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Physiology, study of the physical and chemical processes that take place in living organisms during the performance of life functions. It is concerned with such basic activities as reproduction, growth, metabolism, excitation, and contraction as they are carried out within the fine structure, the cells, tissues, organs, and organ systems of the body.
Physiology is intimately linked with anatomy and was historically considered a part of medicine. Its emphasis on investigating biological mechanisms with the tools of physics and chemistry made physiology a distinct discipline in the 19th century; the tendency today, however, is toward a fragmentation and merging with the many specialized branches of the life sciences. Three broad divisions are recognized: general physiology, concerned with basic processes common to all life forms; the physiology and functional anatomy of humans and other animals, including pathology and comparative studies; and plant physiology, which includes photosynthesis and other processes pertinent to plant life.

The first studies in animal physiology were probably undertaken about 300BC by the Alexandrian physician Herophilus, who reportedly vivisected the bodies of criminals. For about 1900 years thereafter, few physiological studies were performed.

° Beginnings of Modern Physiology
Modern animal physiology dates from the discovery of the circulation of the blood by the English physician William Harvey in 1616. Shortly thereafter, the Flemish chemist Jan Baptista van Helmont developed the concept of gases and suggested the use of alkalies in treating digestive disturbances; the Italian biophysicist Giovanni Alfonso Borelli published studies of animal motion, suggesting that the basis of muscle contraction lay in the muscle fibers; the Dutch microscopist Antoni van Leeuwenhoek gave the first descriptions of red blood cells and spermatozoa; and the Italian histologist Marcello Malpighi demonstrated the existence of capillaries and studied the physiology of the kidney, liver, and spleen. During the second half of the century the study of glands was initiated by the English physician Thomas Wharton, who demonstrated salivary secretion, and by the Danish anatomist Nicolaus Steno, who demonstrated the secretions of the tear glands and salivary glands. The Dutch physician Regnier de Graaf furthered glandular study by his discovery of the follicles in the ovary; he also performed studies on pancreatic juices and bile. The English physician Richard Lower was the first to transfuse blood from one animal to another, and the French physician Jean Baptiste Denis first gave a human being a successful blood transfusion.

In the 17th century advances were made also in the study of respiration. The English physiologist John Mayow showed that air was not a single substance but a mixture of several, not all of which were necessary for life. In the 18th century, the British chemist Joseph Priestley showed that the proportion of oxygen essential for animal life is identical with the proportion of oxygen needed to support combustion. Antoine Laurent Lavoisier, the French chemist, isolated and named oxygen shortly thereafter and showed that the by-product of respiration is carbon dioxide.

° 18th- and 19th-Century Physiology
Modern physiology owes much to the work performed during the 18th century by the Dutch physician Hermann Boerhaave and his pupil, the Swiss scientist Albrecht von Haller. Through their criticism of the iatrochemists (scientists who believed physiology involved only chemical reactions) and the iatrophysicists (scientists who believed physiology involved only physical reactions), they laid the foundation for an integrated study of physiology. Haller was the first scientist to state that all living matter possesses irritability.

During the second half of the 18th century, the Italian physician Luigi Galvani showed that the muscles of a frog's leg could be made to contract by stimulation with an electric current; the Italian physiologist Lazzaro Spallanzani investigated the activity of gastric juice in digestion. Spallanzani also studied fertilization and artificial insemination in lower animals.

The principal figure in animal physiology in the 19th century was the French physiologist Claude Bernard, who investigated carbohydrate metabolism in humans; he also studied the autonomic nervous system and described many of its functions. His greatest contribution was his statement of the principle that living organisms are never at rest but constantly undergo dynamic changes to maintain internal equilibrium. The basis of health, according to Bernard, is the organism's success in maintaining this balance. Bernard's principles were amplified during the first half of the 20th century by the American physiologist Walter Bradford Cannon, who named the dynamic state homeostasis and showed that the body could adjust to meet serious external danger. Cannon demonstrated such processes of the human body as internal regulation of body heat and alkalinity of the blood and preparation of the body for defense by the secretion of epinephrine (also called adrenaline) in the adrenal gland.

During the 19th century, the physiology of the nervous system was studied by the Scottish anatomist Sir Charles Bell, who described the functions of sensory and motor nerves; the French physiologist François Magendie, who described the functions of the spinal nerves and investigated the mechanisms of swallowing and regurgitation; the French physiologist Pierre Flourens, who investigated the functions of the cerebellum and who was a pioneer in the physiological investigation of animal psychology; and the German physiologist Johannes Peter Müller, who showed that perceptions were determined only by the sensory organ that received the sensory impulse. Ernst Heinrich Weber, a German physiologist, discovered that two types of nerve stimulate the human heart: those that increase the heartbeat and those that inhibit it. He was thus one of the first to recognize that the autonomic nervous system is composed of two nerve systems. Weber also conducted investigations into the mechanics of perception.

The first laboratory for the investigation of the physiological basis of psychology was founded by the German physiologist and psychologist Wilhelm Wundt during the last quarter of the 19th century. The first such laboratory in the U.S. was established at the close of the 19th century by the American physiologist Henry Pickering Bowditch.

During the late 19th and early 20th centuries, the impetus of the new science of bacteriology led to research into immunity. The most important figures were the Russian naturalist Élie Metchnikoff, who developed the theory of phagocytosis, the destruction of foreign materials in the blood, and the German bacteriologist and chemist Paul Ehrlich, who evolved a theory of antibody formation.

At about the same time, the physiology of the ductless glands was investigated by the British physiologist Sir Edward Albert Sharpey-Schafer, who showed that an extract of the adrenal glands, later discovered to be epinephrine, raised the blood pressure after injection. Several years later, the British physiologist Sir William Maddock Bayliss and Ernest Henry Starling discovered that an intestinal extract, called secretin, caused the flow of pancreatic juice upon injection. They proposed the term hormones for secretions that could act upon other organs through the bloodstream. Later work on the hormones has yielded important information on the mechanics of growth and reproduction.

Among the most important advances of the 20th century are the discovery of new hormones; recognition of the role of vitamins; discovery of blood types; development of the electrocardiograph and electroencephalograph, to record the activity of the heart and brain; discovery of the cause and cure of pernicious anemia by George Richards Minot, William Parry Murphy, and George Hoyt Whipple, all American physicians; and greater understanding of metabolism, the role of enzymes, and the immune system.

The first part of the 20th century also witnessed great advances in the understanding of reflexes, first elaborated by the French philosopher René Descartes as a philosophic concept to distinguish involuntary reflexes of animals from the more rational reactions of humans. The concept was refined by the work of German zoologists, who described it in physical terms and divided behavior into its component reflexes. Further understanding was facilitated by the British neurophysiologist Sir Charles Sherrington, who showed that reflexes enable the nervous system to function as a unit. The concept of a conditioned response, first described in the 18th century by the Scottish physiologist Robert Whytt, a pioneer in the study of reflex action, culminated in the later work of the Russian physiologist Ivan Pavlov and that of the Russian neuropathologist Vladimir Bekhterev. Although there was no validity to Pavlov's attempt to extend the principles of conditioning-the method by which responses may be elicited more frequently or more predictably by reinforcement-to complex mental processes, his work had great impact on psychology and learning. It was one of the primary influences in the founding of behaviorism by the American psychologist John B. Watson; the work of the American psychologist B. F. Skinner in programmed instruction, the basis of so-called teaching machines, was also based on the theory of conditioning and reinforcement.

The 20th century has also witnessed other fundamental advances in neurology. The British physiologist Lord Edgar Douglas Adrian measured and recorded electric potentials from sense organs and motor nerve fibers. Sherrington investigated the integrative action of the nervous system. Their work was followed by that of the American physiologists Joseph Erlanger and Herbert Spencer Gasser, who demonstrated functional differences in nerve fibers and used the oscilloscope to record the variation of electrical impulses that occurs in these fibers. Later investigations by the American biochemist Julius Axelrod, the Swedish physiologist Ulf von Euler, and the British physician Sir Bernard Katz demonstrated the role of specific chemicals in the transmission of nerve impulses. The importance of these investigations to such basic processes as the control of blood pressure and the mobilization of strength to meet an emergency is shown by the fact that all of the seven investigators mentioned received Nobel Prizes in physiology or medicine.