THE HEAT OF THE BODY 541 



which is in direct contact with external bodies, usually colder than 

 itself, is cooler than the internal organs; its temperature in health 

 is from 36 to 37 C. (96.8-98.5 F.), being warmer in more pro- 

 tected parts, as the hollow of the armpit. In internal organs, as 

 the liver and brain, the temperature is somewhat higher. In the 

 lungs there is loss of the heat carried out by the expired air and 

 that used up in evaporating the water carried out in the breath, 

 so the blood returned to the heart by the pulmonary veins is 

 slightly colder than that carried from the right side of the heart 

 to the lungs. 



The Sources of Animal Heat. Apart from heat received from 

 its surroundings and in hot food and drink, the source of heat in 

 the Body is the oxidation of fuel. The 1,750 Calories which repre- 

 sent the basal metabolism (p. 506) all appear as heat; whenever 

 muscular work is done there is an additional by-product of heat. 

 Moreover, except in those who store surplus fuel as fat, any excess 

 of food consumed is burned, with the production of still more heat. 



The Maintenance of a Uniform Temperature. Obviously if 

 the Body is to preserve the same temperature during any period 

 of time the production of heat within it must exactly balance the 

 loss of heat from it during that time. In ourselves this balance 

 is actually maintained within narrow limits of fluctuation through- 

 out healthy life. Only in fevers, or as the result of prolonged ex- 

 posure to cold, is the balance upset. In fact its preservation is 

 necessary for the continuance of the life of a warm-blooded ani- 

 mal; should the temperature rise above certain limits chemical 

 changes, incompatible with life, occur in the tissues; for example, 

 at about 49 C. (120 F.) the muscles begin to become rigid. On 

 the other hand, death ensues if the Body be cooled down to about 

 19 C. (66 F.). 



Since we live in an environment of constantly varying tem- 

 perature a rather delicate adjustment between heat production 

 and heat loss is required. 



This adjustment is attained through the interaction of two 

 sorts of regulatory devices, one for controlling the loss of heat from 

 the Body, the other its production in the Body. Both of these 

 are partly voluntary and partly involuntary. As regards heat- 

 loss, by far the most important regulating organ is the skin : under 

 ordinary circumstances nearly 90 per cent of the total heat given 



