Anatomy is the branch of biology involving the structure of plants, animals, and other forms of biological organisms. It is related to morphology, which is concerned with the description of organisms, and MORPHOGENESIS, concerned with their development. \Tcomparative anatomy\t is concerned with the difference in structure of animal forms. Anatomy is divided into several subdisciplines. Gross anatomy involves studies on structures that can be seen with the naked eye. \Thistology\t is the study of tissue structure and \Tcytology\t that of cell structure; because histological and cytological studies require the use of a microscope, they are known together as microscopic anatomy. When the word functional is placed before any of these words, as in "functional anatomy," reference is being made to the subject of \Tphysiology\t. Often anatomy cannot be discussed properly without introducing some physiology, and physiology can almost never be discussed properly without an anatomical background. Two other divisions of anatomy are \Tembryology\t and paleoanatomy. Embryology is commonly called developmental anatomy because it is concerned with the genesis and development of a fully differentiated tissue, organ, or organism. Paleoanatomy is the study of the structure of extinct organisms. EARLY HISTORY OF ANATOMY Aristotle The first recorded attempts to study anatomy were made by \TAristotle\t (384-22 BC), although hieroglyphics and papyruses produced from 3000 to 1600 BC indicate that some interest was taken in certain anatomical aspects, or appearances, in mummies. Aristotle, the founder of biological science, dissected plants and animals, although neither he nor \THippocrates\t (460-374 BC), the father of \Tmedicine\t, dissected the human body. Both believed that the \Theart\t is the seat of thought and of the soul. Herophilus and Erasistratus Soon after the death of Aristotle the Ptolemies, kings of Egypt, encouraged dissections. Herophilus (335-280 BC) and his contemporary \TErasistratus\t (310-250 BC) were perhaps the most active practitioners. Herophilus dissected about 600 human bodies and wrote more than one treatise on anatomy, another on the eyes, and a handbook for midwives. His greatest contribution was to dispel the earlier erroneous notions of Aristotle and Hippocrates about the mind, proving that the \Tbrain\t is the center of the \Tnervous system\t and the seat of intelligence by tracing nerves from the brain to the spinal cord and classifying them as voluntary and involuntary. Erasistratus observed lymph carrying fat toward the heart, described the function of the epiglottis in closing the larynx, identified the tricuspid valve of the heart, and was able to distinguish sensory nerves from motor nerves. He studied the \Tcirculatory system\t intensively, but, like his predecessors, he held that the arteries contain air. This was a common belief, since the veins normally contain about 60 percent of the total \Tblood\t, and on death the arteries empty out into the capillary beds and veins. Galen Despite many incorrect observations by the ancients, perhaps as many principles were known by the end of Cleopatra's reign (around 30 BC) as were to be discovered during the next 1,000 years. Shortly after Cleopatra's death, Alexandria became a Roman city and one of the main centers of the Christian church. Its leaders then began to discourage dissections. Leaders in other parts of the world, except Arabia, also denounced them. The fervor for achievement of anatomical knowledge did not die immediately, however, in spite of these difficulties. Most notable among those who strove to advance anatomical understanding was Claudius \TGalen\t (AD 131-200), physician to the Roman emperor Marcus Aurelius. Known historically as the founder of experimental physiology, Galen showed that urine is formed in the \Tkidneys\t, not in the bladder, and that sectioning of the spinal cord results in paralysis to that part of the body below the cut. His monumental work, On the Use of the Parts of the Human Body, served as the standard medical text for 1,400 years. Despite some excellent contributions to experimental medicine, however, Galen may have delayed anatomical progress. Because of religious prejudice and superstition, he was not permitted to dissect human bodies, and many of his conclusions were based on dissections of oxen, dogs, swine, and apes. More important, he perpetuated false beliefs established by his predecessors and contemporaries--that cosmic life is taken into the body with each breath (pneuma) and that three separate spirits dwell in the body: a "natural" spirit in the \Tliver\t, a "vital" spirit in the heart, and an "animal" spirit in the brain. The first of these beliefs gained further credence in that it was established on Galen's experimental approaches. Although Galen's beliefs suited and pleased the leaders of both church and state, they served to misinform the concerned portion of the populace who, although wishing to know the truth, had to accept a fallacious presentation until William \THarvey\t would prove it wrong in the 17th century. Galen's view of the circulatory system was also incorrect. Like his predecessors, and despite numerous studies on his own, Galen traced the passage of blood from the liver--where, he maintained, it was formed--to the heart, where supposedly a breath of air was injected into it as a gaseous material from the \Tlungs\t. One important contribution of his is that the arteries carry blood, not air. According to Galen, the blood then travels to the rest of the body rather than first going to the lungs and back into the heart. This view of the circulatory passage as one-way was also maintained as scientific thought until the 17th century. Avicenna and the Arabs Meanwhile, faced with increasing religious antipathy, anatomical pursuit was virtually abandoned, except for what work was done by the Arabs. Unfortunately, their studies were based on Aristotle's and Galen's works, which the Arabs had translated into their language. \TAvicenna\t (AD 980-1037) was perhaps the most famous of the Arab anatomists. His Canon of Medicine, published in AD 1000, contained information based on observations derived from studies on humans, apes, dogs, and other animals. Neither he nor his peers, however, carried out systematic studies. Dissections were sometimes performed, mainly in an attempt to learn the causes of death, and occasionally public authorities permitted physicians to dissect executed criminals. Vesalius Gradually, as the Renaissance became established, some individuals began to circumvent authoritarian restrictions, and the number of anatomists increased rapidly. Perhaps the most important of these was Andreas \TVesalius\t (1514-64), who ushered in the modern era of anatomy. Rather than accept many of the incorrect observations of Galen and pursue the study of the topic through metaphysical dialectic, he took a straightforward scientific experimental approach. He compared the anatomy of various animals with humans and noted the fallacy of extrapolating from one to the other. His work culminated in On the Structure of the Human Body, which was published the same year (1543) as Copernicus's On the Revolution of Celestial Bodies. Vesalius's work was most significant in that it was the first to contain accurate depictions of the inner structure of the human body. Successors and contemporaries of Vesalius added important details to the basework he had created, and in less than a century they had completed much of the basic studies on gross anatomy. Harvey This basic work culminated in the accomplishments of William Harvey (1578-1657), an English physician who was the first to postulate that the blood moves in a circle from left heart to arteries to veins to right heart. He also postulated the existence of thousands of miles of microscopic blood capillaries, a theory that was proved correct after microscopes became available. At about this time anatomy began to become established as a full-fledged science. Scientific societies were formed, textbooks and atlases began to appear, schools were established, and laws were enacted regulating the acquisition of human bodies for dissection and study. HISTORY OF MICROSCOPIC ANATOMY Rapid strides were made as microscopic anatomy began to be established and, together with gross anatomy, merged with comparative and developmental anatomy. Marcello \TMalpighi\t (1628-94), a young contemporary of the elderly William Harvey, demonstrated that blood went through capillaries in the lungs before reaching the left heart. He described many other important histological features, such as the germinal layer at the base of the outer layer (epidermis) of \Tskin\t and the structure of the functional units of the kidney (nephrons) through which blood is filtered in producing urine. Robert \THooke\t (1635-1703) also was an outstanding physicist. In 1665 he reported in Micrographia his observations on plant anatomy. Based on his microscopic observations of cork, he coined the word cell. His observations later helped other investigators advance the concept that cells are the unit structures of tissues. In particular, about a century later, Caspar Friedrich \TWolff\t (1733-94), as a result of intensive study in embryology, hinted that cells are the building blocks of organs. Later, Robert Brown (1773-1858) discovered the cell nucleus, probably because staining techniques became available. Still later, M. J. Schleiden (1804-81) and Theodor \TSchwann\t (1810-82) in 1840 advanced the theory that all tissues, including bone, tendon, and ligament, are composed of cells. In doing so, they brought cytology into existence as a separate, although interrelated, field of inquiry. This field now comprises the subspecialties of cytochemistry and cytogenetics. Two other investigators figured prominently as contemporaries of Malpighi and Hooke. Anton van \TLeeuwenhoek\t (1632-1723) was the first to observe and describe bacteria, protozoa, and other microorganisms, as well as sperm and the cross striations of skeletal and cardiac \Tmuscle\t. In addition, he provided visual proof that William Harvey's theory on blood circulation was indeed correct. The other notable microanatomist, Jan \TSwammerdam\t (1637-80), is famous for his remarkable work on the developmental anatomy of various insects and is considered the first person to observe and describe (1658) red blood cells. Near the end of the 17th century, histology emerged as a distinct discipline of study. Its development went hand in hand with advances in the microscope itself, the invention of instruments (microtomes) for cutting thin sections of plant and animal tissues, and the introduction of staining procedures. The major advances in each of these technological fields were especially prominent in the 19th century. Long before this, however, Xavier Bichat (1771-1802) distinguished himself as an animal histologist by pointing out the similarities that exist between certain tissues widespread in the body and by classifying these tissues as muscle, nerve, and connective. Today, these tissues, together with epithelial tissues, are recognized as the four principal tissues of animals. HISTORY OF COMPARATIVE ANATOMY Comparative anatomy is the science that concerns evolutionary advances in animal structure and the anatomical and physiological adaptations animals have made in response to environmental demands. As a discipline with these objectives, it may be said to have begun with Aristotle, who dissected numerous animals in an attempt to develop theories on animal classification and evolution. Galen also involved himself with comparative anatomy, although his objective was to obtain a better understanding of human anatomy. Other anatomists, beginning especially in the Renaissance, dissected and described certain anatomical aspects of fish and other animals, but Georges \TBuffon\t (1707-88) was the first to attempt a major compilation of data. The results were published in the first 15 volumes of his 44-volume Natural History, General and Particular (1749-1804; Eng. trans., 20 vol., 1781-1812), a monumental work that took 53 years to complete. Buffon's accomplishments paved the way for Georges Baron \TCuvier\t (1769-1832), the first to attempt the synthesis of comparative anatomy into a framework of principles. Instead of trying to fit fact to preconceived theory, as was commonly done, Cuvier attempted to establish new theories based on the available facts. In his 9-volume work Le Regne animal distribue d'apres son organisation (The Animal Kingdom Distributed According to Its Organization, 1817-30), he included the results of all his research on the structure of extant and fossil animals. Many anatomists followed Cuvier's lead, but none contributed as much as Richard Owen (1804-92), who originated the concept of homology. This concept concerns the anatomical structures in terms of their embryology and evolutionary origin. The wings of a bat, for example, are homologous to the arms of a man, since each pair of these structures is derived from the same germinal centers in the embryo. HISTORY OF DEVELOPMENTAL ANATOMY In the 17th century, embryological studies were conducted by Swammerdam (on invertebrates) and Malpighi (on chicks), but it was not until the 19th century that this science gained considerable momentum. Among the most famous embryologists are E. R. Lankester (1847-1929), Oscar Hertwig (1849-1922), and Richard Hertwig (1850-1937), all of whom worked on the development of body cavities. Equally important were F. M. Balfour (1851-82), who published a 2-volume Treatise of Comparative Embryology (1880-81), the first volume on invertebrates and the second on vertebrates; and Ernest \THaeckel\t (1834-1919), who advanced the useful concept that in the development of an advanced organism, such as a mammal, the embryo proceeds progressively through the stages of its forebears, including fishes and amphibians. This concept is usually summarized as "ontogeny recapitulates phylogeny." It is not strictly correct, although some embryonic developments relate closely to it, such as the events associated with the acquisition of the mammalian jaws, eardrum, thymus, and parathyroid glands. MODERN WORK Anatomical studies today are characterized by their interdisciplinary nature and their emphasis on function, or physiology. At the whole-body level, anatomists with training in physics are attempting to learn the anatomical bases of speed in the locomotion of such animals as cheetahs and horses; of power in digging animals, such as moles; of flight in such diverse animals as bats and birds; and of swimming in animals as different from each other as whales and salmon. In the study of specific organs, neurobiologists are attempting to map out brains in order to correlate complex functions, such as behavior, with networks of neuronal circuitry. Histologists are exploring problems relating to the origin of hormones, the occurrence of enzymes, and the storage depots of trace elements; histochemical procedures, combined with immunoradiography, are commonly employed. Cytologists use a wide variety of approaches, including electron microscopy, ultracentrifugal separation of subcellular organs (organelles), cytochemistry, and biochemistry. Anatomy as a whole has advanced in the course of two millennia from an area of absolute ignorance to a sound science that is integrated with all other biological sciences. \TRoy\t HARTENSTEIN Bibliography: Cole, F. J., A History of Comparative Anatomy (1944; repr. 1975); Green, John, Medical History for Students (1968); Long, Esmond, a History of Pathology (1928); Lund, Fred, Greek Medicine (1936); Persaud, T. V., Early History of Human Anatomy (1984); Singer, Charles, Short History of Anatomy and Physiology (1957); Stenn, Frederick, ed., The Growth of Medicine (1967); Thorwald, Jurgen, Science and Secrets of Early Medicine (1962).