Born Sept. 14 (26), 1849, in Riazan’; died Feb. 27, 1936, in Leningrad. Soviet physiologist. Author of the materialist theory of higher nervous activity and modern ideas on digestion; founder of the major Soviet school of physiology; innovator of methods in experimental physiology that were based on original surgical techniques and that permitted long-term experiments on intact and apparently healthy animals, that is, under conditions as close to nature as possible. Academician of the Academy of Sciences of the USSR (1907; corresponding member, 1901).

After graduating from a theological school in Riazan’ in 1864, Pavlov entered the Riazan’ Theological Seminary. In the next years he came into contact with the ideas of the Russian revolutionary democrats A. I. Herzen, N. G. Chernyshevskii, and N. A. Dobroliubov. During this time he was also influenced by the books of D. I. Pisarev and I. M. Sechenov, especially the latter’s Reflexes of the Brain, which was published in 1863. Pavlov was admitted to the law faculty at the University of St. Petersburg in 1870 but soon switched to the natural sciences division of the physics and mathematics faculty, where he studied animal physiology under I. F. Tsion and F. V. Ovsiannikov. Upon graduating in 1875, Pavlov was admitted as a third-year student to the Medical and Surgical Academy (the present-day Military Medical Academy). From 1876 to 1878, he also worked in the physiology laboratory of K. N. Ustimovich. After graduating in 1879, he remained as head of the physiology laboratory in S. P. Botkin’s clinic. Pavlov defended his doctoral dissertation, Efferent Nerves of the Heart, in 1883, and from 1884 to 1886 he traveled abroad on a fellowship to R. Heidenhain’s laboratory in Breslau (present-day Wroclaw) and K. Ludwig’s laboratory in Leipzig.

In 1890, Pavlov was appointed professor and head of the subdepartment of pharmacology at the Military Medical Academy, and in 1896 head of the department of physiology, which he directed until 1924. Beginning in 1890, he also headed the laboratory of physiology at the newly founded Institute for Experimental Medicine. From 1925 until his death Pavlov directed the Institute of Physiology at the Academy of Sciences of the USSR. In 1904 he won a Nobel Prize for his long-term studies on the mechanisms of digestion. In these experiments, which established the scientific basis for the study of digestive physiology, he used the artificially produced fistulas for which he is so well known, a technique that completely transformed this area of physiology.

Pavlov applied his scientific creativity to revolutionizing the prevailing views on blood circulation and especially digestion. His theory of conditioned reflexes was developed into a systematic, materialist approach to studying higher brain functions in animals and man. After completing his study of pathways of nerves that accelerate heartbeat, Pavlov studied the regulation of pancreatic digestion and suggested that the pancreas is doubly innervated, that is, innervated by two types of fiber. From 1876 to 1878 he conducted research that uncovered the homeostatic mechanism of blood-pressure regulation, which is predicated on the antagonistic pressor-depressor relationship between visceral and cutaneous blood vessels.

While working in the laboratory of Botkin’s clinic, Pavlov made a major discovery that later became the basis of his doctoral dissertation: cardiac activity is regulated by four efferent nerves, each of which either accelerates, decelerates, strengthens, or weakens the heartbeat. After this discovery, he focused on the neural regulation of digestion, thereby continuing his earlier research on the secretory nerves of the pancreas. At this time, he also investigated innervation of the gastric glands, conducting experiments that employed sham feeding as well as esopha-gotomy and vagotomy, or transection of the esophagus and vagus nerve. Pavlov’s brilliant successes in this field were due both to his formulation of the principles of aseptic experimental surgery and to his creative understanding of Sechenov’s and Botkin’s theory of nervism, which explains the nervous system’s role in the physiology and pathophysiology of the regulation of bodily functions.

Experiments with a surgically isolated section of the stomach, called a Pavlov pouch, enabled Pavlov to discover the two phases of gastric secretion. These are the cephalic phase, which is completely regulated by reflexes that bring about the secretion of gastric juice before any food has been ingested, and the gastric stage, whose regulation is humoral. Pavlov studied the liver by producing permanent gallbladder fistulas, and the intestine by isolating a small loop from the rest of the intestine and maintaining normal innervation within that loop. The results of these researches were discussed in his Lectures on the Work of the Primary Digestive Glands, which appeared in 1897.

Pavlov’s decision to take up the study of higher nervous activity proceeded both from the general trend of his research and from his ideas on the adaptive nature of the activity of the digestive glands. A conditioned reflex, according to Pavlov, is the highest and evolutionarily newest mechanism by which an organism adapts to its environment. Whereas an unconditioned reflex is a comparatively stable, innate reaction that is shared by all members of a given species, a conditioned reflex must be acquired as a result of the organism’s individual experiences. One of Pavlov’s greatest contributions is reflected in his approach to the study of higher nervous activity: he consciously and systematically addressed the question of mental phenomena from the standpoint of a “pure” physiologist, that is, an adherent of materialism for whom the body and mind are not distinct entities. In 1903, Pavlov brilliantly defended his position in a lecture entitled “Experimental Psychology and Psychopa-thology in Animals,” which he subsequently elaborated in 1910 in his article “Natural Science and the Brain.”

The range and volume of Pavlov’s scientific pursuits and discoveries were enormous. He studied the neural mechanism behind the temporary association that forms between an unconditioned reflex and any external or internal stimulus. He revealed patterns by which conditioned reflexes arise and are extinguished. He localized the phenomenon of inhibition—the opposite of excitation—in the cerebral cortex and designated two types of inhibition, external and internal. He discovered patterns by which the primary neural processes of inhibition and excitation spread, or irradiate, through the nervous system or become concentrated in a smaller group of nerves. Pavlov studied sleep from the point of view of a proposed mosaic arrangement of excited and inhibited points in the cerebral cortex. His confirmation of the cerebral phases of sleep contributed to an understanding of dreams and hypnosis. He also investigated pathological disturbances of sleep and the protective function of inhibition and proposed a theory that accounts for experimental neurosis in terms of a “clash” between excitation and inhibition.

Pavlov developed the doctrine of types of nervous systems, according to which nervous systems fall into distinct categories, depending on the balance, lability, and strength of inhibition and excitation. Thus, a system of four basic personality types was proposed, corresponding to the four temperaments that were recognized by the Greeks—choleric, phlegmatic, sanguine, and melancholic. In Pavlov’s system, a personality can be strong, balanced, and excitable; strong, balanced, and unexcitable; strong, balanced, and labile; or weak. Thus, the empirical observations of many physicians on temperaments, beginning with Hippocrates, were at last placed on the firm basis of experiment.

Pavlov’s theories on analyzers, localization of functions in the cerebral cortex, and functional systems of the cerebral hemispheres have since been experimentally substantiated and interpreted. Pavlov’s last research was devoted to signal systems. He demonstrated that humans and animals possess a first-signal system, but that humans also possess a second-signal system, which is the sum of audible, pronounceable, and written verbal signals that enable humans to write and speak. According to Pavlov, the existence of two extreme types of mental activity in man—creative and reflective—can be accounted for by the dominance of one signal system over the other.

Pavlov’s scientific legacy is largely responsible for the nature of modern physiology and several allied branches of biology and medicine. It has also left a mark on psychology and pedagogy. His research has been especially important to the development of medicine, for he believed that physiology and medicine are ultimately inseparable. His ideas gave rise to major schools of thought with respect to therapy, surgery, psychiatry, and neuropathology. All of Pavlov’s work was permeated with a warm love for his native land. “Whatever I do,” he wrote, “I am constantly aware that above all I am serving my country and Russian science to the best of my ability. This is both a powerful motivation and a source of deep satisfaction” (Poln. sobr. soch, 2nd ed., vol. 1, 1951, p. 12). In a letter to young people, Pavlov wrote about a scientist’s high responsibility to the motherland (ibid., pp. 22–23).

Lenin greatly admired and valued Pavlov, as is clearly shown by a decree of the Council of People’s Commissars that he signed on Jan. 24, 1921. The decree cited the “exceptional scientific services of Academician I. P. Pavlov, which are of great importance to workers throughout the world.” In 1935 the 15th International Congress of Physiologists, of which Pavlov was president, acknowledged the great scientist as the world’s senior physiologist, not merely in age but also in prestige.

Pavlov was a member or an honorary member of many foreign academies, universities, and societies. Many institutions are named after him, for example, the Institute of Physiology of the Academy of Sciences of the USSR, the First Leningrad Medical Institute, and the Riazan’ Medical Institute. In 1934 the Academy of Sciences of the USSR established the Pavlov Prize for the best scientific work in physiology, and in 1949 the Pavlov Gold Medal for research that elaborates on Pavlov’s teachings.