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 tissues.

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 the 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 body.

later on when we consider experiments on an individual exposed to high temperatures, that this coefficient may become very small.

We shall now attempt to write our physiological equation in the language of mathematics and afterwards to translate it. We have seen that for a warm-blooded animal the sum of the heats expended must equal the sum of the heats produced, for that is the condition of life. We shall endeavor first to tabulate the sources of heat in the body and measure these amounts as far as is possible. In its fullest expression such a problem must be enormously complex, but in general terms we may arrive at an approximate solution.

Ultimately all the heat produced in the body is derived from the potential energy of the food ingested. We can easily measure the total energy of a certain amount of food, but this energy is used by the body in such varying proportions and at such varying rates that little benefit would accrue to our present problem from such an investigation.

An important source of heat in the body is due to the friction of the blood as it circulates in its vessels. All of this resistance, which is overcome by the heart, is transformed directly into heat. We may calculate the amount approximately. If we suppose that 180 ccs. of blood are expelled from the left ventricle at each stroke, under a pressure of one-third of an atmosphere, this would correspond to .6192 kilogramme-metres at each stroke, and at 72 strokes a minute, this would give 44.3124 kilogramme-metres per minute. If we suppose that the right heart does one-quarter the work of the left, or about 10 kilogramme-metres per minute, we have for the total work per minute 54.312 kilogramme-metres, which corresponds to 128 calories per minute.

This is perhaps a rather high estimate for ordinary conditions, but where, as we shall see later on, the heart is forced to pump a much larger quantity of blood in order to maintain the normal temperature, this estimate is probably much exceeded at times. Since this friction takes place largely in the most constricted portions of the circulation, it would be natural to expect that the blood which had been driven through the capillary system of a gland would issue much warmer than it entered and such we find to be the case. Thus the blood of the hepatic vein has been observed to be 40.73 while that in the right heart was 37.7. In the muscles no contraction can take place without an increased flow of blood through them with a simultaneous constriction of the capillaries, which

would naturally give rise to a considerable production of heat-a fact constantly observed.

In the salivary glands during active secretion the saliva may be 1° to 1.5° higher than the blood in the carotid artery. In most text books of physiology this production of heat is explained as due solely to glandular activity (whatever that is), but in view of the preceding discussion we see that a large proportion of it, if not all, must be due to the friction of the blood in the gland capillaries. If a gland during its action performs an anabolism, that is, elaborates a product of a higher potential than the material worked upon, there must be an absorption of heat equal to the potential gained.

On the other hand where a gland secretes katabolically there must be a generation of heat equal to the drop in potential. Considering the muscles as force glands, which in fact they are, there is here undoubtedly katabolism which may be wholly transformed into work, and which would be the case if a muscle were an engine of perfect efficiency. Anabolism also takes place in a muscle, in the heart probably during the return stroke, which would be accompanied by an absorption of heat.

Determinations of the efficiency of muscles, with proper regard for the heat derived from the friction of the blood, have not yet been made. It is probable that a much higher efficiency obtains than is supposed, especially in certain automatic muscles, such as the heart and respiratory muscles which work continually at a constant rhythm.

The heat due to mental activity will not be considered, as it is extremely doubtful if such exists.

To recapitulate, then, we may tabulate as constant and varying sources of heat in the body:

Ist. The katabolism of the food.

2d. The friction due to the circulation, which, though varying, may be considered to average about 180 kilogramme-degrees in the 24 hours.

3d. The heat absorbed through the surface when the surrounding medium is of a higher temperature than the body.

4th. The heat due to ingesta when these are of a higher temperature than the body. This includes the inspired air.

5th. Heat due to compression of air in the lungs.

6th. Whenever in external or internal contact with the body, any substance passes from a gaseous to a liquid state (condensation), or from a liquid to solid state (solidification).

It will be noticed that glandular activity is not included in the above list, since the heat generated in a gland is probably either frictional or katabolic.

Next, inquiring into the various means by which heat is lost to the body, we have:

Ist. The loss through the surfaces when the external temperature is lower than that of the body.

2d. Whenever, in external or internal contact with the body, any substance passes from a solid to a liquid, or from a liquid to a gaseous state. Thus solution of salt or sugar in any liquid is accompanied by a definite absorption of heat. When food is dissolved by the digestive liquids, heat is absorbed and the resulting temperature is that due to the liquefaction plus, of course, the potential energy lost when katobolism takes place. The evaporation of water, whether on the surface or in the lungs, is of course attended with an absorption of heat which is equal to the latent heat of vaporization at the temperature of the body. The amount of heat which may be lost by evaporation of water on the skin varies within very wide limits. It depends on the amount of perspiration secreted, or the temperature and relative humidity of the atmosphere and the velocity of the currents of air to which the body is exposed. When saturation of the atmosphere exists and the external temperature is less than that of the body, evaporation can still take place at the surface, but when the temperature of the surrounding medium is equal or greater than that of the body, no evaporation can take place, and consequently no heat can be lost to the body by this means.

3d. Heat may be lost by the warming of ingesta, and this applies to the inspired air. This is only possible when the ingesta are colder than the body, since as we have already seen in the reverse case heat will be gained. It is stated in most physiologies that heat is lost to the body through the expulsion of the urine and faeces, but a little consideration will show that the temperature cannot be affected by this means, while if the substances from which these products are derived were originally ingested warmer than the body there must be a net gain of heat. Accordingly the urine and faeces have no place in our problem.

4th. Heat may be lost to the body by the expansion of air in the lungs during the process of breathing. Let us consider carefully the changes taking place during the respiratory cycle. For the average individual the capacity of the chest at the beginning of in