The bodies of most animals are considerably warmer than their surroundings. Part of the energy set free by the chemical changes in the animal tissues appears as heat which is devoted to this purpose. Warm-blooded animals are those which habitually preserve an even temperature, independent of the changes which take place in that of the medium in which they live; and, as the term warm blooded implies, their temperature is, as a rule, higher than the surrounding air or water. Cold-blooded animals, on the other hand, are those whose temperature is considerably affected by, or more or less closely follows, that of the medium surrounding them.

The blood of all mammalia has pretty much the same temperature as that of man, about 37.50 C, and probably varies under similar circumstances. But birds, the other class of warm-blooded animals, have a temperature about 40 or 6° C. higher than that of mammals.

The blood of those animals whose temperature follows the changes that occur around them is generally from 1° to 50 C. higher than the medium in which they live. They produce some heat, though it be small in quantity, and since they have no special plan for its regulation, it does not remain at a fixed standard. In every part where active oxidation takes place, heat is produced; so even in invertebrate animals an elevation of temperature occurs; this can be ascertained where they exist in masses, as in bee hives, an active hive sometimes reaching a temperature of 35° C.

Instead of the term "warm-blooded," it is more accurate to apply to animals whose temperature remains uniformly even, and independent of their surroundings, the term "Homceother-mic" (of constant temperature), and to animals with temperatures varying with their surroundings "Poikilothermic" (of changing temperature), instead of the words warm and cold blooded.

Measurement Of Temperature

On account of the slight amount of variation that occurs in the temperature of man, all the changes can be measured with a thermometer having a short scale of some twenty degrees, each degree of which occupies considerable length on the instrument, so that very slight variations may be easily appreciated. Such thermometers, with an arrangement for self-registering the maximum height attained by the column of mercury, are in daily use for clinical observation, for the temperature of the body is now a most important aid to diagnosis and prognosis in a large class of diseases.

As heat is constantly being lost at the surface of the body, the skin is colder than the deeper parts, and in order to avoid variations caused by this surface loss - which depends in a measure on the temperature of the air - special arrangements are necessary to prevent the thermometer being too much influenced by it. The instrument may be brought into close proximity to the deeper parts by being introduced into one of the mucous passages, where it is closely surrounded by vascular tissue. In animals, the rectum is the most convenient part for the application of the thermometer, but in clinical practice it is usually placed under the tongue, or in the armpit, the bulb being held so that on all sides it is in contact with the skin and protected from the cool air.

The variations at different parts of the body are but slight, and the average normal surface temperature in man is found to be about 370 C.

Normal Variations In Temperature

I. The temperature of the whole body normally undergoes certain variations, some of which are: 1. Regular and periodical, depending upon the time of day, the ingestion of food, and the age of the individual. 2. Accidental, such as those caused by mental or bodily exertion.

(a) The temperature is highest between 4 and 5 p.m. and lowest between 2 and 4 A.M., the transition being gradual. This diurnal variation, which normally does not much exceed i° C, is much exaggerated in certain fevers.

(Ji) The temperature rises after a hearty meal and falls during fasting. During starvation the temperature sinks gradually until the death of the individual.

(V) The temperature is highest at birth, and falls about 1° C. between that and the age of 50 years; in extreme old age it is said that it again rises.

(V) Muscular exertion, which gives the individual the sensation of great warmth, only changes the temperature of the blood about.5° C. The very high temperature which accompanies the disease tetanus, where all the muscles are thrown into a state of spasm, probably depends more on pathological changes than on muscular action.

(e) Mental exertion is also said to cause a rise of temperature.

(f) Slight differences in the heat of the blood may be brought about by variations in the surrounding temperature. The abnormally high temperature of fever is much more easily affected by changes in the rate of removal of the heat from the body than is the normal temperature, and hence the therapeutic value of cold applications in this class of disease.

II. The temperature of different parts of the body varies in a slight degree, and depends upon the following circumstances: 1. The amount of blood flowing through them; the blood being the greater carrier of warmth from one part to another, supplying heat where it is lost, and conveying material to those parts where the heat is generated. 2. The amount of heat produced, i. e., the activity of its tissue change. 3. The amount of heat lost, which depends on (a) the extent of surface; (b) the external temperature; (V) the power of conduction of, and the capacity for heat of, the surrounding medium.

From this it is obvious that the deeper parts of the body, where active chemical change takes place, and which are protected from exposure, must be warmer than the exterior, which is constantly giving out its heat. The blood which flows through the surface vessels is cooled, and that which flows through the deeper vascular viscera is warmed. Thus the skin is usually about 370 C, while the mouth beneath the tongue is about 37.5° C, and the rectum about 380 C. The temperature of the blood 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 40.730 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 C02 and H20. In the body the oxidation goes on in a gradual or modified way, and the end products of the process can be recognized as C02 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 C02 and H20, 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 homceothermic 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 CO2 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 C02 and urea.

2. The tissue change giving rise to C02 decreases in a fasting animal, as does also the production of heat.

3. More C02 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.