highly specialized organs that arose in the course of evolution to enable the organism to obtain information about changes in the environment. Sensitivity to light, temperature, chemical agents, and other stimuli is evident even in protozoans. However, the reaction to external influences by lower organisms is usually caused not by special organs but by a general property of living matter—irritability (or excitability). In higher animals, adaptation to the environment, the search for food, reproduction, and escape from enemies are complex activities, effective only if information about the environment is fairly complete and timely. Such information is transmitted by sense organs adapted to perceive signals of a certain kind.

The traditional idea of five specialized sense organs—eye, ear, nose, tongue, and skin—that make possible vision, hearing, smell, taste, and touch was considerablely expanded and deepened with the development of physiology. The vestibular apparatus, the receptor systems of the motor apparatus, and the numerous visceral receptors were investigated in both animals and man, as were the electroreceptors in fish. It was found that the perception of touch, pain, pressure, heat, and cold, combined in the sense of touch, is effected by various receptor structures of the skin. However, light can be perceived by, for example, such different organs as the human eye and the compound eye of insects. The variety of receptor elements of the sense organs led to the idea of the main types of reception, or sensibility—mechanoreception (for example, touch, phonoreception, or perception of sound, and vestibular reception, or perception of the spatial position of the body), chemoreception (taste, smell), and photoreception (light)—and the corresponding perceiving apparatus— the receptors (seeRECEPTOR). In various groups of animals, differing in evolutionary development and ecology, the perception and analysis of environmental signals can be accomplished by structures of different degrees of complexity, while the development and principal use of a particular type of sensibility also depend on the animals’ mode of life and habitat. In man, more than 80 percent of the information about the external world is received through the eyes.

In modern physiology, the sense organs, in the broad meaning of the term, are taken to mean the complex sensory systems (analyzers in I. P. Pavlov’s terminology), including perceiving elements (receptors), conducting nerve pathways, and the corresponding regions in the brain where a signal is converted into sensation (seeSENSATION and ANALYZERS). In the narrow meaning, sense organs are only receptor elements and auxiliary structures, such as the eye and ear, that perceive signals and convert them into nerve impulses.

The development of ideas about the activity of the sense organs and their function of obtaining information about the external world has a long history. The ancient Greek and Roman philosophers did not doubt the reality of external objects and phenomena and the adequacy of its perception by the sense organs. Empedocles was one of the first ancient thinkers to attempt to understand the nature of light and color perception. The scientific study of the sense organs was stimulated by the work of Galileo and R. Descartes, who insisted that the study of natural phenomena be strictly limited and confined to the formulation of questions to which concrete answers could be obtained by experimentation or mathematical calculation. Following these principles, J. Kepler regarded the eye as an optical instrument, and on the basis of the laws of geometrical optics, showed that external objects produce a reverse and reduced image on the retina. In doing so, he deliberately avoided the question of why the world is not perceived upside down. Kepler’s work laid the foundation of physiological optics and paved the way for the development of physiological acoustics and the physiology of other sense organs.

The foundations of modern experimental physiology of the sense organs were laid in the 19th century by the classical studies of H. von Helmholtz, G. T. Fechner, I. M. Sechenov, and other scientists. Pavlov’s conditioned reflex method was of enormous importance for objective research on sensory activity. The sense organs have been successfully studied since the 1920’s by the electrophysiological method, which makes it possible to record electrical phenomena arising in different parts of the sensory systems under the influence of external stimuli. Research on the physicochemical and biochemical bases of visual reception was begun in the late 1930’s, and research on the physicochemical and biochemical bases of olfactory and gustatory reception, in the late 1960’s. However, despite the advances in 20th-century physiology, which have made use of the results of biophysics, biochemistry, cytology, psychology, and other sciences, many problems related to the activity of the sense organs remain unresolved. For example, such major processes as the transformation of the energy of external stimuli into receptor signals in the receptor cells, the coding and decoding in various sensory systems of information contained in the spatial and temporal code of nerve impulses, and the neurophysiological mechanisms of external image recognition have still not been conclusively studied. Lenin’s words are still timely: “there still remains to be investigated and reinvestigated how matter, apparently entirely devoid of sensation, is related to matter which, though composed of the same atoms (or electrons), is yet endowed with a well-defined faculty of sensation” (Poln. sobr. soch., 5th ed., vol. 18, p. 40).

The human sense organs were studied in great detail in the late 1930’s and early 1940’s. Thresholds of stimulation—both the absolute threshold (limits of sensibility) and the differential threshold (the capacity of sense organs to recognize the minimal difference between two stimuli)—were established for all the sense organs. Research of the 1970’s was directed at elucidating the mechanisms controlling the sense organs, thereby making it possible to study the molecular, membranal, and cellular mechanisms of visual reception, the delicate mechanisms of smell and taste, mechanoreception, and electroreception.

Changes in the environment are perceived by the sense organs in the form of light, mechanical (including acoustic), or chemical stimuli. The “signal” interacts with the cellular membrane of a receptor or specialized receptor protein molecule, triggering a chain of ionic, enzymic, and electrical processes. This gives rise to an electrochemical signal or nerve impulse, identical for all types of receptors, that is transmitted to the brain by way of the conducting pathways. A series of such impulses constitutes a sort of code that is decoded in the corresponding nuclei (visual, auditory, and so on) of the cerebral cortex and transformed therein into an image of the external world. Some of the principles and mechanisms of information processing by the sensory systems have been largely elucidated. Sensory analysis at all levels—from the receptors to the cerebral cortex—is a comparative process directed at identifying the characteristics of the signal. The principal neurophysiological mechanism of such comparison is the correlation of the excitatory and inhibitory processes in the nerve networks or at the entry of individual neurons. Specifically, what is involved is the mechanism of lateral inhibition, when the physiological condition of every nerve cell depends on the activity of the adjacent cells. Such inhibition helps strengthen contrasts or contours and localize the site of contact, that is, eliminate redundant information and distinguish the most important information.

The mechanisms of object recognition are, for the most part, still not completely known. However, some neurophysiological data can probably be regarded as providing the first steps toward such knowledge, namely, the discovery of the specific neurons—detectors—capable of reacting selectively to certain biologically important characteristics of objects, for example, only a moving dark spot or only a sound of a certain pitch. Such neurons were first discovered in the visual and then in the other sensory systems. The properties of the detectors become increasingly complex as sensory information is analyzed and transmitted from the receptors to the cortical centers. Specialization of the detectors increases even more from layer to layer in the cortex itself. Thus, visual images, sound patterns, or blends of odors are broken down in the sensory systems into simple constituents and are analyzed separately. The final stage in the analysis of sensory information is its synthesis—the formation of a subjective integral image of the objective external world. Further research in this direction will elucidate the complex mechanisms governing the functioning of the sense organs in the cognitive process.

The elucidation of the mechanisms of activity of the sense organs is not only of great scientific and philosophical interest but is of importance for various practical purposes in medicine, technology, and psychology.