PHYSIOLOGY is the science which treats of the functional working of the body in health, as opposed on the one hand to anatomy which treats of the structure of the body and pathology which treats of its function as distorted by disease. In all forms of animal life except the lowest, the body is made up of organs. With regard to the functions of all the more important organs special articles will be found under their respective names (see HEART; INTESTINE; LIVER; LUNG; etc.).
The activities of these organs are co-ordinated by the nervous system (see BRAIN ; NERVE; SPINAL CORD; SYMPATHETIC SYS TEM) and the hormones (see HORMONES). Nevertheless the body is in a sense more than a number of co-ordinated units. The body has an architecture which has been evolved along well defined lines and which it is the business of the present article to trace as far as may be possible.
The phrase "well defined lines" rather than "well defined prin ciples" has been used advisedly; for it may often be a matter of speculation, whether, even when a frequent repetition of the same sort of architectural conceit occurs, any principle is really involved; or whether merely the same sort of accident has taken place in the same sort of way. To the most reflective type of mind physiology presents no more fascinating problem, than that of the extent to which the structure of the body is the result of some well ordered plan either of "evolution" or "design." In the strict sense of the word "principle" would imply such a plan, but in what follows the word will be used more loosely to include phe nomena that recur with sufficient frequency to simulate an orderly sequence, even though the recurrence may in reality be little more than accidental.
such as the amoeba, exists at a temperature indistinguishable from that of the water in which it lives. Every accident which affects the temperature of the water affects that of the amoeba; the only defence which the creature can put up against a too rigorous en vironment, whether of heat or cold, is to flee if it can to more genial surroundings. Within those limits all the processes of its body, of digestion, of respiration, of movement, must take place at the temperature of the water in which the unicellular organism finds itself. Compare the condition of affairs in man, the most highly organized creature; the temperature of the body is within very narrow limits (I) constant from one time to another; (2) the same over the whole body; and (3) maintained at a level quite different from—and independent of—that of the surround ing medium.
In the development of man, this constancy has been brought about by stages. The root of the matter lies in the fact that the organism itself produces heat; the amoeba is so small and its cooling surface relatively so great, that the heat produced by the organism itself is a negligible factor in determining its tempera ture; but if enough amoebae were packed together the heat which they produced would have less opportunity of escape and so the centre of the mass would attain a higher temperature just as the heat produced in a hay rick raises the temperature of the interior of the rick. Therefore a high body temperature argues a certain considerable size as one of the properties of the body. That the body temperature should be uniform over the whole body is ac complished by the circulation of the blood. The maintenance of the constancy of environment inside the body by the device of circulating the same fluid throughout all its interstices appears very early in the development of animal life and maintains the constancy in many other respects besides that of temperature.