therefore varies within narrow limits according to the part of the body through which it has recently passed. The mean temperature of the blood is higher than that of any tissue. The blood in the hepatic capillaries is the warmest in the body. This reaches 40.73° in the dog, or nearly two degrees higher than that in the aorta of that animal. The cool blood from the extremities and head mingling in the right side of the heart with the unusually warm blood from the liver keeps the blood going to the lungs at the standard temperature. The blood in the left side of the heart is a little cooler than that in the right, probably because the latter lies on the warm liver, as is proved by the substitution of a cold object for this organ, when the blood on the right side becomes colder than the left. It is not because the blood is cooled going through the lungs, for the heat used in warming the respired air is given off by the nose and other air passages, and not by the alveoli of the lungs.

III. The temperature of an organ varies with the state of its activity. During the active state the glands, etc., receive more blood and undergo more active chemical change, so that they become warmer.

MODE OF PRODUCTION OF ANIMAL HEAT.

It has already been indicated that the general effect of the tissue change of the body is a kind of combustion in the tissues of certain substances obtained from the vegetable kingdom, viz., proteid, fat, carbohydrate, etc. The combustible substances are capable of being burned in the open air, or made to unite with oxygen so as to produce a certain amount of heat, being thus converted into CO, and H2O. In the body the oxidation goes on in a gradual or modified way, and the end products of the process can be recognized as CO, eliminated from the lungs, and as water and urea got rid of by the kidneys. The general tendency of the chemical changes in the tissues is such as will set free energy in the form of heat.

The amount of heat that any substance is capable of giving off corresponds to the amount of energy required for the formation from CO, and H2O, etc., of the compounds contained in it,

and this correspondence remains whether the dissociation take place rapidly or slowly. The substances we make use of as food have thus a certain heat value which depends upon their chemical composition.

The high temperature which homoeothermic animals can keep

up in spite of the cold of the atmosphere in which they live is readily accounted for by the chemical change which is constantly occurring in the tissue of their bodies.

The amount of heat produced in any part depends upon the activity of its tissue change, for we find that the temperature varies with the elimination of CO, and urea, which gives a fair estimate of the normal chemical changes of the tissues.

1. The diurnal changes in temperature are accompanied by an afternoon increase and a morning decrease of CO, and urea.

2. The tissue change giving rise to CO, decreases in a fasting animal, as does also the production of heat.

3. More CO, is eliminated after meals, when the temperature also rises.

4. The activity of various organs, such as the muscles and glands, is associated with a local increase of temperature.

INCOME AND EXPENDITURE OF HEAT.

Income. 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 tempera

ture 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:

1. 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.

Balance. As has been said, the exact income of heat is uncertain and variable, because the data upon which the absolute amount can be calculated are not scientifically free from error. According to the most careful estimates, an adult weighing 82 kilo. produces 2,700,000 units of heat in the twenty-four hours, which are expended in the following way :

From this it appears that more than three-quarters of our heat is lost by the skin (77.5 per cent.); by pulmonary evaporation, 14.7 per cent.; in heating the air breathed, 5.2 per cent.; in heating ingesta, 2.6 per cent.

MAINTENANCE OF UNIFORM TEMPERATURE.

In order that the vital processes of man and the other homœothermic animals should go on in a normal manner, it is necessary that their mean temperature remain nearly the same, and we have seen that under ordinary circumstances it varies only about one degree below or above the standard 37° C., notwithstanding the changes taking place in the temperature around us. Thus we can live in any climate, however cold or warm, and if our body temperature remains unaltered, we suffer no immediate injury.

There is a limit, however, to this power of maintaining a uniform standard temperature. If a mammal be kept for some time in a moist medium, where evaporation cannot take place, at a temperature but little higher than its body, say over 45° C., its temperature soon begins to rise, and it dies with the signs of dyspnoea and convulsions (probably from the nervous centres being affected) when its temperature arrives at 43°-45°. If placed in water at freezing point an animal loses its heat quickly, and when its body temperature has fallen to about 20° C. it dies in a condition resembling somnolence, the circulation and respiration gradually failing.

Since a variation of more than one or two degrees in the temperature of our bodies interferes with the vital activities of the controlling tissue in the nervous centres, it is, of course, of the utmost importance that adequate means for the regulation of the mean temperature of our bodies should exist.

The temperature of an animal's body must depend on the relations existing between the amount of heat generated in the