OF ANIMAL HEAT. 93 



176. In the young, the production of animal heat is not 

 equal to what it afterwards becomes, especially in those born 

 with the eyes closed. Thus kittens or puppies left exposed 

 to the air, even in summer, soon die. New-born children 

 also are extremely susceptible of cold. 



177. This production of animal heat is evidently con- 

 nected with the phenomenon of vital combustion, with the 

 absorption of oxygen by the blood, and the production of car- 

 bonic acid gas. It seems, in fact, to be proportional to the 

 amount of oxygen absorbed, and hence is greatest in birds 

 and mammals. 



The production of carbonic acid gas takes place in the 

 capillary vessels, where, in fact, the arterial blood becomes 

 venous ; the production, then, of animal heat is not confined 

 to one spot, as the lungs, but is extended over the whole bod}'. 

 It depends on the arrival of fresh arterial blood, and when the 

 supply of this is cut off or diminished, the temperature is 

 immediately lowered. 



There is a remarkable relation also between the richness of 

 the blood in solid parts and the production of animal heat. 

 It is richest in birds (14 or 15 : 100), in whom the animal 

 heat is greatest ; next in mammals (9 or 12 : 100) ; feeblest 

 in the cold-blooded, as in frogs and fishes, in whom the solid 

 parts of the blood, compared to the liquid or watery, is as 6 

 of globules to 94 of serum. 



It bears a certain relation also to the distance from the 

 heart, and thus the limbs are most exposed to be frost-bitten. 



Thus it is to respiration that is due the production of 

 animal heat, since it is in the lungs that the oxygen is ab- 

 sorbed. But in the higher animals this combustion itself is 

 evidently influenced by another physiological agent, of which 

 we have not } T et spoken the nervous system. 



Numerous experiments have placed this fact beyond a 

 doubt. The late experiments of M. Bernard on the cervical 

 ganglions are in fact not opposed to this view. Toxic agents, 

 which lower the activity of the brain and nervous system, 

 obviously affect the production of animal heat. 



178. Hot-blooded animals have the faculty of resisting 

 external heat when raised above the natural temperature of 

 their bodies ; this is effected by evaporation from the surface 

 of the cutaneous transpiration, by which the temperature of 

 the body is maintained at nearly the same temperature at the 

 equator or within the polar circle. 



