Reference has been made repeatedly to the heat liberated as a result of the oxidation of foods, and of tissue elements. The heat of the body arises from these sources only. As heat is continually leaving the body, it is evident that it must be continually liberated in the body in order to maintain an equable body temperature. It has been estimated that during a twenty-four-hour period the average individual loses not less than 1,800 calories of heat from the surface of the body. Nearly 400 calories are estimated to be carried away from the body through the evaporation of moisture from the surface of the skin, and nearly 200 through the evaporation of moisture from the lungs, while over 50 are lost in the excreta. In round numbers, approximately 2,500 calories of heat leave the body in twenty-four hours. When we consider that the average individual takes in with his food about 3,000 calories per day, it must be evident that about five sixths of all the energy received latent in the foods is consumed in simply maintaining body temperature.

Before proceeding further it may be wise to define the calory. A calory is that amount of heat required to raise 1 kilogram of water 1° centigrade. The same amount of heat would, of course, raise 500 grams of water 2° centigrade, or 100 grams of water 10° centigrade.

There are certain general principles of great importance which must be remembered in all discussions of body temperature. First: Anything which causes an increase of tissue activity causes an increase of liberation of heat in the body, and conversely, anything which causes a decrease of tissue activity causes a decrease of heat liberated in the body. Most important of the things causing increased body heat is muscular work. The reason why muscular work increases body heat is very evident. A large amount of fat, dextrose and proteins are oxidized within the muscles during their activity. Incident to this oxidation, the heat, being liberated from these substances and distributed through the body, tends to raise its temperature. Mental work for similar reasons tends to raise body temperature, though to a very much smaller degree than muscular work. The ingestion of food into the alimentary canal, resulting as it does in the activity of a large number of glandular structures, also increases temperature.

These three causes of increase already mentioned - viz., muscular work, mental work, and the ingestion of food - are all activities of the waking hours. These activities reach a maximum usually in the afternoon, and, as a result, observations on the body ternperature show that it gradually rises from five in the morning to six in the afternoon, covering a total of about 1° C. or 1.8° F. As a rule, body activity decreases from this time on. The partial rest of the evening is followed by complete rest during the hours of sleep. As a result, the temperature gradually falls from six in the evening until five the following morning, when it reaches a minimum.

This fact of the daily wave of temperature must always be remembered by the nurse and physician, as an observation in the early morning hours is certain to be below the average; but though it is below the average, it is not subnormal. If the same temperature, however, were observed at six in the afternoon it might be looked upon as subnormal. In a similar way 0.5° above the recognized average human temperature, when recorded late in the afternoon, must not be looked upon as representing any degree of fever. However, if the same temperature were found at five or six in the morning, it might be looked upon as representing a slight febrile condition.

A second principle always to be remembered is that: Anything which causes an increase of heat radiation or heat conduction from the surface of the body tends to lower the temperature of the body.

Low temperature of the air which comes into contact with the body will carry away heat from the surface of the body, therefore tend to lower its temperature. In order to avoid an actual lowering of temperature through the action of cold air, the body possesses a natural defense in the form of a neuromuscular adaptation, which results in a rapid evolution of heat in the muscles to offset and equalize the heat lost from the surface of the body. If, for any reason, the neuromuscular system is unable to make this response, then subjection to lower external temperatures may be fraught with danger. Perspiration is the most effective means for lowering body temperature. For every gram of water that evaporates from the surface of the body nearly three fifths of a calory of heat is carried away. In this way alone the average individual under average conditions loses, as stated above, nearly 400 calories of heat during twenty-four hours. If a person perspires very freely, as he would do if he were exercising actively in warm weather, this amount would be very greatly increased. Both the methods of decrease mentioned above are natural ones, active almost continuously under usual conditions.

An artificial method of lowering body temperature is the bath, when this is administered in such a way as to leave the body exposed to evaporation of moisture from the wet surface. If a warm-blooded animal is immersed in a bath warmer than its blood temperature, the temperature of the animal will rise for reasons easy to understand. If the temperature of the bath in which the animal is immersed is lower than that of its blood, the temperature will tend to fall at first, but the cold water will stimulate the neuromuscular heat regulation mechanism, and thus quickly bring the temperature back to its original condition before it was subjected to the cold bath. If, now, instead of immersing the animal in the bath, it is given a tepid shower, or a tepid sponge bath, the heat-generating center will not be stimulated into activity and the evaporation of the tepid water from the surface of the bath will cause a marked decrease of body temperature.