OF THE FUNCTIONS OF EELATION. 89 



The production of carbonic acid gas takes place in the ca- 

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

 venous ; the production, then, of animal heat is notconfined to 

 one spot, as the lungs, but is extended over the whole body. 

 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 arid 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 

 evidentl} 1 " influenced by another physiological agent of which 

 we have not yet 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. 



H._OF THE FUNCTIONS OF RELATION. 



179. Hitherto we have been occupied with those func- 

 tions which have for their object the preservation of the 

 individual ; let us now attend to those intended to make him 

 acquainted with surrounding objects. 



180. Observe carefully the movements of an animal, 



