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A. N. BELI, A.M., M.D., Editor.
, Associate Editors.
BROOKLYN, N. Y.: A. N. BELL.
Entered according to Act of Congress, A. D, 1897, by A. N. BELL, in the office of the
Librarian of Congress, at Washington.
ASTOR, LENOX AND TILDEN FOUNDATIONS.
THERMICS AND THERMO-DYNAMICS OF THE BODY.
By F. J. B. CORDEIRO, M. D., P. A. SURGEON, U. S. Navy.
The animal kingdom, in regard to heat, can be divided broadly into two classes-cold-blooded animals and warm-blooded animals. The former conform to the temperature of their suroundings, their vital-chemical reactions taking place with nearly equal facility through a wide range of temperatures. The latter possess a certain definite temperature which they maintain at all times and which may be considered a life function of the species. The difference is entirely one of equlibrium.
All bodies, whether living or dead, must lose or gain heat from the surrounding medium according as their temperatures are higher or lower than that of the medium. In cold blooded animals this loss of heat is greater than the production, and consequently an equilibrium is not reached until the temperature of the body has nearly coincided with that of its envelope. In warmblooded animals, on the other hand, since life can exist only within certain narrow limits of temperature, the expenditure and gain of heat must at all times be nicely balanced. Under different conditions it will be seen that the members of such an equation may vary widely, though the equality must always be maintained. The body, then, can be considered a thermostat set to a certain vital temperature.
In studying the movement of heat in a body it is essential to know the specific heat, the penetrability and permeability to heat, of its various parts. Such constants have unfortunately (so far as the writer knows), not been determined for living bodies or for most animal tissues. The total quantity of heat in an idividual would be the sum of the specific heats of its parts multiplied by their masses, into the absolute temperature, which we may consider as 310. The average specific heat of the body might be de
termined by observing the quantity of heat given out by a dead body in cooling between two fixed temperatures.*
The writer, for his own satisfaction, has made certain observations regarding the thermal constants of animal tissues, but since proper facilities were lacking, they can be considered scarcely more than surmises. In dealing with the problems shortly to be taken up it will be necessary to have some idea in regard to these constants, and the following determinations will serve to fix our ideas. According to certain measurements the specific heat of the blood is somewhat less than that of water, though considerable. Its conductivity may be considered as sensibly equal to that of water. This is as we should expect, since it is composed so largely of water (90 per cent.), and the direct consequence is that, owing to its unceasing circulation the body has a very high permeability (interior conductivity). Whatever heat or cold is received externally or internally is quickly diffused throughout the body and the internal equilibrium is nearly maintained at all times. Not that the temperature of the body is everywhere exactly equal, for such is not the case. The exterior is hotter or colder to a slight extent than the interior, according as the surrounding medium is above or below the vital temperature. Certain parts also, during activity, may be warmer (muscles, glands), and as we shall see later, cooler (lungs), than the neighboring parts, but in general the average temperature is preserved nearly uniform.
We see then that the blood, owing to its high specific heat, conductivity and rapid circulation, is eminently fitted for receiving large quantities of heat (or cold), and distributing it. It thus performs the function of maintaining the internal equilibrium.
According to certain rough determinations the specific heat of the proteids is relatively small, that of muscle being perhaps .08 of water, while that of the bones is less. Of the solid tissues fat stands alone as possessing a very high specific heat, perhaps equal to that of blood, while its conductivity is small. We may thus make an estimate of the thermal capacity of the body as possibly one-third that of water, though in the absence of exact determinations this can be considered as scarcely more than a guess. As fat is mainly situated directly under the skin, it would seem, from the foregoing properties of storing large quantities of heat and parting with it slowly, to be eminently adapted to keeping the sur
* On the plausible supposition that there is no marked difference between the specific heats of living and dead t’ssues.
face warm and preventing rapid losses at the exterior. However, rapid losses may take place at the suface under the following circumstances. The skin, which in itself may be supposed to have a very small penetrability, is extremely vascular, and when a large volume of blood is flowing, we may say, in contact with the surface, it may give out or absorb a considerable amount of heat according as this surface is hotter or colder than the surrounding medium. The coefficient of penetrability of the surface may therefore vary from a very small to a very large quantity. It has been observed* that, on exposure to a low temperature, the naked body loses heat from the surface at first rapidly, but that soon the peripheral circulation ceases almost entirely; the skin becomes blanched and, paradoxical as it may seem, the bodily temperature rises above the normal. In this case it is plausible to suppose that trie considerable reduction of the expenditure of heat caused by the cessation of the surface circulation causes a temporary storing up of heat. It will be seen subsequently that under the circumstances there is an increased production of heat, also due to compression of air in the lungs, so that the increase of temperature is readily acounted for. The writer has also found that by immersing the body in hot water (41°) there was at first a rapid absorption of heat by the body! (200 calories per second), but that shortly the surface became blanched, showing a cessation of the peripheral circulation, and that under similar conditions of temperature, the amount entering the body from the water (due allowance being made at all times for the loss to the air), became too small to be measured.
The exterior of the body is then, composed of a cushion of fat, itself non-vascular and pierced by a few large blood vessels which ramify extensively directly in contact with the surface.—This cushion of fat has a high specific heat and low conductivity. The skin itself has an extremely low conductivity (no direct measurements have been made), but when filled with a rapidly flowing blood current is capable of emitting and absorbing a considerable amount of heat. Such an apparatus is an ideal one for the admission or exclusion of heat according to circumstances. It is extremely desirable that a series of accurate calorimetrical experiments should be undertaken for the determination of the coefficient of penetrability of the skin under different circumstances. We shall see
* Foster, Text Book of Physiology. Shown by the cooling of the water, not by increase of temperature of the bndy.