The chemical changes which give rise to heat cause a certain waste of the tissues, which have again to be renewed by the assimilation of various nutrient materials. Food is thus the fuel of the animal body, and the peculiarity of the combustion is that the tissues assimilate or convert into their own substance the fuel, and then themselves undergo a kind of partial combustion, by means of which they perform their several functions, among others heat production.

As already mentioned, heat is produced most abundantly in those tissues which undergo most active chemical changes, hence the protoplasmic cells of glands, and the contractile substance of muscle, must be looked upon as the chief agents in setting heat free.

The possible heat income depends on the amount of nutrient matter assimilated. As each kind of food has a certain heat value, i. e., the number of heat units its combustion will produce, we ought to be able to estimate the amount of heat produced by ascertaining this value and subtracting the calorific value of the various excreta, and the energy used in producing the muscular movements of the body. Since, practically, the temperature of the body remains the same, the amount of heat lost during a given time should correspond to the income estimated from the number of heat units of the food. So far, however, attempts to make the calculated heat income correspond with the expenditure have not been productive of satisfactory results, the calorific value of the food being hardly sufficient to produce the heat calculated to be given off, and the other work done by the body in the form of muscular movement, etc.

Since the activity of muscle and gland tissue is constantly undergoing variations in intensity, the amount of chemical change differs at different times, so that the amount of heat produced must also vary. We know that the heat set free by any organ, such as a gland or a muscle, increases in proportion to the increase of its functional activity, but we cannot say that the calorific activity can vary independently of other circumstances. Without such a special calorific function of some tissues, such as muscle, the actual net heat income must vary with circumstances which are accidental, and therefore irregular.

Since we know that the nervous system controls the tissue activities which are accompanied by the setting free of heat, we can see how the nerve centres can materially influence the heat production of the body. The more active the muscles, glands, etc., which are under the control of nerves, the greater is the amount of heat produced in a given time. That the nervous system can cause in any tissue a chemical change, giving rise to a greater production of heat, without any other display of functional activity, we do not know, but many facts seem to point to such a possibility.

The effect of nerve influence on the production of heat is greatly complicated by the power exercised by the vasomotor nerves over the blood supply to the great viscera, etc., for the 37 temperature of any given part is so intimately related to the amount of blood flowing through it that the former has been accepted as an adequate measure of the latter.

For the present, therefore, we are not in a position to speak with decision of nerves with a purely thermic action.

The Expenditure of the heat may be classed under the following headings: i. In warming ingesta: As a rule, the food and drink we use, as well as the oxygen we breathe, are colder than the body, and before they pass out they are raised to the body temperature.

2. Radiation and Conduction: From the surface of the body a quantity of heat is being expended in warming the surrounding medium, which is habitually colder than our bodies. The colder the medium, the greater its capacity for heat, and the more quickly it comes in contact with new portions of the surface, the more warmth it robs us of. Water or damp air takes up much more heat from our surface than dry air of the same temperature, and the quantity of heat lost is still further increased if the medium be in motion, so that the relatively colder fluid is constantly renewed.

3. Evaporation: (a) From the larger air passages: a quantity of water passes into the vaporous state and saturates the tidal air, and this change of condition from liquid to that of vapor absorbs much heat; (b) From the skin: surface evaporation is always going on, even when no moisture is perceptible on the skin, and much fluid of which we are not sensible is lost in this way. The quantity of heat lost by evaporation from the skin will depend on the temperature and the degree of moisture of the air in proportion to that of the surface of the body.