ANIMAL HEAT. 



453 



producing tissue. It will shortly be pointed out that the manifesta- 

 tion of muscular energy is always accompanied by the evolution of heat 

 and the production of carbon dioxide. This production of carbon 

 dioxide goes on while the muscles are at rest, only in a less degree to 

 that which is noticed during muscular activity, and so it is certain that 

 an active metabolism is going on in resting as well as in contracting 

 muscles. This metabolism is a source of much heat, and so the total 

 amount of heat produced in the muscular tissues per diem must be very 

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

 by the quiet metabolism of muscular tissue, the amount of heat gener- 

 ated by muscular activity would supply the principal part of the total 

 heat produced within the body. (2.) The Secreting glands, and prin- 

 cipally the liver, as being the largest and most active, come next to the 

 muscles as heat-producing tissue. 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, as we have 

 seen, the more active the metabolism the greater the heat produced. 

 (3.) The Brain; the venous blood has a higher temperature than the 

 arterial. It must be remembered, however, that although the organs 

 above 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 metabolism, and, 

 therefore, of the production of heat; but the share which it takes in 

 this respect, apart from the tissues in which it circulates, is very incon- 

 siderable. There are two other means by which the heat produced by 

 metabolism of the tissues is added to in slight degree, viz., by friction, 

 i.e., in the movements of muscles, in the circulation of blood, and else- 

 where. This contributes a slight but undetermined amount of heat, 

 and by the taking in of warm foods, solid or liquid, a further small 

 amount of heat is at the same time acquired. 



REGULATION OF THE TEMPERATUEE OF THE HUMAN BODY. 



The average temperature of the body is maintained under different 

 conditions of external circumstances by mechanisms which permit of 

 (1) variation in the loss of heat, and (2) variations in the production of 

 icat. In healthy warm-blooded animals the loss and gain of heat are so 

 nearly balanced one by the other that, under all ordinary circumstances, 

 an 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 (1) by radiation 

 and conduction from its surface, and by means of the (2) constant evapo- 

 ation of water from the same part, heat being thus rendered latent, and 



