Physiology

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.

see Anatomy; Medicine and other branches of science mentioned. See also separate articles for those scientists whose names are not followed by dates.