a rudimentary principle of oxygen exchange and condemned bloodletting as a form of treatment.  
  Erasistratus dissected and examined the human brain, noting the convolutions of the outer surface, and observed that the organ is divided into larger and smaller portions (the cerebrum and cerebellum). He compared the human brain with those of other animals and made the correct hypothesis that the surface area/volume complexity is directly related to the intelligence of the animal.  
  Fabricius, Geronimo (1537–1619) Latinized name of Girolamo Fabrizio, Italian anatomist and embryologist. He made a detailed study of the veins and discovered the valves that direct the blood flow toward the heart. He also studied the development of chick embryos.  
  Fabricius also investigated the mechanics of respiration, the action of muscles, the anatomy of the larynx (about which he was the first to give a full description) and the eye (he was the first to correctly describe the location of the lens and the first to demonstrate that the pupil changes size).  
  Fabricius publicly demonstrated the valves in the veins of the limbs 1579, and in 1603 published the first accurate  




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  description, with detailed illustrations, of these valves in De Venarum Ostiolis/On the Valves of the Veins. He mistakenly believed, however, that the valves' function was to retard the flow of blood to enable the tissues to absorb nutriment.  
  Flourens, Pierre Jean Marie (1794–1867) French physiologist who experimented widely on the effects of the removal of various parts of the central nervous system. He determined the function of different parts of the mammalian brain and the role of the semi-circular canals of the inner ear in balance.  
  Flourens removed the cerebral hemispheres in the brain of a pigeon and observed that this made the bird blind. When one cerebral hemisphere was removed, the bird lost the sight from its opposite eye. Flourens therefore demonstrated that vision depends on the integrity of the cerebral cortex. He next removed only the cerebellum and determined that while the bird could see and hear well, it stood, walked, and flew in an indecisive manner. The bird's equilibrium was almost entirely abolished. Flourens later demonstrated the same results on a dog. Injury to the cerebellum therefore causes loss of coordination. Flourens introduced the idea of nervous coordination to physiology.  
  Flourens also did important work on the role of the semicircular canals of the inner ear on balance and demonstrated that the respiratory center is situated in the brain stem, the medulla oblongata, the area in the brain that is responsible for the involuntary contraction of the respiratory muscles.  
  Gasser, Herbert Spencer (1888–1963) U.S. physiologist who shared the 1944 Nobel Prize for Physiology or Medicine with Joseph Erlanger for their discoveries regarding the specialized functions of nerve fibers. Gasser was also one of the first to demonstrate the chemical transmission of nerve impulses.  
  Gasser and Erlanger found that the smaller nerve fibers were responsible for the conduction of pain and that the speed of electrical transmission by a nerve depends upon its diameter. Gasser also performed a great deal of experiments attempting to prove that chemical transmission occurs between nerves. He was one of the first to demonstrate that the injection of acetylcholine into bird muscles or denervated mammalian muscles results in slow contraction. Acetylcholine is now known to be a neurotransmitter, a chemical that carries nerve impulses across synapses between nerves.  
  Golgi, Camillo (1843–1926) Italian cell biologist who produced the first detailed knowledge of the fine structure of the nervous system. He shared the 1906 Nobel Prize for Physiology or Medicine with Santiago Ramón y Cajal, who followed up Golgi's work. Golgi's use of silver salts in staining cells proved so effective in showing up the components and fine processes of nerve cells that even the synapses—tiny gaps between the cells—were visible. The Golgi apparatus, a series of flattened membranous cavities found in the cytoplasm of cells, was first described by him in 1898.  
  From his examinations of different parts of the brain, Golgi put forward the theory that there are two types of nerve cells, sensory and motor cells, and that axons are concerned with the transmission of nerve impulses. He discovered tension receptors in the tendons—now called the organs of Golgi.  
  Graaf, Regnier de (1641–1673) Dutch physician and anatomist who discovered the ovarian follicles, which were later named Graafian follicles. He named the ovaries and gave exact descriptions of the testicles. He was also the first to isolate and collect the secretions of the pancreas and gall bladder.  
  Hall, Marshall (1790–1857) English physician and physiologist who distinguished between voluntary and involuntary reflex muscle contractions, proving that the spinal cord is more than a passive nerve trunk transmitting voluntary signals from the brain and sensory signals to the brain.  
  Hall is best known for his work on the nervous system of frogs. He showed that if the spinal cord of a frog was severed between the front and back limbs, then the front limbs could still be moved voluntarily but the back limbs were useless. He further showed that the back legs could be stimulated to move artificially, but only once for each stimulus. These were reflex (involuntary) muscle contractions. Pain stimuli applied to the back legs were not felt by the animals. From these experiments Hall deduced that the nervous system is made up of a series of reflex arcs. In the intact spinal cord these reflex arcs are coordinated by the ascending and descending pathways in the cord to form movement patterns.  
  Hall also demonstrated that stimulus could not be put into the cord through a sensory nerve without it resulting in effects beyond the anatomical segment to which that nerve belongs.  
  Haller, Albrecht von (1708–1777) Swiss physician and scientist, founder of neurology. He studied the muscles and nerves, and concluded that nerves provide the stimulus that triggers muscle contraction. He also showed that it is the nerves, not muscle or skin, that receive sensation.  
  Tracing the pathways of nerves, he was able to demonstrate that they always lead to the spinal cord or the brain, suggesting that these regions might be where awareness of sensation and the initiation of answering responses are located. While carrying out his experiments, Haller discovered several processes of the human body, such as the role of bile in digesting fats. He also wrote a report on his study of embryonic development.  
  Harvey, William (1578–1657) English physician who discovered the circulation of blood. In 1628 he published his book De motu cordis/On the Motion of the Heart and the Blood in Animals. He also explored the development of chick and deer embryos. Harvey's discovery marked the beginning of the end of medicine as taught by Greek physician c0016-01.gifGalen, which had been accepted for 1,400 years.  
  Examining the heart and blood vessels of mammals, Harvey deduced that the blood in the veins must flow only towards the heart. He also calculated the amount of blood that left the heart at each beat, and realized that the same blood must be circulating continuously around the body. He reasoned that it passes from the right side of the heart to the left through the lungs (pulmonary circulation).  




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  Herophilus of Chalcedon (c. 330–c. 260 B.C.) Greek physician, active in Alexandria. His handbooks on anatomy make pioneering use of dissection, which, according to several ancient sources, he carried out on live criminals condemned to death.  
  Hodgkin, Alan Lloyd (1914– ) British physiologist engaged in research with Andrew Huxley on the mechanism of conduction in peripheral nerves 1945–60. He devised techniques for measuring electric currents flowing across a cell membrane. In 1963 they shared the Nobel prize.  
  Hodgkin and Huxley managed for the first time to record electrical changes across the cell membrane, and Hodgkin then built on these findings working with Bernard c0016-01.gifKatz, another cell physiologist. They proposed that during the resting phase a nerve membrane allows only potassium ions to diffuse into the cell, but when the cell is excited it allows sodium ions (which are positively charged) to enter and potassium ions to move out. The extrusion of sodium is probably dependent on the metabolic energy supplied either directly or indirectly in the form of ATP (adenosine triphosphate). The amount of sodium flowing in equals that of the potassium flowing out.  
  Huxley, Andrew Fielding (1917– ) English physiologist, awarded the Nobel prize in 1963 with Alan Hodgkin for work on nerve impulses, discovering how ionic mechanisms are used in nerves to transmit impulses.  
  In 1945 at Cambridge, Hodgkin and Huxley began to measure the electrochemical behavior of nerve membranes. They experimented on axons of the giant squid—each axon is about 0.7 mm/0.03 in in diameter. They inserted a glass capillary tube filled with sea water into the axon to test the composition of the ions in and surrounding the cell, which also had a microelectrode inserted into it. Stimulating the axon with a pair of outside electrodes, they showed that the inside of the cell was at first negative (the resting potential) and the outside positive, and that during the conduction of the nerve impulse the membrane potential reversed.  
  Hyrtl, Joseph (1810–1894) Austrian anatomist who, in part, was responsible for the success of the New Vienna School of Medicine in the 19th century. He was the most popular and successful teacher of anatomy in Europe. His textbooks of general anatomy were widely used including his seminal work The Handbook of Topographical Anatomy 1845. He is also the author of scholarly works on anatomical terminology and on Hebrew and Arabic elements of anatomy.  
  Katz, Bernard (1911– ) British biophysicist. He shared the 1970 Nobel Prize for Physiology or Medicine for work on the biochemistry of the transmission and control of signals in the nervous system, vital in the search for remedies for nervous and mental disorders.  
  In the 1940s, Katz joined in the Nobel-prizewinning research of Alan c0016-01.gifHodgkin and Andrew c0016-01.gifHuxley on the electrochemical behavior of nerve membranes. During the 1950s, Katz found that minute amounts of acetylcholine were randomly released by nerve endings at the neuromuscular junction, giving rise to very small electrical potentials; he also found that the size of the potential was always a multiple of a certain minimum value. These findings led him to suggest that acetylcholine was released in discrete "packets" (analogous to quanta) of a few thousand molecules each, and that these packets were released relatively infrequently while a nerve was at rest but very rapidly when an impulse arrived at the neuromuscular junction.  
  Kuhne, Wilhelme (1837–1900) German physiologist who coined the term "enzyme" (a substance produced by cells that is capable of speeding up chemical reactions) and was the first to show the reversible effect of light on the retina of the eye.  
  Kuhne began his research on substances responsible for the breakdown of foodstuffs in the digestive system. After working on the active substances in pancreatic juices, Kuhne coined the term "enzyme," which he derived from the Greek words en which means "in" and zyme which means ''leaven." He also worked on the photosensitive proteins present in the retina of the frog's eye and was the first to show the reversible effect of light on the activity of a colored pigment (later called rhodopsin) in the retina.  
  Langley, John Newport (1852–1925) English physiologist who investigated the structure and function of the autonomic nervous system, the involuntary part of the nervous system, that controls the striated and cardiac muscles and the organs of the gastrointestinal, cardiovascular, excretory, and endocrine systems. He went on to divide up the autonomic nervous system into the sympathetic and parasympathetic branches, with specific functions being apportioned to each.  
  Langley did a great deal of research on the structure and function of sympathetic nerve fibers and ganglia. Ganglia are clusters of the cell bodies of sensory nerve cells in the peripheral nervous system which lie just outside the spinal cord. Langley blocked nervous impulses by applying various chemicals, such as nicotine, to ganglia. The cell bodies of motor nerve cells in the autonomic nervous system also lie in these ganglia.  
  Levi-Montalcini, Rita (1909– ) Italian neurologist who discovered nerve-growth factor, a substance that controls how many cells make up the adult nervous system. She shared the 1986 Nobel Prize for Physiology or Medicine with her coworker, U.S. biochemist Stanley c0016-01.gifCohen.  
  Levi-Montalcini first discovered nerve-growth factor in the salivary glands of developing mouse embryos, and later in many tissues. She established that it was chemically a protein, and analyzed the mechanism of its action. Her work has contributed to the understanding of some neurological diseases, tissue regeneration, and cancer mechanisms.  
  Lucas, Keith (1879–1916) English neurophysiologist who investigated the transmission of nerve impulses. He demonstrated that the contraction of muscle fibers follows the "all or none" law: a certain amount of stimulus is needed in order to induce a nerve impulse and subsequent muscle contraction. Any stimulus below that threshold has no effect regardless of its duration.  
  Lucas showed that when two successive stimuli are given, the response to the second stimulus cannot be evoked if the first nerve impulse is still in progress. He also demonstrated that following a contraction there is a period of diminished excitability during which the muscle cannot be induced to contract again. This is due to the chemical transmission of impulses over synaptic clefts (the junction between two individual nerve cells).  




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  Malpighi, Marcello (1628–1694) Italian physiologist who made many anatomical discoveries in his pioneering microscope studies of animal and plant tissues. For example, he discovered blood capillaries and indentified the sensory receptors (papillae) of the tongue, which he thought could be nerve endings.  
  Studying the lungs of a frog, Malpighi found them to consist of thin membranes containing fine blood vessels covering vast numbers of small air sacs. This discovery made it easier to explain how air (oxygen) seeps from the lungs to the blood vessels and is carried around the body.  
  Mechnikov, Ilya Ilich (1845–1916) Russian-born French zoologist who discovered the function of white blood cells and phagocytes (amoebalike blood cells that engulf foreign bodies). He also described how these "scavenger cells" can attack the body itself (autoimmune disease). He shared the Nobel Prize for Physiology or Medicine 1908.  
  While studying the transparent larvae of starfish, Mechnikov observed that certain cells surrounded and engulfed foreign particles that entered the bodies of the larvae. Later he demonstrated that phagocytes exist in higher animals, and form the first line of defense against acute infections.  
  Meyerhof, Otto (1884–1951) German-born U.S. biochemist who carried out research into the metabolic processes involved in the action of muscles. For this work he shared the 1922 Nobel Prize for Physiology or Medicine.  
  In 1920 Meyerhof showed that, in anaerobic conditions, the amounts of glycogen metabolized and lactic acid produced in a contracting muscle are proportional to the tension in the muscle. He also demonstrated that 20–25% of the lactic acid is oxidized during the muscle's recovery period and that energy produced by this oxidation is used to convert the remainder of the lactic acid back to glycogen. Meyerhof introduced the term "glycolysis" to describe the anaerobic degradation of glycogen to lactic acid, and showed the cyclic nature of energy transformations in living cells.  
  Michaelis, Leonor (1875–1949) German-born, U.S. biochemist who derived a mathematical model to describe the kinetics of how enzymes catalyze (trigger) reactions. The work of Michaelis and German scientist Maude Menton enabled several subsequent generations of biochemists to correctly assess the nature and efficiency of the key enzyme-driven steps in cell metabolism.  
  In their mathematical calculations they correctly assumed that an enzyme works by rapidly and reversibly binding to a specific molecule (called the substrate) to form an enzyme—substrate complex, triggering a reaction that generates a product molecule. Once the reaction has occurred the enzyme is released unchanged. Michaelis and Menton then correlated the speed of the enzymatic reaction with the concentrations of both the enzyme and the substrate.  
  Pavlov, Ivan Petrovich (1849–1936) Russian physiologist who studied conditioned reflexes in animals. His work had a great impact on behavioral theory and learning theory. He was awarded Nobel Prize for Physiology or Medicine 1904.  
  Studying the physiology of the circulatory system and the regulation of blood pressure, Pavlov devised animal experiments such as the dissection of the cardiac nerves of a living dog to show how the nerves that leave the cardiac plexus control heartbeat strength.  
  Pavlov's work relating to human behavior and the nervous system also emphasized the importance of conditioning. He deduced that the inhibitive behavior of a psychotic person is a means of self-protection. The person shuts out the world and, with it, all damaging stimuli. Following this theory, the treatment of psychiatric patients in Russia involved placing a sick person in completely calm and quiet surroundings.  
  Sakmann, Bert (1942– ) German cell physiologist who shared the 1991 Nobel Prize for Physiology or Medicine with Erwin Neher for their studies of the electrical activity of nerve cell membranes. They also determined the role of the neurohormone beta-endorphin.  
  In 1976 Sakmann worked with Neher to develop a technique called the patch-clamp technique, which greatly enhanced the ability of researchers to measure the electrical activity of nerves and revolutionized the study of ion channels in membranes.  
  Using the patch-clamp technique Neher and Sakmann also investigated the role of beta-endorphin. Beta-endorphin is a neurohormone which is secreted by the pituitary gland and reaches all body tissues carried in the blood. It is a peptide opiate that has been found to play a clinical role in the perception of pain, behavioral patterns, obesity and diabetes, and psychiatric disorders. They demonstrated that beta-endorphin acts not only to regulate the release of neurotransmitter substances by nerves in the brain, but also, via calcium channels, on the walls of the arteries of the brain.  
  Samuelsson, Bengt Ingemar (1934– ) Swedish biochemist who shared the 1982 Nobel Prize for Physiology or Medicine with Sune Bergström and John Vane for the purification of prostaglandins, chemical messengers produced by the prostate gland.  
  Ulf Muller originally discovered that human semen and extracts of sheep seminal vesicular glands had peculiar properties. Both substances caused contraction of smooth muscle in vitro (in an artificial environment, such as a test-tube) and sharp decreases in the blood pressure in experimental animals. Muller called the active agents in these substances prostaglandins, because they were primarily made in the prostate gland.  
  The purification of the prostaglandins was complicated by the very low amounts present in seminal fluid and their extremely short half lives. In 1957, Bergström and Samuelsson managed to obtain crystals from two prostaglandins, alprostadil (PGE1) and PGF1a, which cause the contraction of smooth muscle. They reported the chemical characterization of these two prostaglandins 1962.  
  Servetus, Michael (Miguel Serveto) (1511–1553) Spanish Christian Anabaptist theologian and physician. He was a pioneer in the study of the circulation of the blood and found that it circulates to the lungs from the right chamber of the heart. He was burned alive by the church reformer Calvin in Geneva, Switzerland, for publishing attacks on the doctrine of the Trinity.  
  Vesalius, Andreas (1514–1564) Belgian physician who revolutionized anatomy by performing post mortem dissections and making use of illustrations to teach anatomy. Vesalius  




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  upset the authority of c0016-01.gifGalen, and his book—the first real textbook of anatomy—marked the beginning of biology as a science. His dissections of the human body (then illegal) enabled him to discover that c0016-01.gifGalen's system of medicine was based on fundamental anatomical errors. Vesalius disproved the widely held belief that men had one rib less than women. He also believed, contrary to Aristotle's theory of the heart being the center of the mind and emotion, that the brain and the nervous system were the center.  
  Vesalius's book De humani corporis fabrica/On the Structure of the Human Body of 1543 employed talented artists to provide the illustrations and is one of the great books of the 16th century. The quality of anatomical depiction introduced a new standard into all illustrated works, especially into medical books, and highlighted the need to introduce scientific method into the study of anatomy. Together with the main work of astronomer Copernicus, published in the same year, On the Structure of the Human Body marked the dawn of modern science.  
the part of the body below the c0016-01.gifthorax, containing the digestive organs and female reproductive organs. It is separated from the thorax by the c0016-01.gifdiaphragm, a sheet of muscular tissue.
the ability of the c0016-01.gifeye to focus on near or far objects by changing the shape of the lens.
ACh, chemical that serves as a c0016-01.gifneurotransmitter, communicating nerve impulses between the cells of the nervous system. It is largely associated with the transmission of impulses across the c0016-01.gifsynapse (junction) between nerve and muscle cells, causing the muscles to contract.
  action potential
change in the potential difference (voltage) across the membrane of a nerve cell when an impulse passes along it. A change in potential (from about -60 to +45 millivolts) accompanies the passage of sodium and potassium ions across the membrane.
  active transport
in cells, the use of energy to move substances, usually molecules or ions, across a membrane.
masses of lymphoid tissue, similar to c0016-01.giftonsils, located in the upper part of the throat, behind the nose. They are part of a child's natural defenses against the entry of germs but usually shrink and disappear by the age of ten.
abbreviation for antidiuretic hormone, part of the system maintaining a correct salt/water balance in vertebrates.
  adipose tissue
c0016-01.gifconnective tissue that serves as an energy reserve, and also pads some organs. It is commonly called fat tissue, and consists of large spherical cells filled with fat. Major layers are in the inner layer of skin and around the kidneys and heart.
  adrenal gland, or suprarenal gland,
triangular gland situated on top of the c0016-01.gifkidney. The cortex (outer part) secretes various steroid hormones and other hormones that control salt and water metabolism and regulate the use of carbohydrates, proteins, and fats. The medulla (inner part) secretes the hormones adrenaline and noradrenaline which, during times of stress, prepare the body for "fight or flight."
  adrenaline, or epinephrine,
hormone secreted by the medulla of the c0016-01.gifadrenal glands. Adrenaline is synthesized from a closely related substance, noradrenaline, and the two hormones are released into the bloodstream in situations of fear or stress.
  adrenocorticotrophic hormone
hormone secreted by the anterior lobe of the c0016-01.gifpituitary gland.
  alimentary canal
tube adapted for c0016-01.gifdigestion, through which food passes. It is a complex organ, consisting of the mouth cavity, pharynx, esophagus, stomach, and the small and large intestines.
(plural alveoli) one of the many thousands of tiny air sacs in the c0016-01.giflungs in which exchange of oxygen and carbon dioxide takes place between air and the bloodstream.
one of a group of c0016-01.gifenzymes that break down starches into their component molecules (sugars) for use in the body. It is found in saliva (as ptyalin) and in pancreatic juices.
general name for any male sex hormone, of which c0016-01.giftestosterone is the most important.
  antagonistic muscles
pair of muscles allowing coordinated movement of the skeletal joints. The individual components of antagonistic pairs can be classified into extensors (muscles that straighten a limb) and flexors (muscles that bend a limb).
protein molecule produced in the blood by c0016-01.giflymphocytes in response to the presence of foreign or invading substances (c0016-01.gifantigens); such substances include the proteins carried on the surface of infecting microorganisms.
  antidiuretic hormone
pituitary hormone that prevents excessive fluid loss. See c0016-01.gifADH.
  antigen any substance that causes the production of c0016-01.gifantibodies by the body's immune system.  
opening at the end of the alimentary canal that allows undigested food and other waste materials to pass out of the body, in the form of feces.
a short, blind-ended tube attached to the c0016-01.gifcaecum. It has no known function in humans.
  aqueous humor
watery fluid found in the chamber between the cornea and lens of the vertebrate eye. Similar to blood plasma in composition, it is constantly renewed.
vessel that carries blood from the heart to the rest of the body.
process by which absorbed food molecules, circulating in the blood, pass into the cells and are used for growth, tissue repair, and other metabolic activities. The actual destiny of each food molecule depends not only on its type, but also on the body requirements at that time.
either of the two upper chambers of the heart.
  auditory canal
tube leading from the outer c0016-01.gifear opening to the eardrum.




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  autonomic nervous system
part of the nervous system that controls involuntary functions, including the heart rate and activity of the intestines.
long threadlike extension of a c0016-01.gifnerve cell that conducts electrochemical impulses away from the cell body toward other nerve cells, or toward an effector organ such as a muscle. Axons terminate in c0016-01.gifsynapses, junctions with other nerve cells, muscles, or glands.
  ball-and-socket joint
joint allowing considerable movement in three dimensions, for instance the joint between the pelvis and the femur. To facilitate movement, such joints are rimmed with cartilage and lubricated by synovial fluid.
  bicuspid valve,
or mitral valve, in the left side of the c0016-01.gifheart, a flap of tissue that prevents blood flowing back into the atrium when the ventricle contracts.
brownish alkaline fluid produced by the liver. Bile is stored in the gall bladder and is intermittently released into the duodenum (small intestine) to aid digestion.
hollow elastic-walled organ that stores the urine produced in the kidneys. Urine enters the bladder through two ureters, one leading from each kidney, and leaves it through the urethra.
transport medium composed of cells and fluid that circulates in the arteries, veins, and capillaries.
  blood clotting
production of a semisolid mass of protein fibers and blood cells that prevents excessive bleeding after injury.
  blood group
any of the types into which blood is classified according to the presence or otherwise of certain c0016-01.gifantigens on the surface of its red cells.
  blood pressure
pressure, or tension, of the blood against the inner walls of blood vessels, especially the arteries, due to the muscular pumping activity of the heart.
  blood vessel
tube that conducts blood either away from or toward the heart. The principal types are arteries, veins, and capillaries
hard connective tissue, composed of a network of collagen fibers impregnated with mineral salts, that makes up the skeleton.
  bone marrow
soft tissue in the center of some large bones that manufactures red and white blood cells.
mass of interconnected nerve cells, contained within the skull, that forms the anterior part of the central nervous system, whose activities it coordinates and controls.
region where the top of the spinal cord merges with the undersurface of the brain, consisting largely of the medulla oblongata and midbrain.
muscular movements whereby air is taken into the lungs and then expelled, a form of gas exchange.
small air tube found in the lung responsible for delivering air to the respiratory surface. Bronchioles lead off from the larger bronchus and branch extensively before terminating in the many thousand alveoli that form the bulk of lung tissue.
one of a pair of large tubes (bronchi) branching off from the windpipe and passing into the lung. Apart from their size, bronchi differ from the bronchioles in possessing cartilaginous rings, which give rigidity and prevent collapse during breathing movements.
narrowest type of blood vessel; it forms the main area of exchange between the fluid (c0016-01.giflymph) bathing body tissues and the blood.
flexible bluish-white c0016-01.gifconnective tissue that makes up the embryonic skeleton and, in adults, occurs in the discs between the vertebrae, at bone endings, and in the larynx, nose, and external ear.
in the c0016-01.gifdigestive system, a blind-ending tube branching off from the first part of the large intestine, terminating in the appendix. It has no function in humans.
  central nervous system
(CNS) the brain and spinal cord, as distinct from other components of the c0016-01.gifnervous system. The CNS integrates all nervous function.
part of the brain that controls muscle tone, movement, balance, and coordination.
part of the brain that coordinates all voluntary activity. It is made up of two paired cerebral hemispheres, separated by a central fissure, and is covered with a deeply folded layer of gray matter, the cerebral cortex.
layer found at the rear of the c0016-01.gifeye beyond the retina. By absorbing light that has already passed through the retina, it stops back-reflection and so prevents blurred vision.
general term for the stomach contents. Chyme resembles a thick creamy fluid and is made up of partly digested food, hydrochloric acid, and a range of enzymes.
  ciliary muscle
ring of muscle surrounding and controlling the lens inside the eye, used in focusing. Suspensory ligaments, resembling spokes of a wheel, connect the lens to the ciliary muscle and pull the lens into a flatter shape when the muscle relaxes. When the muscle is relaxed the lens has its longest focal length and focuses rays from distant objects. On contraction, the lens returns to its normal spherical state and therefore has a shorter focal length and focuses images of near objects.
another name for the collar bone.
another term for c0016-01.gifblood clotting, the process by which bleeding is stopped in the body.
part of the inner c0016-01.gifear. It is equipped with thousands of hair cells that move in response to sound waves and thus stimulate nerve cells to send messages to the brain. In this way they turn vibrations of the air into electrical signals.
protein that is the main constituent of c0016-01.gifconnective tissue. Collagen is present in skin, cartilage, tendons, and ligaments. Bones are made up of collagen, with the calcium salts providing increased rigidity.
the main part of the large intestine, between the cecum and rectum. Water and mineral salts are absorbed from undigested food in the colon, and the residue passes as feces toward the rectum.




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membrane covering the front of the eye. It is continuous with the epidermis of the eyelids, and lies on the surface of the cornea.
  connective tissue
tissue made up of a noncellular substance, the c0016-01.gifextracellular matrix, in which some cells are embedded. Skin, bones, tendons, cartilage, and adipose tissue (fat) are the main connective tissues. There are also small amounts of connective tissue in organs such as the brain and liver, where they maintain shape and structure.
transparent front section of the eye. The cornea is curved and behaves as a fixed lens, so that light entering the eye is partly focused before it reaches the lens.
the outer part of a structure such as the brain, kidney, or adrenal gland.
any of several steroid hormones secreted by the cortex of the c0016-01.gifadrenal glands.
the dome-shaped area of the skull that protects the brain. It consists of eight bony plates fused together by sutures (immovable joints).
slender filament projecting from the cell body of a c0016-01.gifnerve cell or neuron. Dendrites receive incoming messages from many other nerve cells and pass them on to the cell body.
thin muscular sheet separating the thorax from the abdomen. Arching upward against the heart and lungs, the diaphragm is important in the mechanics of breathing. It contracts at each inhalation, moving downward to increase the volume of the chest cavity, and relaxes at exhalation.
the resting period between beats of the heart when blood is flowing into it.
process whereby food is broken down mechanically, and chemically by enzymes, mostly in the stomach and intestines, to make the nutrients available for absorption and cell metabolism.
  ductless gland
alternative name for an c0016-01.gifendocrine gland.
short length of c0016-01.gifalimentary canal found between the stomach and the ileum.
  endocrine gland
ductless gland that secretes hormones into the bloodstream to regulate body processes.
fluid found in the inner ear, filling the central passage of the cochlea as well as the semicircular canals.
natural substance that modifies the action of nerve cells. Endorphins are produced by the pituitary gland and hypothalamus. They lower the perception of pain by reducing the transmission of signals between nerve cells.
biological catalyst produced in cells, and capable of speeding up the chemical reactions necessary for life. Enzymes are large, complex proteins, and are highly specific, each chemical reaction requiring its own particular enzyme.
small flap that closes off the end of the trachea (windpipe) during swallowing to prevent food from passing into it and causing choking.
tissue of closely packed cells that forms a surface or lines a cavity or tube. Epithelium may be protective (as in the skin) or secretory (as in the cells lining the wall of the gut).
another name for c0016-01.gifred blood cell.
muscular tube by which food travels from mouth to stomach.
any of a group of hormones, principally estradiol, produced by the c0016-01.gifovaries. Estrogens control female sexual development, promote the growth of female secondary sexual characteristics, stimulate egg production, and prepare the lining of the uterus for pregnancy.
  eustachian tube
small air-filled canal connecting the middle ear with the back of the throat. It equalizes the pressure on both sides of the eardrum.
the removal of the waste products of metabolism from the body. It is achieved by specialized excretory organs, principally the kidneys, which excrete nitrogenous wastes such as urea and excess salts and water. Water and metabolic wastes are also excreted in the feces and through the sweat glands in the skin; carbon dioxide and water are removed via the lungs.
  exocrine gland
gland that discharges secretions, usually through a tube or a duct, on to a surface. Examples include sweat glands which release sweat on to the skin, and digestive glands which release digestive juices on to the walls of the intestine
muscle that straightens a limb; compare with c0016-01.gifflexor.
another name for the thigh bone.
insoluble protein involved in blood clotting.
in the leg, the rear lower bone; it is paired with a smaller front bone, the tibia.
any muscle that bends a limb. Flexors usually work in opposition to other muscles, the extensors, an arrangement known as antagonistic.
  follicle-stimulating hormone,
FSH, a c0016-01.gifhormone produced by the pituitary gland. It affects the ovaries in women, stimulating the production of an egg cell. In men, FSH stimulates the testes to produce sperm.
  gall bladder
small muscular sac situated on the underside of the liver and connected to the small intestine by the bile duct. It stores bile from the liver.
(plural ganglia) solid cluster of nervous tissue containing many cell bodies and Osynapses, usually enclosed in a tissue sheath.
  gas exchange
movement of gases between the body and the atmosphere, principally oxygen and carbon dioxide.
specialized organ of the body that manufactures and secretes enzymes, hormones, or other chemicals.
in the kidney, the cluster of blood capillaries at the threshold of the nephron, or kidney tubule, from which waste products and water pass, ultimately to become urine.




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hormone secreted by the islets of Langerhans in the pancreas, which increases the concentration of glucose in the blood by promoting the breakdown of glycogen in the liver.
  gray matter
those parts of the brain and spinal cord that are made up of interconnected and tightly packed nerve cell nucleuses. This is in contrast to white matter, which is made of the axons of nerve cells.
  growth hormone
(GH) or somatotrophin,
hormone from the anterior c0016-01.gifpituitary gland that promotes growth of long bones and increases protein synthesis.
muscular organ that rhythmically contracts to force blood around the body.
protein used for oxygen transport. Oxygen combines readily and reversibly with hemoglobin. It occurs in red blood cells (erythrocytes), giving them their color.
  hinge joint
joint where movement occurs in one plane only. Examples are the elbow and knee.
study of tissue by visual examination, usually with a c0016-01.gifmicroscope.
chemical secretion of the endocrine glands and other specialized cells that affects the activity of other distant tissues.
the upper bone of the arm.
region of the brain below the cerebrum that regulates rhythmic activity and physiological stability within the body, including water balance and temperature. It regulates the production of the pituitary gland's hormones and controls that part of the c0016-01.gifnervous system governing the involuntary muscles.
part of the small intestine, between the duodenum and the colon, that absorbs digested food.
one of four sharp teeth found at the front center of each jaw. Incisors are used for biting.
process of taking food into the mouth.
hormone, produced by the islets of Langerhans in the pancreas, that stimulates the conversion of glucose to glycogen in the liver, thereby lowering blood glucose levels.
the digestive tract from the stomach outlet to the anus.
colored muscular diaphragm that controls the size of the pupil in the eye. It contains radial muscle that increases the pupil diameter and circular muscle that constricts the pupil diameter. Both types of muscle respond involuntarily to light intensity.
  islets of Langerhans
groups of cells within the pancreas responsible for the secretion of the hormone insulin. They are sensitive to blood glucose levels, producing more hormone when glucose levels rise.
one of two bony structures that form the framework of the mouth. They consist of the upper jawbone (maxilla), which is fused to the skull, and the lower jawbone (mandible), which is hinged at each side to the bones of the temple by c0016-01.gifligaments.
point where two bones meet.
one of a pair of organs responsible for excreting waste products such as urea and regulating the water and salt content of blood.
small lymph vessel responsible for absorbing fat in the small intestine.
cavity at the upper end of the trachea (windpipe) containing the vocal cords. It is stiffened with cartilage and lined with mucous membrane.
another name for a white blood cell.
strong, flexible connective tissue, made of the protein c0016-01.gifcollagen, which joins bone to bone at moveable joints and sometimes encloses the joints.
enzyme responsible for breaking down fats into fatty acids and glycerol.
large organ, situated in the upper abdomen, that has many regulatory and storage functions. It receives the products of digestion, converts glucose to glycogen (a long-chain carbohydrate used for storage), and then back to glucose when needed. It removes excess amino acids from the blood, converting them to urea, which is excreted by the kidneys. The liver also synthesizes vitamins, produces bile and bloodclotting factors, and removes damaged red cells and toxins such as alcohol from the blood.
one of a pair of organs in the chest cavity, used for bringing inhaled air into close contact with the blood so that oxygen can pass into the body and waste carbon dioxide can be passed out.
  luteinizing hormone
hormone produced by the pituitary gland. In males, it stimulates the testes to produce androgens (male sex hormones). In females, it works together with follicle-stimulating hormone to initiate production of egg cells by the ovary. If fertilization occurs, it plays a part in maintaining the pregnancy by controlling the levels of the hormones estrogen and progesterone in the body.
fluid found in the lymphatic system of vertebrates. Lymph is drained from the tissues by lymph capillaries, which empty into larger lymph vessels (lymphatics). These lead to lymph nodes (small, round bodies chiefly situated in the neck, armpit, groin, thorax, and abdomen), which process the c0016-01.giflymphocytes produced by the bone marrow, and filter out harmful substances and bacteria.
  lymph node
mass of lymphatic tissue that occurs at various points along the major lymphatic vessels. Examples are the tonsils and adenoids. As lymph passes through the nodes, it is filtered, and bacteria and other microorganisms are engulfed by cells known as macrophages.
type of white blood cell with a large nucleus, produced in the bone marrow. B lymphocytes, or B cells, are responsible for producing antibodies; T lymphocytes, or T cells, have several roles in the mechanism of immunity.
type of white blood cell that specializes in the removal of bacteria and other microorganisms, or of cell debris after injury. Macrophages are found throughout the




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  body, but mainly in the lymph and connective tissues, and especially the lungs, where they ingest dust, fibers, and other inhaled particles.  
enzyme that breaks down the disaccharide maltose into glucose.
the central part of a structure such as the kidney, or adrenal gland. In the brain, the medulla oblongata is the posterior region responsible for the coordination of basic activities, such as breathing and temperature control.
one of the 12 large teeth, used for crushing food, found at the back of the mouth. The structure of the jaw, and the relation of the muscles, allows a massive force to be applied to molars.
type of white blood cell. Monocytes are found in the tissues, the lymphatic and circulatory systems where their purpose is to remove foreign particles, such as bacteria and tissue debris, by ingesting them.
  motor nerve
any nerve that transmits impulses from the central nervous system to muscles or organs.
  mucous membrane
thin skin lining all body cavities and canals that come into contact with the air (for example, eyelids, breathing and digestive passages, genital tract). It secretes mucus, a moistening, lubricating, and protective fluid.
contractile tissue that produces locomotion and power, and maintains the movement of body substances. It is made up of long fibers that can contract to between one-half and one-third of their relaxed length.
  myelin sheath
insulating layer that surrounds nerve cells and serves to speed up the passage of nerve impulses.
microscopic unit in the kidney that is responsible for forming urine.
bundle of nerve cells enclosed in a sheath of connective tissue and transmitting nerve impulses to and from the brain and spinal cord.
  nerve cell, or neuron,
elongated cell, the basic functional unit of the c0016-01.gifnervous system, that transmits information rapidly between different parts of the body.
another name for a c0016-01.gifnerve cell.
chemical that diffuses across a synapse, and thus transmits impulses between nerve cells, or between nerve cells and effector organs (such as muscles). Common neurotransmitters are noradrenaline (which also acts as a hormone) and acetylcholine, the latter being most frequent at junctions between nerve and muscle.
  noradrenaline, or norepinephrine,
chemical that acts both as a hormone, produced by the adrenal gland, and as a neurotransmitter, secreted by nerve cells. It acts directly on specific receptors to stimulate the sympathetic nervous system.
upper entrance of the respiratory tract; the organ of the sense of smell. The whole nasal cavity is lined with a c0016-01.gifmucous membrane that warms and moistens the air as it enters and ejects dirt. In the upper parts of the cavity the membrane contains 50 million olfactory receptor cells (cells sensitive to smell).
  optic nerve
large nerve passing from the eye to the brain, carrying visual information.
part of the body, such as the liver or brain, that has a distinctive function or set of functions. An organ is composed of a group of coordinated c0016-01.giftissues.
process whereby the water content is maintained at a constant level. If the water balance is disrupted, the concentration of salts will be too high or too low, and vital functions, such as nerve conduction, will be adversely affected.
in females, the organ that generates the c0016-01.gifovum (egg cell) and secretes the hormones responsible for female secondary sexual characteristics, such as smooth, hairless facial skin and enlarged breasts.
hormone that stimulates the uterus in late pregnancy to initiate and sustain labor. After birth, it stimulates the uterine muscles to contract, reducing bleeding at the site where the placenta was attached.
  pacemaker, or sinoatrial node
group of muscle cells in the wall of the heart that contracts spontaneously and rhythmically, setting the pace for the contractions of the rest of the heart.
sense that gives an awareness of harmful effects on or in the body. It may be triggered by stimuli such as trauma, inflammation, and heat. Pain is transmitted by specialized nerves and also has psychological components controlled by higher centers in the brain.
accessory gland of the digestive system located close to the duodenum. When stimulated by the hormone secretin, it releases enzymes into the duodenum that digest starches, proteins, and fats. It contains groups of cells called the islets of Langerhans, which secrete the hormones insulin and glucagon that regulate the blood sugar level.
  parasympathetic nervous system
division of the autonomic nervous system responsible for slowing the heart rate, decreasing blood pressure, and stimulating the digestive system.
one of a pair of small c0016-01.gifendocrine glands, located behind the thyroid gland. They secrete parathormone, a hormone that regulates the amount of calcium in the blood.
  patella, or kneecap,
flat bone embedded in the knee tendon, which protects the joint from injury.
relating to the upper chest; associated with the muscles and bones used in moving the arms.
lower area of the abdomen featuring the bones and muscles used to move the legs or hindlimbs. The pelvic girdle is a set of bones that allows movement of the legs in relation to the rest of the body and provides sites for the attachment of relevant muscles.
enzyme that breaks down proteins during digestion.
wavelike contractions, produced by the contraction of smooth muscle, that pass along tubular organs, such as the intestines.




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membrane lining the abdominal cavity and digestive organs.
excretion of water and dissolved substances from the c0016-01.gifsweat glands of the skin. It has two main functions: body cooling by the evaporation of water from the skin surface, and excretion of waste products such as salts.
type of white blood cell, or leukocyte, that can engulf a bacterium or other invading microorganism. Phagocytes are found in blood, lymph, and other body tissues, where they also ingest foreign matter and dead tissue.
the engulfing of foreign bodies or food by white blood cells.
muscular cavity behind the nose and mouth, extending downward from the base of the skull. The internal nostrils lead backward into the pharynx, which continues downward into the esophagus and (through the epiglottis) into the windpipe. On each side, a Eustachian tube enters the pharynx from the middle ear cavity.
  pituitary gland
major endocrine gland, situated in the center of the brain. The posterior lobe is an extension of the hypothalamus, and is in effect nervous tissue. It stores two hormones synthesized in the hypothalamus: c0016-01.gifADH and c0016-01.gifoxytocin. The anterior lobe secretes six hormones, some of which control the activities of other glands (thyroid, gonads, and adrenal cortex); others are direct-acting hormones affecting milk secretion and controlling growth.
the liquid component of the c0016-01.gifblood. It is a strawcolored fluid, largely composed of water (around 90%), in which a number of substances are dissolved. These include a variety of proteins (around 7%) such as fibrinogen (important in blood clotting), inorganic mineral salts such as sodium and calcium, waste products such as urea, traces of hormones, and antibodies to defend against infection.
tiny disk-shaped structure found in the blood, which helps it to clot. Platelets are not true cells, but membrane-bound cell fragments without nuclei that bud off from large cells in the bone marrow.
one of the eight large teeth, used for crushing, found toward the back of the mouth. Unlike molars, they are present in milk dentition as well as in the permanent dentition of adults.
steroid hormone that regulates the menstrual cycle and pregnancy. Progesterone is secreted by the corpus luteum (the ruptured Graafian follicle of a discharged ovum).
any of a group of complex fatty acids present in the body that act as messenger substances between cells. Effects include stimulating the contraction of smooth muscle (for example, of the uterus during birth), regulating the production of stomach acid, and modifying hormonal activity.
general term for a digestive enzyme capable of splitting proteins. Examples include pepsin, found in the stomach, and trypsin, found in the small intestine.
pertaining to the c0016-01.giflungs.
the lower opening of the stomach.
one of the two bones in the lower arm (the other is the ulna).
discrete area of cell membrane or an area within a cell with which neurotransmitters, hormones, and drugs interact. Such interactions control the activities of the body. For example, adrenaline transmits nervous impulses to receptors in the sympathetic nervous system which initiates the characteristic response to excitement and fear in an individual.
lowest part of the large intestine, which stores feces prior to elimination (defecation).
  red blood cell, or erythrocyte,
the most common type of blood cell, responsible for transporting oxygen around the body. It contains hemoglobin, which combines with oxygen from the lungs to form oxyhemoglobin.
very rapid involuntary response to a particular stimulus. A reflex involves only a few nerve cells, unlike the slower but more complex responses produced by the many processing nerve cells of the brain.
pertaining to the c0016-01.gifkidneys.
  rennin, or chymase,
enzyme found in the gastric juice of babies, used in the digestion of milk.
light-sensitive area at the back of the eye connected to the brain by the optic nerve. It has several layers and contains over a million rods and cones, sensory cells capable of converting light into nervous messages that pass down the optic nerve to the brain.
  rhesus factor
group of c0016-01.gifantigens on the surface of red blood cells that characterize the rhesus blood group system. Most individuals possess the main rhesus factor (Rh+), but those without this factor (Rh–) produce c0016-01.gifantibodies if they come into contact with it. The name comes from rhesus monkeys, in whose blood rhesus factors were first found.
one of 12 pairs of long, usually curved bone that extends laterally from the vertebral column. The ribs protect the lungs and heart, and allow the chest to expand and contract easily.
alkaline secretion from the salivary glands that aids the swallowing and digestion of food in the mouth. It contains the enzyme ptyalin (amylase), which converts starch to sugar.
  salivary gland
one of three pairs of glands—the parotid, sublingual, and the submandibular—responsible for the manufacture of saliva and its secretion into the mouth.
  scapula, or shoulder blade,
large, flat, triangular bone that lies over the second to seventh ribs on the back, forming part of the pectoral girdle, and assisting in the articulation of the arm with the chest region. Its flattened shape allows a large region for the attachment of muscles.
hormone produced by the small intestine that stimulates the production of digestive secretions by the pancreas and liver.
the covering of the body. The outer layer (epidermis) is dead and its cells are constantly being rubbed away and replaced from below; it helps to protect the body from infection and to prevent dehydration. The lower layer (dermis) contains blood vessels, nerves, hair roots, and sweat and




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  sebaceous glands, and is supported by a network of fibrous and elastic cells.  
collection of flat and irregularly shaped bones that enclose the brain and the organs of sight, hearing, and smell, and provide support for the jaws.
  smooth muscle
involuntary muscle capable of slow contraction over a period of time. It is present in hollow organs, such as the intestines, stomach, bladder, and blood vessels.
  spinal cord
major component of the central nervous system, encased in the vertebral column.
synonym for the vertebral, or spinal column.
organ, situated on the left side of the body, under the stomach, that helps to process c0016-01.giflymphocytes. It also regulates the number of red blood cells in circulation by destroying old cells, and stores iron.
  sternum or breastbone,
large flat bone at the front of the chest, joined to the ribs. It gives protection to the heart and lungs.
the first cavity in the digestive system, a bag of muscle situated just below the diaphragm.
  suspensory ligament
in the eye, a ring of fiber supporting the lens.
junction between two c0016-01.gifnerve cells, or between a nerve cell and a muscle (a neuromuscular junction), across which a nerve impulse is transmitted. The two cells are separated by a narrow gap, which is bridged by a chemical c0016-01.gifneurotransmitter, released by the nerve impulse.
  synovial fluid
viscous colorless fluid that bathes movable joints between the bones. It nourishes and lubricates the c0016-01.gifcartilage at the end of each bone.
contraction of the heart. It alternates with diastole, the resting phase of the heart beat.
sense that detects some of the chemical constituents of food. The human tongue can distinguish only four basic tastes (sweet, sour, bitter, and salty) but it is supplemented by the sense of smell.
salty fluid exuded by lacrimal glands in the eyes. The fluid contains proteins that are antibacterial, and also absorbs oils and mucus. Apart from cleaning and disinfecting the surface of the eye, the fluid supplies nutrients to the cornea, which does not have a blood supply.
  tendon, or sinew,
cord of very strong, fibrous connective tissue that joins muscle to bone.
plural testes, organ that produces c0016-01.gifsperm in the male. It is one of a pair of oval structures that descend from the body cavity during development, to hang outside the abdomen in a sac called the scrotum. The testes also secrete the male sex hormone testosterone.
hormone secreted chiefly by the testes, but also by the ovaries and the cortex of the adrenal glands. It promotes the development of secondary sexual characteristics in males.
  thorax, or chest,
the part of the body containing the heart and lungs, and protected by the ribcage.
another name for a c0016-01.gifplatelet.
organ situated in the upper chest cavity. The thymus processes c0016-01.giflymphocyte cells to produce T-lymphocytes (T denotes "thymus-derived"), which are responsible for binding to specific invading organisms and killing them or rendering them harmless.
endocrine gland, situated in the neck in front of the trachea. It secretes several hormones, principally thyroxine, an iodine-containing hormone that stimulates growth, metabolism, and other functions of the body.
  tibia, or shinbone,
the front bone in the leg between the ankle and the knee.
cellular fabric in the body. Several kinds of tissue can usually be distinguished (for example, nerve and muscle), each consisting of cells of a particular kind bound together by extracellular matrix.
masses of lymphoid tissue situated at the back of the mouth and throat (palatine tonsils), and on the rear surface of the tongue (lingual tonsils). The tonsils contain many c0016-01.giflymphocytes and are part of the body's defense system against infection.
sensation produced by specialized nerve endings in the skin. Some respond to light pressure, others to heavy pressure. Temperature detection may also contribute to the overall sensation of touch.
  trachea, or windpipe,
tube that forms part of the airway between the throat and the lungs. It is strong and flexible, and reinforced by rings of c0016-01.gifcartilage. In the upper chest, the trachea branches into two tubes: the left and right bronchi, which enter the lungs.
  tricuspid valve
flap of tissue situated on the right side of the c0016-01.gifheart between the atrium and the ventricle. It prevents blood flowing backward when the ventricle contracts.
enzyme responsible for the digestion of protein molecules.
one of the two bones found in the lower arm (the other is the radius).
2)2 waste product formed in the liver when nitrogen compounds are broken down. It is filtered from the blood by the kidneys, and stored in the bladder as urine prior to release.
tube connecting the kidney to the bladder. Its wall contains fibers of smooth muscle whose contractions aid the movement of urine out of the kidney.
tube connecting the bladder to the exterior. It carries urine and, in males, semen.
amber-colored fluid filtered out by the kidneys from the blood. It contains excess water, salts, proteins, waste products in the form of urea, a pigment, and some acid.
structure for controling the direction of the blood flow. Valves prevent backflow in the heart, ensuring that its




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  contractions send the blood forward into the arteries. The tendency for low-pressure venous blood to collect at the base of the legs under the influence of gravity is counteracted by a series of small valves within the veins.  
any vessel that carries blood from the body to the heart.
  vena cava
either of the two great veins of the trunk, returning deoxygenated blood to the right atrium of the c0016-01.gifheart. The superior vena cava, beginning where the arches of the two innominate veins join high in the chest, receives blood from the head, neck, chest, and arms; the inferior vena cava, arising from the junction of the right and left common iliac veins, receives blood from all parts of the body below the diaphragm.
either of the two lower chambers of the heart that force blood to circulate by contraction of their muscular walls.
irregularly shaped bone that forms part of the vertebral column.
  vertebral column, or spine,
series of bones, or vertebrae, that support the body and protect the spinal cord.
plural villi, small fingerlike projection of the wall of the small intestine. It serves to increase the wall's surface area for the absorption of digested nutrients.
general term for the organs contained in the chest and abdominal cavities.
  vitreous humor
transparent jellylike substance behind the lens of the eye. It gives rigidity to the spherical form of the eye and allows light to pass through to the retina.
  vocal cords
the paired folds of tissue within the larynx (voice box). Air constricted between the folds makes them vibrate, producing sounds. Muscles in the larynx change the pitch of the sounds produced, by adjusting the tension of the vocal cords.
another name for the c0016-01.giftrachea.
  Further Reading  
  Austin, C. R., and Short, R. V. (eds.) Reproduction in Mammals 1982  
  Baker, Robin Sperm Wars 1996  
  Bolt, Robert J. The Digestive System 1983  
  Bryan, Jenny The Pulse of Life: the Circulatory System 1992  
  Chaplin, Martin F, and Bucke, Christopher Enzyme Technology 1990  
  Chivers, David John, and Langer, Peter The Digestive System in Mammals: Food, Form and Nutrition 1994  
  Cohen, Jack Reproduction 1977  
  Conn, P Michael, and Melmed, Shlomo Endocrinology: Basic and Clinical Principles 1997  
  Dulbecco, Renato Encyclopedia of Human Biology 1991  
  Friday, Adrian, and Ingram, David S. The Cambridge Encyclopedia of Life Sciences 1985  
  Gould, Stephen Jay The Mismeasure of Man 1981  
  Halton, Frances The Digestive System 1994  
  Kimbrell, Andrew The Human Body Shop 1993  
  Miller, Jonathan The Body in Question 1978  
  Miller, Jonathan, and Pelham, David Human Body 1994  
  Mines, Allan H. Respiratory Physiology 1993  
  Norman, Anthony W., and Litwack, Gerard Hormones 1997  
  Prange, Henry D. Respiratory Physiology: Understanding Gas Exchange 1996  
  Ridley, Matt The Red Queen 1993  
  Ryall, Robert James The Digestive System 1984  
  Smith, James John, and Kampine, John P. Circulatory Physiology: the Essentials 1990  
  Suckling, C. J. Enzyme Chemistry: Impact and Applications 1990  
  Sutcliffe, Margaret Do You Understand the Circulatory System? 1991  
  Vines, Gail Raging Hormones: Do They Rule Our Lives? 1993  
  Vroon, Piet Smell: The Secret Seducer 1998  
  Warnock, Mary A Question of Life 1984  
  West, John B. Respiratory Physiology: the Essentials 1995  
  Wolpert, Lewis The Triumph of the Embryo 1991  
  Wong, C. H., and Whitesides, G. M. Enzymes in Synthetic Organic Chemistry 1994