FISIOLOGIA. — From the Greek φύσις = nature and λογέω = I investigate, physiology is the science of the functioning of the living organism. Eminently experimental in nature, it has undergone considerable development only in recent times.
The origin of physiology in history must be sought at the point when, from the simple description of living organisms, human beings passed to attempting to explain the significance of the structures observed. Galen (born at Pergamum in A.D. 129) certainly addressed himself to 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 circulatory system. The definitive explanation of the mechanism of circulation, however, came much later through the contributions of numerous observers (R. Colombo, 1559; F. D'Acquaperedente, 1574; W. Harvey, 1628; M. Malpighi, 1661; L. Spallanzani, 1773), and was one of the first achievements of modern physiology (v. CIRCOLAZIONE).
Through 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 achievements of human surgery and greatly facilitated by the discovery of narcotics and anaesthetics, has made possible an infinite number of experiments capable of enhancing or diminishing the activities of organs, so that their function might thus be studied. The possibility, moreover, of keeping organs and tissues isolated from the organism alive has made it possible to study functional performance directly, under conditions that can be precisely controlled at will and without interference from other organs or tissues. With an increasingly refined and exact recording technique, it has been possible to follow through time and space not only the gross mechanical performances of organs, but also movements imperceptible to the naked eye and performances of another nature (electrical, thermal, chemical), enormously expanding our capacity to analyse physiological phenomena. With the valuable assistance of chemical technology, the complicated chemical mechanism by which chemical energy is transformed into other forms of energy (mechanical, electrical, thermal) in living organisms has also been brought to light, as has the validity of the laws of thermodynamics in these transformations.
From a scholastic point of view, a physiology of the organs of vegetative life and a physiology of the organs of relational life may be distinguished.
The first comprises the physiology of the circulatory system, the respiratory system, the digestive system, the urogenital system, and the glands of internal secretion. The second essentially comprises the physiology of the central and peripheral nervous system and of the organs of sense (sensibility), as well as of the muscular and skeletal system (motility).
A distinction corresponding instead to practical needs and based on the different methods employed is the following: physiological chemistry is the physiological science that makes use of chemical methods and therefore tends toward the solution of questions that can be resolved only by chemical methods, such as the problem of nutrition, the chemistry of respiration, intermediate chemical transformations, total energy expenditure, etc.; physiological chemistry has subsequently given rise to further specialized branches such as enzymology, vitaminology, etc. The expression physiology properly so called, on the other hand, refers to that part of physiology based on the classical techniques of physiology: vivisection and the recording of mechanical, thermal, and electrical performances; specialized branches have also developed within physiology, such as electrophysiology, endocrinology, etc.
The present state of our knowledge and the questions that trouble modern physiology reflect the present state of experimental technique and its possibilities for development. Every advance in technique has marked an advance in physiological knowledge, channeling research toward those problems whose solution had been made possible by the new acquisitions.
In the field of physiological chemistry, it has been precisely demonstrated that the material and energetic transformations in living organisms (metabolism) obey the law of the conservation of matter and energy; it has been demonstrated that organisms function as chemical machines and not as heat machines, obtaining 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 adult living organism may be regarded as a stationary system with periodic and aperiodic accelerations of its metabolism.
The applicability of the physical and chemical laws to living organisms has made the question of defining living beings in relation to non-living beings a pressing one. In this regard, it must be said that it is incontestable—and accepted by most—that a teleological ordering must be present in the formation and functioning of living beings; this is not in contradiction with the validity, in living beings as well, of the laws of the inanimate world. The study of the chemical mechanism of energetic transformations has brought to light the fact that chemical energy is released little by little through an infinite number of intermediate reactions, whose velocity is regulated by innumerable catalysts (ferments); in particular, the chemistry of muscular function is well known, although to this day it is not known how the transformation of chemical energy into mechanical energy occurs; this is a problem currently subjected to intensive research.
The need for food, initially understood simply as a need for energy, is now understood instead not only as the meeting of energy requirements but also as the replenishment of particular substances (nitrogenous substances, fats, vitamins) that the organism necessarily consumes and is unable to synthesize; this chapter too remains open.
In the field of physiology properly so called, the mechanical, electrical, thermal, and other phenomena of the various organs and tissues have been thoroughly examined, as have the correlations among them, through the nervous pathways or through the circulation, by means of substances elaborated by special glands (glands of internal secretion). Nervous correlation, that is, correlation through separate nervous pathways between one organ and another, makes possible a very fine regulation in intensity, time, and space, which is not possible with correlation mediated by the products of the glands of internal secretion (humoral correlation). It remains a matter of debate, at least in certain fields, whether the transmission of the nervous impulse along the nervous pathways is electrical in nature (diffusion of potential) or chemical in nature (diffusion of a substance).
Sensitivity to environmental stimuli is ensured by organs specialized in receiving and analysing these stimuli: the cutaneous nerve endings (tactile, thermal, and pain-sensitive) and the sense organs (sight, hearing, taste, and smell); the excitation produced in these organs is transmitted through nervous pathways (sensory pathways) to the nervous centres (nevraxes), where the motor or secretory response is elaborated and then transmitted to the effectors—that is, to the organs that must carry it out (muscles, glands)—through other nervous pathways (motor pathways). The entire mechanism which, as is apparent, governs all our relations with the external world is called a reflex.
As regards the relations between the energy of environmental stimuli and the intensity of sensory excitation, the stage of empirical laws (Weber and Fechner’s law) has now been surpassed, and an attempt is being made to frame these relations in terms of quantum-statistical concepts.
I fenomeni della coscienza e della conoscenza restano completamente sottratti alla possibilità di una spiegazione fisiologica, anche se presuppongono, per il loro estrinsecarsi, la presenza di centri nervosi particolarmente differenziati (certeccia cerebrale umana).
BIBLI, R. Hoeber, Phytophthische Chemie der Zelle und der Gewebe, 6ª ed., Lipsia 1926; A. P. Matthews, Physiological Chemistry, 5ª ed., Londra 1930; H. Rein, Physiologie des Menschen, 5ª ed., Berlino 1940; C. H. Best-N, B. Taylor, The Physiological basis of Medical Practice, 3ª ed., Baltimore 1943; M. Flockan, Biochimie humaine, Parigi e Lieut 1944; C. Lovatt Evans, Principles of Human Physiology, 9ª ed., Londra 1945; W. R. Fearon, An Introduction to Biochemistry, 3ª ed., ivi 1946; L. V. Heilbrunn, An Outline of General Physiology, 2ª ed., Filadelfia e Londra 1947; R. Margaria, Principi di fizico-chimica e chimica-fisiologica, 6ª ed., Milano 1947; R. Margaria-L. De Caro, Principi di F. umana, ivi 1948.