Kids Research Express

. Thyroxine, also tetraiodothyronine, main hormone secreted by the thyroid gland. The function of this and the other thyroid hormone, triiodothyronine, is to increase the cellular rate of carbohydrate metabolism and of protein synthesis and breakdown. The hormones are synthesized in the thyroid by combining iodine with the amino acid tyrosine and are secreted into the blood as a complex with the protein plasma globulin. Both synthesis and secretion are regulated by, and in turn regulate the formation of, a hormone secreted by the pituitary gland . Thyroxine was first isolated in 1919 by the American biochemist Edward Calvin Kendall and was synthesized in 1927 by the British biochemist Charles Harington. Synthetic thyroxine is now used in treating thyroid-deficiency conditions such as cretinism and goiter .

. Adrenocorticotropic Hormone (ACTH), also known as corticotropin, hormone secreted by the anterior part of the pituitary gland . The specific function of ACTH is to stimulate the growth and secretions of the cortex (outer layer) of the adrenal gland . One of these secretions is cortisone, a hormone involved in carbohydrate and protein metabolism . ACTH is used medically for its anti-inflammatory action to alleviate symptoms of allergies and arthritis . ACTH is a complex protein molecule containing 39 amino acids . Its molecular weight is approximately 5000. The biological activity of the ACTH of various animal species is similar to that of humans, but the sequence of amino acids has been found to vary somewhat among species. ACTH production is controlled in part by the hypothalamus and in part by the existing levels of adrenal gland hormones. ACTH levels increase in response to stress, disease, and decreased blood pressure.

. Testosterone, principal male hormone, or androgen , produced mainly in the Leydig cells in the male testes. The Leydig cells also produce two other androgens of less potency and in much smaller quantities. Testosterone stimulates the development of the male secondary sex characteristics after puberty , causing growth of the beard and pubic hair, development of the penis, and change of voice. The hormone also aids in the growth, muscular development, and masculine body contour of the adult male. See Hormone . If, before puberty, little or no testosterone secretion occurs, secondary sexual characteristics fail to develop. In addition, the long bones continue to grow abnormally and give the patient a characteristic tall but effeminate build. If testicular failure follows puberty, less obvious changes occur, although gradual recession of beard, weakening of muscles, increased deposition of fat, and change in voice may develop slowly, with infertility usually present and decreased libido ...

Androgen, term embracing any of the male sex hormones, substances that induce and maintain secondary sex characteristics in males. The principal androgen is testosterone, produced in the testes of males and in the adrenal glands of both sexes. Testosterone circulates in the blood and is excreted in the urine. Androgens function principally, beginning at puberty, in the stimulation of such male secondary sex characteristics as development of the genital organs and maturation of sperm, growth of body hair, and changes in the larynx that lower the voice. Androgens have an anabolic effect—that is, they stimulate protein synthesis and inhibit protein breakdown, enhancing the growth of muscle and bone tissue in the developing male. An excess of androgens can lead to a variety of disorders, including the accentuation of male secondary sex characteristics in both males and females. Less frequently, an excess of androgens may cause testicular or ovarian tumors. For a discussion of the related f...

About 55 percent of the blood is composed of a liquid known as plasma . The rest of the blood is made of three major types of cells: red blood cells (also known as erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes). See also Blood Type .

Plasma consists predominantly of water and salts. The kidneys carefully maintain the salt concentration in plasma because small changes in its concentration will cause cells in the body to function improperly. In extreme conditions this can result in seizures, coma, or even death. The pH of plasma, the common measurement of the plasma’s acidity, is also carefully controlled by the kidneys within the neutral range of 6.8 to 7.7. Plasma also contains other small molecules, including vitamins, minerals, nutrients, and waste products. The concentrations of all of these molecules must be carefully regulated. Plasma is usually yellow in color due to proteins dissolved in it. However, after a person eats a fatty meal, that person’s plasma temporarily develops a milky color as the blood carries the ingested fats from the intestines to other organs of the body. Plasma carries a large number of important proteins, including albumin, gamma globulin , and clotting factors. Albumin is the main prot...

Red Blood Cell or Erythrocyte, the oxygen-carrying component of the blood. Also referred to as red corpuscles, they are by far the most numerous type of blood cell. Red blood cells make up almost 45 percent of the blood volume. Their primary function is to carry oxygen from the lungs to every cell in the body. Red blood cells are composed predominantly of a protein and iron compound, called hemoglobin , that captures oxygen molecules as the blood moves through the lungs, giving blood its red color. As blood passes through body tissues, hemoglobin then releases the oxygen to cells throughout the body. Red blood cells are so packed with hemoglobin that they lack many components, including a nucleus, found in other cells. The membrane, or outer layer, of the red blood cell is flexible, like a soap bubble, and is able to bend in many directions without breaking. This is important because the red blood cells must be able to pass through the tiniest blood vessels, the capillaries, to deliver...

There are several types of red blood cells and each person has red blood cells of just one type. Blood type is determined by the occurrence or absence of substances, known as recognition markers or antigens, on the surface of the red blood cell. Type A blood has just marker A on its red blood cells while type B has only marker B. If neither A nor B markers are present, the blood is type O. If both the A and B markers are present, the blood is type AB. Another marker, the Rh antigen (also known as the Rh factor), is present or absent regardless of the presence of A and B markers. If the Rh marker is present, the blood is said to be Rh positive, and if it is absent, the blood is Rh negative. The most common blood type is A positive—that is, blood that has an A marker and also an Rh marker. More than 20 additional red blood cell types have been discovered. Blood typing is important for many medical reasons. If a person loses a lot of blood, that person may need a blood transfusion to re...

White blood cells only make up about 1 percent of blood, but their small number belies their immense importance. They play a vital role in the body’s immune system—the primary defense mechanism against invading bacteria, viruses, fungi, and parasites. They often accomplish this goal through direct attack, which usually involves identifying the invading organism as foreign, attaching to it, and then destroying it. This process is referred to as phagocytosis . White blood cells also produce antibodies, which are released into the circulating blood to target and attach to foreign organisms. After attachment, the antibody may neutralize the organism, or it may elicit help from other immune system cells to destroy the foreign substance. There are several varieties of white blood cells, including neutrophils, monocytes, and lymphocytes, all of which interact with one another and with plasma proteins and other cell types to form the complex and highly effective immune system.

The smallest cells in the blood are the platelets, which are designed for a single purpose—to begin the process of coagulation, or forming a clot, whenever a blood vessel is broken. As soon as an artery or vein is injured, the platelets in the area of the injury begin to clump together and stick to the edges of the cut. They also release messengers into the blood that perform a variety of functions: constricting the blood vessels to reduce bleeding, attracting more platelets to the area to enlarge the platelet plug, and initiating the work of plasma-based clotting factors, such as fibrinogen. Through a complex mechanism involving many steps and many clotting factors, the plasma protein fibrinogen is transformed into long, sticky threads of fibrin. Together, the platelets and the fibrin create an intertwined meshwork that forms a stable clot. This self-sealing aspect of the blood is crucial to survival. See also Blood Diseases: Anemia ; leukemia , hemophilia

Blood Type, in medicine, classification of red blood cells by the presence of specific substances on their surface. Typing of red blood cells is a prerequisite for blood transfusion . In the early part of the 20th century, physicians discovered that blood transfusions often failed because the blood type of the recipient was not compatible with that of the donor. In 1901 the Austrian pathologist Karl Landsteiner classified blood types and discovered that they were transmitted by Mendelian heredity (see Mendel's Laws ). The four blood types are known as A, B, AB, and O. Blood type A contains red blood cells that have a substance A on their surface. This type of blood also contains an antibody directed against substance B, found on the red cells of persons with blood type B. Type B blood contains the reverse combination. Serum of blood type AB contains neither antibody, but red cells in this type of blood contain both A and B substances. In type O blood, neither substance is present ...

Serum, clear, yellowish fluid that remains as the liquid portion of blood after clotting has taken place. Before clotting occurs, the liquid of the blood is called plasma . Both plasma and serum contain proteins, salts, sugars, waste products, vitamins, minerals, fats, and hormones. But plasma also contains the protein fibrinogen and certain other elements necessary for clotting, and it therefore clots as easily as whole blood. Serum, which lacks these elements, does not clot. In order to isolate serum for chemical tests and other medical uses, whole blood is placed in a glass tube and allowed to clot. The clotted blood is then placed in a centrifuge, a machine that spins the blood at high speeds, causing the clot and the cellular parts of the blood to separate from the serum and sink to the bottom of the tube. The serum remains at the top of the tube. Serum from animals like horses and sheep can be used to provide human beings with protection against infections or poisons. Animals ar...

Rh Factor, term applied to any of the 30 or more substances, called agglutinogens, found on the surfaces of red blood cells. They are distinct from the main blood types, but their composition is unknown. The Rh factors were named by the American pathologists Karl Landsteiner and Alexander Solomon Wiener, who discovered the first of them in the blood of the rhesus monkey in 1937. This first Rh agglutinogen, corresponding to the one now designated Rh0, is present in the blood of about 85 percent of all humans. Blood reactions involving Rh factors are now chiefly of interest in obstetrics. The presence of Rh factors in the blood is controlled by the laws of heredity. An individual who possesses one gene for the Rh factor will express the factor on the red blood cells. If a woman is Rh-negative, that is, if she has two recessive genes for the Rh0 factor, and a man has two genes that express the Rh-positive factor, then all of their children will be Rh-positive. But if the Rh-positive man h...

Cerebrum (the Latin word for brain)—two large, almost symmetrical hemispheres which make up approximately 85 percent of the brain's weight. The exterior surface of the cerebrum, the cerebral cortex, is a convoluted, or folded, grayish layer of cell bodies known as the gray matter. The gray matter covers an underlying mass of fibers called the white matter. The convolutions are made up of ridgelike bulges, known as gyri, separated by small grooves called sulci and larger grooves called fissures. Approximately two-thirds of the cortical surface is hidden in the folds of the sulci. The extensive convolutions enable a very large surface area of brain cortex to fit within the cranium. The cerebrum receives information from all the sense organs and sends motor commands (signals that result in activity in the muscles or glands) to other parts of the brain and the rest of the body. Motor commands are transmitted by the motor cortex, a strip of cerebral cortex extending from side to side a...

Cerebellum (“little brain”)—two smaller hemispheres located at the lower back of the brain beneath the occipital lobes, which coordinates body movements. It is divided into two lateral (side-by-side) lobes connected by a fingerlike bundle of white fibers called the vermis. The outer layer, or cortex, of the cerebellum consists of fine folds called folia. As in the cerebrum , the outer layer of cortical gray matter surrounds a deeper layer of white matter and nuclei (groups of nerve cells). Three fiber bundles called cerebellar peduncles connect the cerebellum to the three parts of the brain stem —the midbrain, the pons, and the medulla oblongata. The cerebellum coordinates voluntary movements by fine-tuning commands from the motor cortex in the cerebrum. The cerebellum also maintains posture and balance by controlling muscle tone and sensing the position of the limbs. All motor activity, from hitting a baseball to fingering a violin, depends on the cerebellum.

Brain stem, the lowest part of the brain , a central core that gradually becomes the spinal cord, exiting the skull through an opening at its base called the foramen magnum. It serves as the path for messages traveling between the upper brain and spinal cord but is also the seat of basic and vital functions such as breathing, blood pressure, and heart rate, as well as reflexes like eye movement and vomiting. The brain stem has three main parts: the medulla , pons , and midbrain . A canal runs longitudinally through these structures carrying cerebrospinal fluid. Also distributed along its length is a network of cells, referred to as the reticular formation, that governs the state of alertness.

Midbrain, the topmost structure of the brain stem that contains major relay stations for neurons transmitting signals to the cerebral cortex, as well as many reflex centers—pathways carrying sensory (input) information and motor (output) commands. Relay and reflex centers for visual and auditory (hearing) functions are located in the top portion of the midbrain. A pair of nuclei called the superior colliculus control reflex actions of the eye, such as blinking, opening and closing the pupil, and focusing the lens. A second pair of nuclei, called the inferior colliculus, control auditory reflexes, such as adjusting the ear to the volume of sound. At the bottom of the midbrain are reflex and relay centers relating to pain, temperature, and touch, as well as several regions associated with the control of movement, such as the red nucleus and the substantia nigra.

Pons, the prominent bulge in the brain stem , continuous with and below the midbrain and directly in front of the cerebellum. It consists of large bundles of nerve fibers that connect the two halves of the cerebellum and also connect each side of the cerebellum with the opposite-side cerebral hemisphere. The pons serves mainly as a relay station linking the cerebral cortex and the medulla oblongata.

Medulla Oblongata, the long, stalklike lowermost portion of the brain stem . At the top, it is continuous with the pons and the midbrain ; at the bottom, it makes a gradual transition into the spinal cord at the foramen magnum. Sensory and motor nerve fibers connecting the brain and the rest of the body cross over to the opposite side as they pass through the medulla. Thus, the left half of the brain communicates with the right half of the body, and the right half of the brain with the left half of the body.

Creatine, substance in the bloodstream of vertebrate animals that is used for supplementary energy transport in certain body systems. (Arganine serves a similar role in invertebrate animals.) Adenosine triphosphate , or ATP, is the main immediate source of usable energy in the body, but brain , heart , and muscle cells require enormous amounts of energy that creatine also helps to supply. Produced in the kidneys and liver , the chemical is absorbed into cells and picks up the high-energy phosphate group from excess ATP at cellular bodies called mitochondria, then shuttles back and forth to other cells to supply the energy.

Fertilization is the process in which gametes —a male's sperm and a female's egg or ovum—fuse together, producing a single cell that develops into an adult organism. Fertilization occurs in both plants and animals that reproduce sexually—that is, when a male and a female are needed to produce an offspring (see Reproduction ). Fertilization is a precise period in the reproductive process. It begins when the sperm contacts the outer surface of the egg and it ends when the sperm's nucleus fuses with the egg's nucleus. Fertilization is not instantaneous—it may take 30 minutes in sea urchins and up to several hours in mammals. After nuclear fusion, the fertilized egg is called a zygote. When the zygote divides to a two-cell stage, it is called an embryo. Fertilization is necessary to produce a single cell that contains a full complement of genes. When a cell undergoes meiosis , gametes are formed—a sperm cell or an egg cell. Each gamete contains only half the genetic materia...

. Skeleton (anatomy), term applied to all the rigid or semirigid structures supporting the soft tissues of an animal's body and providing leverage for muscular action. In vertebrates, the skeleton is known as the endoskeleton and is formed within the body. Some invertebrate animals, such as insects and crustaceans, have skeletons known as exoskeletons on the outside of the body. EXOSKELETON A form of exoskeleton is the shell of calcium or silica secreted by certain protozoans known as foraminiferans. Commercial sponges have an exoskeleton consisting of spongin, which is a tough, elastic substance. Cnidarians secrete a wide variety of exoskeletal substances, ranging from the elastic covering of the jellyfish to the stony material deposited by coral. The familiar shells of most mollusks are composed of calcium carbonate and an organic ground substance known as conchiolin. Among insects, each of the three principal divisions of the body—the head, the thorax, and the abdomen—is enclose...

. Pregnancy and Childbirth, terms for the gestation period of the human reproductive cycle (see Reproduction ). PREGNANCY Pregnancy starts when a male’s sperm fertilizes a female’s ovum (egg), and the fertilized ovum implants in the lining of the uterus (see Fertilization ; Reproductive System ). Because pregnancy changes a woman’s normal hormone patterns, one of the first signs of pregnancy is a missed menstrual period (see Menstruation ). Other symptoms include breast tenderness and swelling, fatigue, nausea or sensitivity to smells, increased frequency of urination, mood swings, and weight gain. Some women also experience cravings for unusual substances such as ice, clay, or cornstarch; this condition, called pica, can indicate a dietary deficiency in iron or other nutrients. By the 12th week of pregnancy many of these symptoms have subsided, but others appear. For example, a woman’s breasts usually increase in size, and her nipples darken. The most obvious symptom is weight gain; m...

. Progesterone, hormone formed by the granulosa cells of the corpus luteum of the ovary. The corpus luteum is a structure in the ovary that develops at the site where a mature egg was released at ovulation. Therefore the level of progesterone rises in the second half of the menstrual cycle (see Menstruation ). If the released ovum is not fertilized, production of progesterone falls just before the onset of the next menstrual cycle and the corpus luteum degenerates. Progesterone was isolated and crystallized by three independent groups of investigators in 1934. It is a steroid hormone, a compound possessing the same chemical nucleus as the female estrogenic hormones and the male androgenic hormones, as well as cholesterol and adrenal steroid hormones. The principal function of progesterone is the preparation of the mucous membrane of the uterus for the reception of the ovum. The hormone also stimulates the formation of saclike structures in the lacteal glands in preparation for thei...

Mammary Glands, source of milk for the young and a characteristic of all mammals, including the egg-laying monotremes. Although the mammary glands evolved recently in the development of vertebrates, their origin is uncertain. In monotremes, they resemble sweat glands in structure, leading to speculation that they are sweat or sebaceous glands modified through evolution. In higher orders, the glands at maturity are developed only in females; in monotremes they evolve equally in both sexes. The organ in human males occasionally develops functional ability, giving support to the belief that lactation occurred in both sexes in prehistoric mammals.

Hip, one of two projecting portions on opposite sides of the body between the lowest ribs and the thigh. The hip is made up of the hipbone, or innominate bone (a part of the pelvis), and of the skin, fat, muscles, and membranes overlying it. The hip is a ball-and-socket joint; the ball is the spherical head of the thighbone, or femur, and the socket is a region on the side of the hipbone known as the acetabulum (Latin, “vinegar cup”). Friction between the bones of the hip joint is reduced by a coating of cartilage and by a lubricating agent known as synovial fluid. The center of gravity of the human body is located behind the hip joint, tending to throw the body backward. With the aid of various muscles from the pelvis and thigh, the femur is able to rotate in the acetabulum and move in any direction, the extent of its movement being limited only by certain supporting ligaments. The hip and hip joint are subject to a variety of disorders and injuries. Congenital disorders of the hip ar...

. Hydrocortisone, also cortisol, common names for 17-hydroxy-corticosterone, the principal hormone secreted by the outer layer, or cortex, of the adrenal gland . Hydrocortisone affects the metabolism of carbohydrate, protein, and fat; the maturation of white blood cells; the retention of salt and water in the body; the activity of the nervous system; and the regulation of blood pressure. Secretion of hydrocortisone from the adrenal cortex is stimulated by the pituitary hormone ACTH. Because of their widespread effects, hydrocortisone and related compounds, called corticosteroids, or corticoids, are employed for many medical purposes. They are used to treat a deficiency of adrenal cortical hormones, a condition called congenital adrenal hyperplasia, rheumatoid disease that is not helped by milder drugs, and to counter severe non-infectious inflammations. Corticosteroids suppress the immune response, so they are used to increase acceptance of transplants ( Medical Transplantation ). Othe...

Paranasal Sinuses, air-filled, mucus-lined cavities in the head and cheekbones that drain into the nasal cavity. The largest are the two maxillary sinuses found directly under the eye sockets. Others are the two frontal sinuses, found in the frontal bone just above and between the eye sockets, and the sphenoid and ethmoid sinuses, located deeper in the skull over the nasal cavity. The sinuses produce mucus, which is propelled by small, hairlike processes called cilia into the nasal cavity. Their function is not clearly understood, but they are believed to help warm and moisten inhaled air.

Prostate Gland, chestnut-shaped male organ located next to the bladder and surrounding the urethra (the tube that carries urine from the bladder to the penis). The prostate gland produces a secretion known as prostate fluid that makes up most of the liquid part of semen, which is discharged from the penis during sexual orgasm. Measuring about 3 cm (about 1.2 in) across, the prostate gland is composed of both glandular tissue that produces prostate fluid and muscle tissue that helps in male ejaculation. Prostate fluid also helps to keep sperm, which is found in semen, healthy and lively, thereby increasing the chances that fertilization will occur

. Pelvis, lower part of the trunk of the body, bounded at the front and on either side by the hipbone, and at the back by the sacrum and the coccyx, the lowest part of the spinal column (see Hip ; Sacroiliac Joint ). The hipbone is composed of three separate bones: the ilium, the ischium or lower part of the hipbone, and the pubis, the central pubic bone that unites with the ischium at either side. In early life, the three bones are separate, but when a person is in the late teens or early 20s, they unite to form a single structure called the innominate bone. At the lower end of the hipbone is a cup-shaped depression called the acetabulum in which the femur, or thighbone, rotates. The pelvis thus acts as a unit in all bodily movements. The weight of the trunk is transferred from the spine through the sacrum and then through the hipbone to the thighbone and the lower extremities. Conversely, all forces acting on the lower limbs are transmitted to the trunk by the same route. The cavity ...

Breast, one of a pair of glandular organs in mammals that secrete milk for newborns and infants. Also known as a mammary gland , particularly in nonhuman mammals, the breast is unique to mammals and is not found in any other type of animal. In humans, the female breast has a distinctive rounded shape while the male breast remains flat and undeveloped. Only mature females can produce milk, which is rich in nutrients, antibodies, and other substances that aid in a baby’s growth and development. The surface of a human breast has a circular, pigmented area in the center called the areola. The nipple, a rounded protruding structure, lies in the center of the areola. In mature women the breast contains a collection of 10 to 15 tubes, called ducts, that connect to the nipple. These ducts branch out from the nipple into the interior of the breast, ending in clusters of rounded cells, called lobules, that produce the milk. In addition to the structures directly connected with the production a...

Uterus or Womb, flattened, pear-shaped, hollow organ in the pelvis of the human female and most other mammals. In pregnancy, it is the organ that holds the unborn developing child (see Pregnancy and Childbirth ). The uterus consists of a body, a base or fundus, a neck or cervix, and a mouth. Suspended in the pelvis, it lies with the base directed upward and forward and the cervix directed slightly backward. It is connected to the vagina by the cervix. On either side of the uterus lies an ovary. Eggs produced by the ovaries reach the uterus through the fallopian tubes. In the unimpregnated condition the uterus is about 7.6 cm (about 3 in) in length, 5 cm (2 in) in breadth, and 2.5 cm (1 in) in thickness. In pregnancy the fertilized egg implants itself in the lining of the uterus, where it grows to maturity; the walls of the uterus are elastic and stretch during pregnancy to hold the developing child. The uterus body consists of a firm outer coat of muscle, known as the myometrium, ...

Tonsil, name applied to any of a number of masses of lymphoid tissue forming a ring around the walls of the pharynx, or throat. These masses consist of cells, similar to the lymphocytes of the bloodstream, embedded in fibrous connective tissue and covered by a single layer of epithelium. The lymphoid cells are phagocytic (see Phagocytosis ) and help protect the pharynx from invasion by disease-producing bacteria. Tonsils may become inflamed and acutely or chronically infected. Such a condition, called tonsillitis, is often caused by streptococcus infection. Tissues surrounding the tonsils frequently form pus during acute attacks of tonsillitis, particularly streptococcal, causing the tonsils to show white specks or even be coated with whitish exudate. This condition is called quinsy. Pharyngeal tonsils, located at the back of the throat, may become abnormally large when inflamed. They are called adenoids . Acute cases of tonsillitis are often treated by administration of such antibiot...

. Spinal Cord, that part of the central nervous axis contained within the vertebral or neural canal. In the adult human it extends from the base of the skull to the second lumbar vertebra. Below this it tapers off as a narrow thread containing little nervous matter. Above the foramen magnum, in the base of the skull, it is continuous with the medulla oblongata. The cord, like the brain, is encased in a triple sheath of membranes and is partially divided into two lateral halves by a median fissure toward the front and a median partition toward the rear; 31 pairs of spinal nerves arise, each by an anterior and a posterior root, from each side of the cord. The spinal cord functions in the transmission of ascending impulses to the brain and of descending impulses from the brain to the cord. Thus, it conveys to higher centers information brought to it by peripheral nerves from many parts of the body; in addition, it is acted on by impulses from the brain itself. The spinal cord relays impul...

. Alimentary Canal, in anatomy , the principal part of the digestive system . It begins at the mouth and extends to the anus, having, in humans, an average length of about 9 m (about 30 ft). Passing through the head, neck, and body, it includes the mouth, pharynx, esophagus, stomach , small intestines, cecum, and large intestines.

. Achilles Tendon, in the legs of mammals, powerful, cordlike band of connective tissue that joins the fused muscles of the calf to the bone of the heel. Because bones are moved by muscles pulling on the connecting tendon, severance or rupture of the Achilles tendon results in immediate loss of the normal use of the leg and foot . The name of the tendon is derived from the legendary Greek warrior Achilles.

Sacroiliac Joint, fibrous joint in the lower back between the lumbar vertebrae and the coccyx, the bones at the lower end of the spinal column. It is composed of five fused vertebrae that form a solid triangular bone (the sacrum), and the ilium parts of the two hip bones. Once believed to be the site of sprain and lower back pain, it is in fact bound by powerful ligaments that avert injury. Such pain is more generally caused by a displaced vertebral disk ( Spinal Column ). The sacroiliac joint is subject to a number of diseases embraced by the term spondylitis, an inflammation of the vertebrae that is often rheumatic in character. Spondylitis is frequently accompanied by ankylosis, a chronic disease that progresses up the spine, fusing the vertebrae. The different forms of spondylitis are controlled by early recognition, diagnosis by X ray, and treatment by deep radiation therapy, cortisone ( Hydrocortisone ), exercises, and orthopedic procedures.

. Spinal Column, common name applied to the structure of bone or cartilage surrounding and protecting the spinal cord in vertebrate animals (see Animal). It is also called a vertebral column, spine, or backbone, ANATOMY AND PHYSIOLOGY The spinal column forms the major part of the skeleton. To it are attached the skull, shoulder bones, ribs, and pelvis. In very primitive animals having a vertebral column, the spine consists of a solid cartilaginous rod known as the notochord. Although remnants of the notochord persist in the cartilages that form part of the apparatus connecting adjoining vertebrae, in higher animals the notochord is replaced by a series of separate bones called vertebrae. The shape and number of vertebrae vary among different animals. In general, the vertebrae are stacked like a column of poker chips and are held together by ligaments, the connective tissue that holds bones together at a joint. In humans the spinal column contains 33 vertebrae: 7 cervical vertebrae in t...

Sense Organs, in humans and other animals, faculties by which outside information is received for evaluation and response. This is accomplished by the effect of a particular stimulus on a specialized organ, which then transmits impulses to the brain via a nerve or nerves. Aristotle classified five senses as follows: 1.  Hearing , a sense by which sound waves are perceived by the organ of hearing—the ear —in vertebrate animals. The process of sound perception is called audition. The physical stimulus of auditory sensation is the vibration of some material object. The vibration is transmitted from the object to the ear, under ordinary conditions, by a wave movement of air particles. 2. Sight (vision ) , is the ability to see the features of objects we look at, such as color, shape, size, details, depth, and contrast. Vision is achieved when the eyes and brain work together to form pictures of the world around us. Vision begins with light rays bouncing off the surface of obje...

. Mastoid Process, conical prominence of the temporal bone of the human skull , situated behind the ear . It commonly becomes infected in cases of suppurative otitis media. The inner ear adjoins the hollow, spongy spaces within the mastoid process so that infection of the ear easily spreads to that area, causing pain and swelling. Surgical drainage of pus and injection of antibiotics usually eliminate mastoid infection and prevent its spread to nearby areas of the brain.

Cephalic Index, basic measure of the shape of the skull , used by scientists of the 19th and early 20th centuries in classifying humans. Devised by Swedish anatomist Anders Retzius in the 1840s, the cephalic (or cranial) index expresses the width of the skull as a percentage of the length from front to back. Craniologists distinguished three basic classifications of skulls based on the cephalic index: dolichocephalic (long skulls), with widths of less than 75 percent of the length; mesocephalic (medium skulls), with indexes of 75 to 80; and brachycephalic (broad skulls), with indexes of more than 80 percent.

. Skull, term applied to the rigid cartilaginous or bony structure located anterior to the spinal column in all vertebrate animals, and serving to encase and protect the brain and to provide attachment for the muscles of the face and mouth. In lower vertebrates, such as cyclostomes and elasmobranchs, the skull is composed of cartilage throughout the lifetime of the organism; in teleost fishes and in more highly developed vertebrates, including humans, the skull is cartilaginous during embryonic and fetal life, becoming ossified early in postnatal life. The adult human skull is divided into two regions, the cranial and the facial. The cranial region is the portion of the skull directly surrounding the brain; the facial region includes all the other bones of the skull. Exteriorly the cranial bones include the two frontal bones, which constitute the forehead and which fuse together in adulthood; the two parietal bones, which constitute the top of the head and which in early childhood are ...

. Periosteum, tough, fibrous membrane that surrounds each bone. It is highly vascular and is the means by which the outer layers of the shafts and the greater part of the spongy portions of the bones are supplied with blood. It consists of an outer fibrous layer and an inner osteogenetic layer. The inner layer is very vascular and contains many protoplasmic cells called osteoblasts. Numerous experiments show that the formation of bone is essentially due to the action of the periosteum and that, by transplanting detached portions of periosteum into muscular or other tissues, bony tissue is generated in those parts. It appears to be the curative agent in the case of bone breakage. In most cases in which the periosteum has become detached as a result of a wound or of disease, the exposed bone perishes (except in the instance of the skull, which derives most of its nutrient matter from the dura mater, actually the periosteum of the inner surface of the skull). Inflammation of the perioste...

. Appendix, a worm-shaped tube branching off the cecum, the first part of the large intestine. It is located on the lower right side of the abdomen and is usually about 9 cm (about 3.5 in) long, with a thick wall. Only humans and apes have an appendix. It has no known function in human biology, but it does contain a large amount of lymphoid tissue, which may provide a defense against local infection. Many scientists believe that the human appendix at one time served a useful purpose that has gradually been lost through evolution. For reasons not fully understood, the appendix can become infected and filled with pus—particularly in children, teenagers, and young adults—resulting in appendicitis . Symptoms of appendicitis include pain and cramps in the area between the right hip bone and the navel, fever, nausea and vomiting, constipation, and diarrhea. The treatment is surgical removal of the appendix, known as appendectomy. If the appendix wall ruptures, infection may spread to the abd...

. Bladder, in anatomy, organ that stores urine after its formation by the kidneys. It consists of three layers: a lining of mucous membrane, called epithelium; an intermediate layer of involuntary muscle, with fibers arranged in three layers, each running in a different direction; and an outer layer of connective tissue, covered by the peritoneum above and to the back and blending with the connective tissues of the body wall in front and below. Urine enters the bladder from the kidneys through two ureters and is discharged through the urethra. The openings of the ureters lie in the basal part of the bladder, each about 3.8 cm (about 1.5 in) from the opening of the urethra, which is located in the midline of the bladder at its lowest point. Average normal capacity of the adult bladder is about half a liter (about 1 pt). See also Urinary System .

. Spleen, flattened, oblong organ that removes disease-producing organisms and worn-out red blood cells from the bloodstream. The spleen is situated in the upper left abdominal cavity, in contact with the pancreas, the diaphragm, and the left kidney. It is supported by bands of fibers that are attached to the peritoneum (the membrane lining the abdominal cavity). The spleen varies in size but, in adult humans, it is about 13 cm (about 5 in) long, up to 10 cm (4 in) wide and 3.8 cm (1.5 in) thick, and weighs about 200 g (about 7 oz). The spleen is fed by the splenic artery, and venous blood from the spleen enters the liver. The spleen removes iron from the hemoglobin of red blood cells for use in the body. It also removes such waste materials as bile pigments for excretion as bile by the liver. The spleen produces antibodies against various disease organisms and manufactures a variety of blood cells. In some mammals (but not in humans) it stores red blood cells and feeds them into the ...

. Pancreas, conglomerate gland lying transversely across the posterior wall of the abdomen. It varies in length from 15 to 20 cm (6 to 8 in) and has a breadth of about 3.8 cm (about 1.5 in) and a thickness of from 1.3 to 2.5 cm (0.5 to 1 in). Its usual weight is about 85 gm (about 3 oz), and its head lies in the concavity of the duodenum. The pancreas has both an exocrine and an endocrine secretion. The exocrine secretion is made up of a number of enzymes that are discharged into the intestine to aid in digestion. The endocrine secretion, insulin , is important in the metabolism of sugar in the body (see Sugar Metabolism ). Insulin is produced in small groups of especially modified glandular cells in the pancreas; these cell groups are known as the islets of Langerhans. The failure of these cells to secrete sufficient amounts of insulin causes diabetes (see Diabetes Mellitus ). Diseases of the pancreas are not common. Hemorrhage in the pancreas and acute pancreatitis are, however, seri...

. Thymus Gland, name applied to a structure located just beneath the upper portion of the sternum in almost all vertebrates. The thymus gland consists chiefly of lymphatic tissue and contains a few small areas of epithelial tissue known as Hassall's corpuscles. The human thymus gland increases in weight in the first two years of life, and from then until puberty it grows slowly to a weight of about 43 g (about 1.5 oz). After puberty, it shrinks gradually and the lymphatic tissue of the thymus gland is replaced by fat. In the adult human the organ is chiefly composed of fatty tissue. Scientists generally agree that the thymus gland plays an important role in the development of immune responsiveness in early life (see Immune System ). It is a site of formation of lymphocytes and a site of antibody production. Whether or not it has any other endocrine functions is uncertain. Clearly the adult animal is not affected by its removal. Abnormal enlargement of the thymus or development of t...

. Varicose Vein, dilated (enlarged) and often twisted vein just below the skin that develops when the valves in the vein no longer function properly or when blood volume in the vein increases. Varicose veins develop most commonly in the legs, but also occur in the anus ( hemorrhoids ), esophagus, and testes in males (varicocele). Varicose veins in the legs are characterized by a purplish-blue color. These veins may become prominent and readily visible. Varicose veins may be painful and cause swelling of the ankles and ulcerations on the skin of the legs. Venous thromboses (blood clots) may develop within varicose veins. If these blood clots break off from the vein and enter the circulatory system, they may cause a dangerous obstruction elsewhere, as in the smaller arteries of the lungs. Simple superficial varicose leg veins are treated by applying pressure all along their length with an elastic stocking. Larger varicose veins may be removed by a physician with a chemical solution or su...

Vein, in anatomy , blood vessel that conducts the deoxygenated blood from the capillaries back to the heart. Three exceptions to this description exist: the pulmonary veins return blood from the lungs, where it has been oxygenated, to the heart; the portal veins receive blood from the pyloric, gastric, cystic, superior mesenteric, and splenic veins and, entering the liver, break up into small branches that pass through all parts of that organ; and the umbilical veins convey blood from the fetus to the mother's placenta. Veins enlarge as they proceed, gathering blood from their tributaries. They finally pour the blood through the superior and inferior venae cavae into the right atrium of the heart. Their coats are similar to those of the arteries, but thinner, and often transparent. See Circulatory System ; Heart ; Varicose Vein .

. Tongue (anatomy), muscular organ in the mouth, the primary organ of taste and important in the formation of speech and in the chewing and swallowing of food. The tongue, which is covered by a mucous membrane, extends from the hyoid bone at the back of the mouth upward and forward to the lips. Its upper surface, borders, and the forward part of the lower surface are free; elsewhere it is attached to adjacent parts of the mouth. The extrinsic muscles attach the tongue to external points, and the intrinsic muscle fibers, which run vertically, transversely, and longitudinally, allow it great range of movement. The upper surface is covered with small projections called papillae, which give it a rough texture. The color of the tongue, usually pinkish-red but discolored by various diseases, is an indication of health. The tongue serves as an organ of taste, with taste buds scattered over its surface and concentrated toward the back of the tongue. In chewing, the tongue holds the food again...

. Throat, in the human body, passage leading to the lungs and the stomach, located in the neck in front of the spinal column. It contains the larynx, pharynx, and the upper parts of the esophagus and the trachea. Several large arteries and veins also pass through this region.

. Humans are diphyodont—that is, they develop two sets of teeth during their lives. The first set of teeth are the deciduous teeth, 20 small teeth also known as baby teeth or milk teeth. Deciduous teeth start developing about two months after conception and typically begin to erupt above the gumline when a baby is six or seven months old. Occasionally a baby may be born with one or more deciduous teeth at birth, known as natal teeth. By the time a child is six years old, a second set of 32 larger teeth, called permanent teeth, start to erupt, or push out of the gums, eventually replacing the deciduous teeth. Human tooth development occurs in stages. The hard tissue of the deciduous teeth, or the dentin, forms while the fetus is in the womb. After the child is born, tooth enamel develops in stages. Front tooth enamel, for example, is usually complete around one month after birth, while the enamel on the second molars is not completely developed until a child is about a year and a half ...

. Kidney The kidneys filter the blood and rid the body of wastes. Approximately one million nephrons (right) compose each bean-shaped kidney (left). The filtration unit of the nephron, called the glomerulus, regulates the concentration within the body of important substances such as potassium, calcium, and hydrogen, and removes substances not produced by the body such as drugs and food additives. Cancers that originate in the filtration tissues of the kidney, called renal cell cancer, account for 85 percent of all cancers of the kidney. A small percentage of cancers originate in the renal pelvis, a cavity in the center of each kidney. The kidneys lie embedded in fat tissue on either side of the backbone at about waist level. Each fist-sized kidney is reddish-brown, weighs 140 to 160 g (5 to 6 oz), and is similar in shape to the kidney beans. On the inner border of each kidney is a depression called the hilum, where the renal artery, the renal vein, and the ureter connect with the kidne...

. Along the course of the lymphatic vessels are situated the lymph nodes, more commonly called the lymph glands. These nodes are bean-shaped organs containing large numbers of leukocytes, embedded in a network of connective tissue. All the lymph being returned along the lymphatics to the bloodstream must pass through several of these nodes, which filter out infectious and toxic material and destroy it. The nodes serve as a center for the production of phagocytes, which engulf bacteria and poisonous substances (see Immune System ). During the course of any infection, the nodes become enlarged because of the large number of phagocytes being produced; these nodes are often painful and inflamed. The swollen glands most often observed are located on the neck, in the armpit, and in the groin. Certain malignant tumors tend to “travel” along the lymphatics; surgical removal of all nodes that are suspected of being involved in the spread of such malignancies is an accepted therapeutic procedure...

. These vessels are found in all body tissues except the central nervous system, which has a circulatory system known as the cerebrospinal system. The lymphatic capillaries run together to form larger ducts that intertwine about the arteries and veins . The lymph in these larger ducts, which are similar to thin, dilated veins, is moved along by the muscular movements of the body as a whole; it is prevented from moving back through the ducts by valves located along them at intervals. The ducts from the lower limbs and abdomen come together at the dorsal left side of the body to form a channel, known as the cisterna chyli, that gives rise to the chief lymphatic vessel of the body, the thoracic duct. This vessel receives lymph from the left side of the thorax, the left arm, and the left side of the head and neck; it empties into the junction of the left jugular and left subclavian veins. Another, smaller vessel, known as the right lymphatic duct, receives lymph from the right side of the...

. Lymphatic System, common name for the circulatory vessels or ducts in which the fluid bathing the tissue cells of vertebrates is collected and carried to join the bloodstream proper (see Lymph ). The lymphatic system is of primary importance in transporting digested fat from the intestine to the bloodstream; in removing and destroying toxic substances; and in resisting the spread of disease throughout the body. The portions of the lymphatic system that collect the tissue fluids are known as lymphatic capillaries and are similar in structure to ordinary capillaries . The lymphatic capillaries that pick up digested fat in the villi of the intestine are known as lacteals. The lymphatic capillaries are more permeable than ordinary capillaries and allow passage of larger particles than would ordinarily pass through capillary walls; large-molecule proteins, produced as a result of tissue breakdown, pass into the lymphatics for transport away from the tissues. Among the abnormal conditions ...

. Sexual dysfunctions are problems with sexual response that cause distress. Erectile dysfunction (impotence) refers to the inability of a man to have or maintain an erection. Premature ejaculation occurs when a man is not able to postpone or control his ejaculation. Inhibited male orgasm, or retarded ejaculation, occurs when a man cannot have an orgasm despite being highly aroused. Female orgasmic dysfunction (anorgasmia, or inhibited female orgasm) refers to the inability of a woman to have an orgasm. Orgasmic dysfunction may be primary, meaning that the woman has never experienced an orgasm; secondary, meaning that the woman has had orgasms in the past but cannot have them now; or situational, meaning that she has orgasms in some situations but not in others. Vaginismus refers to a spastic contraction of the outer third of the vagina, a condition that can close the entrance of the vagina, preventing intercourse. Dyspareunia refers to painful intercourse in either women or men. Low s...

. There are a number of pressing sexually related public health and social policy issues facing countries around the world today. According to the United States Centers for Disease Control and Prevention, in the United States a teen becomes pregnant every 30 seconds, and every 13 seconds a teen contracts a sexually transmitted disease (STD). For most people in the United States, engaging in heterosexual intercourse without the use of a condom is the behavior that puts them at greatest risk for infection with human immunodeficiency virus (HIV), which can lead to acquired immunodeficiency syndrome (AIDS) and is often ultimately fatal. Although there is currently no cure for AIDS, there are medications that can help delay the onset of symptoms. Another serious sexually transmitted disease is syphilis, which if left untreated for many years, can lead to paralysis, psychiatric illness, and death. Gonorrhea and chlamydia may produce no obvious symptoms in a woman, but they can lead to ste...