PHYSIOLOGY. – From Gr. φύσις = nature and λογέω = I investigate, physiology is the science of the functioning of the living organism. Of a markedly experimental nature, it has only recently attained great development.
The origin of physiology in history must be sought at the point when man moved from the simple description of living organisms to the attempt to explain the significance of the structures observed. Galen (b. at Pergamon in AD 129) certainly addressed these problems; he initiated vivisection for experimental purposes, and to him are owed the first observations on the function of the nervous system and the cerebro-spinal system. The definitive explanation of the mechanism of circulation, however, came much later through the contributions of numerous observers (R. Colombo, 1559; F. D'Acquapendente, 1574; W. Harvey, 1628; M. Malpighi, 1661; L. Spallanzani, 1773) and was one of the first achievements of modern physiology (v. CIRCULATION).
With the use of ever-new experimental methods, this science has developed enormously, occupying one of the foremost places among the biological sciences.
Vivisection, often preceding the most important advances in human surgery, greatly facilitated by the discovery of narcotics and anaesthetics, has made possible the execution of countless experiments designed to enhance or diminish the activities of organs so that their function might be studied. The ability to keep organs and tissues alive when isolated from the organism has allowed the direct study of functional performance under conditions that can be precisely controlled and without interference from other organs or tissues. With increasingly refined and exact recording techniques, it has been possible not only to follow over time and space the gross mechanical performances of organs, but also movements imperceptible to the naked eye, as well as performances of another nature (electrical, thermal, chemical), thereby greatly enlarging our capacity for analysing physiological phenomena. With the valuable aid of chemical techniques, moreover, the complex chemical mechanism of the transformation of chemical energy into other forms of energy (mechanical, electrical, thermal) in living organisms has been elucidated, as has the validity of the laws of thermodynamics in these transformations.
From a scholarly standpoint, a distinction can be made between physiology of the organs of vegetative life and physiology of the organs of relation.
The former comprises the physiology of the circulatory system, the respiratory system, the digestive system, the urogenital system, and the glands of internal secretion. The latter essentially comprises the physiology of the central and peripheral nervous system, the organs of sense (sensitivity), and the muscular and skeletal systems (mobility).
A distinction based instead on practical needs and on the different methodologies employed is as follows: physiological chemistry is that branch of physiology that employs chemical methods and thus seeks to resolve questions that can only be answered by chemical means, such as the problem of nutrition, the chemistry of respiration, intermediate chemical transformations, overall energy expenditure, etc.; physiological chemistry has in turn given rise to further specialized branches such as enzymology and vitaminology, etc. By physiology *sensu stricto* is meant, instead, that part of physiology based on the classical techniques of physiology—vivisection, recording of mechanical, thermal, and electrical performances; even in this field specialized branches have developed, such as electrophysiology and endocrinology, etc.
The present state of our knowledge and the questions that preoccupy modern physiology reflect the current state of experimental technique and its potential for development. Every advance in technique has marked a corresponding advance in physiological knowledge, directing research towards problems whose solution has become possible through new acquisitions.
In the field of physiological chemistry it has been conclusively demonstrated that material and energy transformations in living organisms (metabolism) obey the law of the conservation of matter and energy; it has been shown that organisms function as chemical machines rather than as heat engines and obtain the necessary energy by oxidizing the carbon and hydrogen of foodstuffs into carbon dioxide and water.
The efficiency of the transformation of chemical energy into mechanical energy generally does not exceed 25–30%.
The living organism can be regarded as a steady-state system with periodic and aperiodic accelerations of its metabolism.
The applicability of physical and chemical laws to living organisms has revived the question of defining living beings in relation to non-living ones. On this point it must be said to be incontestable (and acknowledged by almost all) that a teleological ordering must govern the formation and functioning of living beings; this is not in contradiction with the validity, even in living beings, of the laws of the inanimate world. The study of the chemical mechanism of energy transformations has revealed that chemical energy is gradually released through an infinite number of intermediate reactions, the speed of which is regulated by countless catalysts (ferments); in particular, the chemistry of muscular function is well understood, although it remains unknown how the transformation of chemical energy into mechanical energy occurs—a problem currently the subject of assiduous research.
The need for food, initially understood merely as a need for energy, is now understood not only as covering energy requirements but also as the replenishment of particular substances (nitrogenous substances, unsaturated fats, vitamins) that the organism necessarily consumes and is incapable of synthesizing; this chapter too remains open.
In the field of physiology *sensu stricto*, the mechanical, electrical, thermal, and other phenomena of the various organs and tissues, and their correlations through nervous pathways or through the circulation by means of substances elaborated by special glands (glands of internal secretion), have been thoroughly examined. Nervous correlation, i.e., through separate nervous pathways between organ and organ, makes possible a very fine regulation in intensity, time, and space that is not possible through correlation by means of the products of glands of internal secretion (humoral correlation). It is still a matter of debate, at least in certain areas, whether the transmission of the nervous impulse along nervous pathways is of an electrical nature (spread of potential) or of a chemical nature (spread of a substance).
Sensitivity to environmental stimuli is ensured by specialized organs for receiving and analysing these stimuli: cutaneous nerve endings (tactile, thermal, painful) and the sense organs (sight, hearing, taste, smell); the excitation that arises in these organs is transmitted via nervous pathways (sensory pathways) to nervous centres (neural axis) where the motor or secretory response is elaborated and then transmitted to the effectors, i.e., the organs that will carry it out (muscles, glands) via other nervous pathways (motor pathways). The entire
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Mechanism that, as is evident, governs all our relations with the external world, is called a reflex.
As regards the relationship between the energy of environmental stimuli and the intensity of sensory excitation, the stage of empirical laws (Weber’s and Fechner’s law) has now been surpassed, and an attempt is now being made to frame these relationships in terms of quantum-statistical concepts.
The phenomena of consciousness and knowledge remain completely beyond the possibility of a physiological explanation, even though they presuppose, for their manifestation, the presence of particularly differentiated nerve centers (the human cerebral cortex).