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Anatomy (Greek anatomÁ,"dissection"), branch of natural science dealing with the structural organization of living things. It is an old science, having its beginnings in prehistoric times. For centuries anatomical knowledge consisted largely of observations of dissected plants and animals. The proper understanding of structure, however, implies a knowledge of function in the living organism. Anatomy is therefore almost inseparable from physiology, which is sometimes called functional anatomy. As one of the basic life sciences, anatomy is closely related to medicine and to other branches of biology.

It is convenient to subdivide the study of anatomy in several different ways. One classification is based on the type of organisms studied, the major subdivisions being plant anatomy and animal anatomy. Animal anatomy is further subdivided into human anatomy (see below) and comparative anatomy, which seeks out similarities and differences among animal types (see Animal). Anatomy can also be subdivided into biological processes-for example, developmental anatomy, the study of embryos, and pathological anatomy, the study of diseased organs. Other subdivisions, such as surgical anatomy and anatomical art, are based on the relationship of anatomy to other branches of activity under the general heading of applied anatomy. Still another way to subdivide anatomy is by the techniques employed-for example, microanatomy, which concerns itself with observations made with the help of the microscope (see the section below on the history of anatomy).

The human body operates through systems that are summarized below. This encyclopedia contains separate articles on each of the systems and organs mentioned to which the reader is referred for more comprehensive discussions. References to articles discussing the senses and perception are listed in Sense Organs.

° Musculoskeletal System
The human skeleton consists of more than 200 bones bound together by tough and relatively inelastic connective tissues called ligaments. The different parts of the body vary greatly in their degree of movement. Thus, the arm at the shoulder is freely movable, whereas the knee joint is definitely limited to a hingelike action. The movements of individual vertebrae are extremely limited; the bones composing the skull are immovable. Movements of the bones of the skeleton are effected by contractions of the skeletal muscles, to which the bones are attached by tendons. These muscular contractions are controlled by the nervous system. See Muscle; Skeleton.

° Nervous System
The nervous system has two divisions: the somatic, which allows voluntary control over skeletal muscle, and the autonomic, which is involuntary and controls cardiac and smooth muscle and glands. The autonomic nervous system has two divisions: the sympathetic and the parasympathetic. Many, but not all, of the muscles and glands that distribute nerve impulses to the larger interior organs possess a double nerve supply; in such cases the two divisions may exert opposing effects. Thus, the sympathetic system increases heartbeat, and the parasympathetic system decreases heartbeat. The two nervous systems are not always antagonistic, however. For example, both nerve supplies to the salivary glands excite the cells of secretion. Furthermore, a single division of the autonomic nervous system may both excite and inhibit a single effector, as in the sympathetic supply to the blood vessels of skeletal muscle. Finally, the sweat glands, the muscles that cause involuntary erection or bristling of the hair, the smooth muscle of the spleen, and the blood vessels of the skin and skeletal muscle are actuated only by the sympathetic division.

Voluntary movement of head, limbs, and body is caused by nerve impulses arising in the motor area of the cortex of the brain and carried by cranial nerves or by nerves that emerge from the spinal cord to connect with skeletal muscles. The reaction involves both excitation of nerve cells stimulating the muscles involved and inhibition of the cells that stimulate opposing muscles. A nerve impulse is an electrical change within a nerve cell or fiber; measured in millivolts, it lasts a few milliseconds and can be recorded by electrodes.

Movement may occur also in direct response to an outside stimulus; thus, a tap on the knee causes a jerk, and a light shone into the eye makes the pupil contract. These involuntary responses are called reflexes. Various nerve terminals called receptors constantly send impulses into the central nervous system. These are of three classes: exteroceptors, which are sensitive to pain, temperature, touch, and pressure; interoceptors, which react to changes in the internal environment; and proprioceptors, which respond to variations in movement, position, and tension. These impulses terminate in special areas of the brain, as do those of special receptors concerned with sight, hearing, smell, and taste.

Muscular contractions do not always cause actual movement. A small fraction of the total number of fibers in most muscles are usually contracting. This serves to maintain the posture of a limb and enables the limb to resist passive elongation or stretch. This slight continuous contraction is called muscle tone.

° Circulatory System
In passing through the system, blood pumped by the heart follows a winding course through the right chambers of the heart, into the lungs, where it picks up oxygen, and back into the left chambers of the heart. From these it is pumped into the main artery, the aorta, which branches into increasingly smaller arteries until it passes through the smallest, known as arterioles. Beyond the arterioles, the blood passes through a vast amount of tiny, thin-walled structures called capillaries. Here, the blood gives up its oxygen and its nutrients to the tissues and absorbs from them carbon dioxide and other waste products of metabolism. The blood completes its circuit by passing through small veins that join to form increasingly larger vessels until it reaches the largest veins, the inferior and superior venae cavae, which return it to the right side of the heart. Blood is propelled mainly by contractions of the heart; contractions of skeletal muscle also contribute to circulation. Valves in the heart and in the veins ensure its flow in one direction.

° Immune System
The body defends itself against foreign proteins and infectious microorganisms by means of a complex dual system that depends on recognizing a portion of the surface pattern of the invader. The two parts of the system are termed cellular immunity, in which lymphocytes are the effective agent, and humoral immunity, based on the action of antibody molecules.

When particular lymphocytes recognize a foreign molecular pattern (termed an antigen), they release antibodies in great numbers; other lymphocytes store the memory of the pattern for future release of antibodies should the molecule reappear. Antibodies attach themselves to the antigen and in that way mark them for destruction by other substances in the body's defense arsenal. These are primarily complement, a complex of enzymes that make holes in foreign cells, and phagocytes, cells that engulf and digest foreign matter. They are drawn to the area by chemical substances released by activated lymphocytes.

Lymphocytes, which resemble blood plasma in composition, are manufactured in the bone marrow and multiply in the thymus and spleen. They circulate in the bloodstream, penetrating the walls of the blood capillaries to reach the cells of the tissues. From there they migrate to an independent network of capillaries that is comparable to and almost as extensive as that of the blood's circulatory system. The capillaries join to form larger and larger vessels that eventually link up with the bloodstream through the jugular and subclavian veins; valves in the lymphatic vessels ensure flow in one direction. Nodes at various points in the lymphatic network act as stations for the collection and manufacture of lymphocytes; they may become enlarged during an infectious disease. In anatomy, the network of lymphatic vessels and the lymph nodes are together called the lymphatic system; its function as the vehicle of the immune system was not recognized until the 1960s.

° Respiratory System
Respiration is carried on by the expansion and contraction of the lungs; the process and the rate at which it proceeds are controlled by a nervous center in the brain.

In the lungs, oxygen enters tiny capillaries, where it combines with hemoglobin in the red blood cells and is carried to the tissues. Simultaneously, carbon dioxide, which entered the blood in its passages through the tissues, passes through capillaries into the air contained within the lungs. Inhaling draws into the lungs air that is higher in oxygen and lower in carbon dioxide; exhaling forces from the lungs air that is high in carbon dioxide and low in oxygen. Changes in the size and gross capacity of the chest are controlled by contractions of the diaphragm and of the muscles between the ribs.

° Digestive and Excretory Systems
The energy required for maintenance and proper functioning of the human body is supplied by food. After it is broken into fragments by chewing (see Teeth) and mixed with saliva, digestion begins. The food passes down the gullet into the stomach, where the process is continued by the gastric and intestinal juices. Thereafter, the mixture of food and secretions, called chyme, is pushed down the alimentary canal by peristalsis, rhythmic contractions of the smooth muscle of the gastrointestinal system. The contractions are initiated by the parasympathetic nervous system; such muscular activity can be inhibited by the sympathetic nervous system. Absorption of nutrients from chyme occurs mainly in the small intestine; unabsorbed food and secretions and waste substances from the liver pass to the large intestines and are expelled as feces. Water and water-soluble substances travel via the bloodstream from the intestines to the kidneys, which absorb all the constituents of the blood plasma except its proteins. The kidneys return most of the water and salts to the body, while excreting other salts and waste products, along with excess water, as urine.

° The Endocrine System
In addition to the integrative action of the nervous system, control of various body functions is exerted by the endocrine glands. An important part of this system, the pituitary, lies at the base of the brain. This master gland secretes a variety of hormones, including the following: (1) a hormone that stimulates the thyroid gland and controls its secretion of thyroxine, which dictates the rate at which all cells utilize oxygen; (2) a hormone that controls the secretion in the adrenal gland of hormones that influence the metabolism of carbohydrates, sodium, and potassium and control the rate at which substances are exchanged between blood and tissue fluid; (3) substances that control the secretion in the ovaries of estrogen and progesterone and the creation in the testicles of testosterone; (4) the somatotropic, or growth, hormone, which controls the rate of development of the skeleton and large interior organs through its effect on the metabolism of proteins and carbohydrates; and (5) an insulin inhibitor-a lack of insulin causes diabetes mellitus.

The posterior lobe of the pituitary secretes vasopressin, which acts on the kidney to control the volume of urine; a lack of vasopressin causes diabetes insipidus, which results in the passing of large volumes of urine. The posterior lobe also elaborates oxytocin, which causes contraction of smooth muscle in the intestines and small arteries and is used to bring about contractions of the uterus in childbirth. Other glands in the endocrine system are the pancreas, which secretes insulin, and the parathyroid, which secretes a hormone that regulates the quantity of calcium and phosphorus in the blood.

° The Reproductive System
Reproduction is accomplished by the union of male sperm and the female ovum. In coitus, the male organ ejaculates more than 250 million sperm into the vagina, from which some make their way to the uterus. Ovulation, the release of an egg into the uterus, occurs approximately every 28 days; during the same period the uterus is prepared for the implantation of a fertilized ovum by the action of estrogens. If a male cell fails to unite with a female cell, other hormones cause the uterine wall to slough off during menstruation. From puberty to menopause, the process of ovulation, and preparation, and menstruation is repeated monthly except for periods of pregnancy. The duration of pregnancy is about 280 days. After childbirth, prolactin, a hormone secreted by the pituitary, activates the production of milk.

° Skin
The skin is an organ of double-layered tissue stretched over the surface of the body and protecting it from drying or losing fluid, from harmful external substances, and from extremes of temperature. The inner layer, called the dermis, contains sweat glands, blood vessels, nerve endings (sense receptors), and the bases of hair and nails. The outer layer, the epidermis, is only a few cells thick; it contains pigments, pores, and ducts, and its surface is made of dead cells that it sheds from the body. (Hair and nails are adaptations arising from the dead cells.) The sweat glands excrete waste and cool the body through evaporation of fluid droplets; the blood vessels of the dermis supplement temperature regulation by contracting to preserve body heat and expanding to dissipate it. Separate kinds of receptors convey pressure, temperature, and pain. Fat cells in the dermis insulate the body, and oil glands lubricate the epidermis.

The oldest known systematic study of anatomy is contained in an Egyptian papyrus dating from about 1600 BC. The treatise reveals knowledge of the larger viscera but little concept of their functions. About the same degree of knowledge is reflected in the writings of the Greek physician Hippocrates in the 5th century BC. In the 4th century BC Aristotle greatly increased anatomical knowledge of animals. The first real progress in the science of human anatomy was made in the following century by the Greek physicians Herophilus and Erasistratus, who dissected human cadavers and were the first to distinguish many functions, including those of the nervous and muscular systems. Little further progress was made by the ancient Romans or by the Arabs. The Renaissance first influenced the science of anatomy in the latter half of the 16th century.

Modern anatomy began with the publication in 1543 of the work of the Belgian anatomist Andreas Vesalius. Before the publication of this classical work anatomists had been so bound by tradition that the writings of authorities of more than 1000 years earlier, such as the Greek physician Galen, who had been restricted to the dissection of animals, were accepted in lieu of actual observation. Vesalius and other Renaissance anatomists, however, based their descriptions on their own observations of human corpses, thus setting the pattern for subsequent study in anatomy.

° Morphology
For many years anatomists (even those of the modern era) were concerned mainly with the accumulation of a vast amount of information known as descriptive morphology. Descriptive morphology has been supplemented, and to a certain extent supplanted, by the development of experimental morphology, which attempts to identify the hereditary and environmental determinants in morphology and their relationships by controlled-environment and grafting experiments on embryos. Ideally, anatomical investigation consists of a combination of descriptive and experimental approaches. Present-day anatomy involves scrutiny of the structure of organisms at many levels of observation. For example, the anatomist studies the cells and tissues of organisms with the unaided eye, with simple and compound lenses, with various kinds of microscopes, and by chemical methods of analysis.

° Microscopic Anatomy
The 17th-century invention of the compound microscope led to the development of microscopic anatomy, which is divided into histology, the study of tissues, and cytology, the study of cells. Under the leadership of the Italian anatomist Marcello Malpighi, the study of the microscopic structure of animals and plants flourished during the 17th century. Many great anatomists of the period were reluctant to accept microscopic anatomy as part of their science. By contrast, modern anatomy is studied usually with the aim of correlating the structure of organisms as seen by the naked eye with their structure as revealed by more refined methods of observation.

Pathological anatomy was established as a branch of the science by the Italian physician Giovanni Morgagni, and in the late 18th century comparative anatomy was systematized by the French naturalist Georges Cuvier.
In the late 18th and early 19th centuries restrictive legislation limiting the use of unclaimed human bodies for the study of anatomy and surgery gave rise in England and the United States to an era of body snatching. The scandals arising from this practice forced the repeal of the English restrictions in 1832 and the enactment of more advanced legislation.
Microscopic anatomy developed rapidly in the 19th century. During the second half of the century many basic facts about the fine structure of organisms were discovered, largely as a result of greatly improved optical microscopes and of new methods that made cells and tissues easy to study with this instrument. The method of microtomy, the cutting of tissue into thin, practically transparent slices, was perfected. Microtomy was rendered incomparably more valuable by the application to the tissue slices of various types of dyes and stains that make it much easier to see various parts of the cell.

Knowledge of microscopic anatomy was greatly expanded during the 20th century as a result of the development of microscopes that provided much greater resolution and magnification than had conventional instruments, thus revealing formerly unclear or invisible detail; and expanded laboratory techniques helped facilitate observation. The ultraviolet microscope allows the observer to see more because the wavelengths of its probing rays are shorter than those of visible light (the resolving power of a microscope is inversely proportional to the wavelength of the light used). It also is used to emphasize particular details through selective absorption of certain ultraviolet wavelengths. The electron microscope gives even greater magnification and resolution. These tools have opened up formerly unexplored fields of anatomical investigation. Other modern microscopes have made visible unstained and living materials that would be invisible under the conventional microscope. Two examples are the phase-contrast microscope and the interference microscope. Through utilization of ordinary light beams, both these instruments differentiate parts of living, unstained cells.
The discovery of X rays by the German physicist Wilhelm Conrad Roentgen enabled anatomists to study tissues and organ systems in living animals. The first X-ray photograph, taken in 1896, was of a human hand. Today's techniques permit three-dimensional X-ray photographs of the soft tissues of the viscera after ingestion of special opaque fluids, and of "slices" of the body with computer-aided X-ray beams. See Radiology. The latter is called computerized tomography, or CT scanning. Other noninvasive techniques that have been developed include the use of ultrasonic waves for imaging soft tissues and the application of nuclear magnetic resonance systems to research and diagnosis.

Another 20th-century technique of anatomical investigation is tissue culture, which involves the cultivation of cells and tissues of complex organisms outside the body. The technique permits the isolation of living units so that the investigator can directly observe the processes of growth, multiplication, and differentiation of cells. Tissue culture thus has added a new dimension to anatomical science.

° Histochemistry and Cytochemistry
The closely related techniques of histochemistry and cytochemistry are concerned with the investigation of chemical activities of tissues and cells. For example, the presence of certain colors within cells indicates that particular chemical reactions have occurred. In addition, the density of the color reaction may serve as an index of the intensity of the reaction. Histochemical methods have been particularly successful in the study of enzymes, catalytic substances that control and direct many of the cell's activities. Much knowledge of enzymes was gained in studies carried out after removal of the enzymes from their cells of origin, but not until the advent of histochemistry could the anatomist see through the microscope which cells carry specific enzymes or gauge how active these enzymes are in different cells under various conditions.

An important technique of histochemistry involves the use of radioactive isotopes of various chemical elements that are present in cells and tissues (see Isotope; Radioimmunoassay; Isotopic Tracer). Elements or compounds "tagged" or "labeled" with radioactive isotopes are administered to living materials, permitting the investigator to trace the pathways taken by these substances through the various tissues. The degree of concentration and dilution of elements within specific cellular constituents may be estimated by measuring the radiations emanating from these tissues. The technique of labeling compounds with radioactive isotopes makes it possible to study the distribution and concentration of isotopes in tissue slices similar to those studied routinely under the microscope. This study, called autoradiography, is accomplished by bringing the radioactive tissue slices into contact with photographic films and emulsions that are sensitive to radiation.

Another technique of localizing chemical compounds within tissue slices is microincineration: the heating of microscopic sections to the point at which the organic materials present are destroyed and only the mineral skeleton remains. The remaining minerals can then be identified by special chemical and microscopic procedures. Thus, microincineration provides still another way of locating specific chemical elements within particular cell or tissue components.

Another development in the field of histochemistry is microspectrophotometry, a precise method of color analysis. In this process the colors within a tissue slice are analyzed with a spectrophotometer, an instrument that measures the intensity of each color as a function of wavelength. Microspectrophotometry can be used to estimate the characteristics of unstained cells and tissues by measuring their absorption of particular wavelengths. Another application permits precise judgments to be made concerning the nature and intensity of color reactions. These judgments provide, in turn, accurate information about the location and intensity of chemical reactions in the components of living organisms.
See Physiology.



Sense Organs
Isotopic Tracer