434 ANIMAL HEAT 



heat-producing tissue. The manifestation of muscular energy is always ac- 

 companied by the evolution of heat and the production of carbon dioxide. 

 This production of carbon dioxide goes on while the muscles are in mechanical 

 rest, only in a less degree than that which is noticed during muscular activity, 

 and so it is certain that an active catabolism is going on in resting as well as 

 in contracting muscles. This catabolism is a source of much heat, and so 

 the total amount of heat produced in the muscular tissues per day must be 

 very great. It has been calculated that, even neglecting the heat produced 

 by the quiet catabolism of muscular tissue, the amount of heat generated by 

 muscular activity would supply the principal part of the total heat produced 

 within the body. The heart, as a special muscle, deserves particular mention 

 since it is in constant vigorous activity. All its energy is ultimately converted 

 into heat, accounting for about 5 per cent of the total heat of the body. The 

 secreting glands, and principally the liver as being the largest and most ac- 

 tive, come next to the muscles and heart as heat-producing tissues. It has 

 been found by experiment that the blood leaving the glands is considerably 

 warmer than that entering them. The metabolism in the glands is very 

 active; and the more active the catabolism, the greater the heat produced. 



It must be remembered, however, that although the organs mentioned are 

 the chief heat-producing parts of the body, all living tissues contribute their 

 quota, and this in direct proportion to their activity. The blood itself is also 

 the seat of catabolism, and, therefore, of the production of heat; but the 

 share which it takes in this respect, apart from the tissues in which it circu- 

 lates, is very inconsiderable. 



Regulation of the Temperature of the Human Body. The average 

 temperature of the body is maintained under different conditions of external 

 circumstance by mechanisms which permit of (i) variation in the loss of heat, 

 and (2) variations in the production of heat. In healthy warm-blooded ani- 

 mals the loss and gain of heat are so nearly balanced one by the other that, 

 under all ordinary circumstances, a uniform temperature, within a degree or 

 two, is preserved. 



Variation in the Loss of Heat. The loss of heat from the human 

 body is principally regulated by the amount given off by radiation and con- 

 duction from its surface, by means of the constant evaporation of water from 

 the same part, heat being thus rendered latent, and to a much less degree by 

 loss from the air-passages. In each act of respiration, heat is lost to a greater 

 or less extent according to the temperature of the atmosphere; unless indeed 

 the temperature of the surrounding air exceeds that of the blood. We must 

 remember, too, that all food and drink which enter the body at a lower tem- 

 perature abstract a small measure of heat; while the urine and feces which 

 leave the body at about its own temperature are also means by which a certain 

 small amount of heat is lost. 



Heat Lost from the Surface of the Body. By far the most impor- 



