SOURCES OF ANIMAL HEAT. 339 



being observed to come forth from their winter quarters, with 

 the return of spring, in a very lean condition. 



413. The consumption of oxygen and the production of 

 carbonic acid are found to bear, in every animal, a very close 

 relation to the amount of heat liberated at the time. Thus in 

 warm-blooded animals, the respiratory function is much more 

 active than in the cold-blooded ; but when the former are 

 reduced to the state of cold-blooded animals, as occurs in 

 hybernation ( 309), their respiration is proportionately low. 

 On the other hand, whenever the temperature of an animal 

 is quickly raised by any extraordinary stimulus, above that 

 which it was previously maintaining, it is always by means of 

 increased activity of the respiratory movements, and augmented 

 consumption of oxygen. Thus during the incubation of Bees 

 ( 411), the insect, by accelerating its respiration, causes the 

 evolution of heat and the consumption of oxygen to take place 

 at least twenty times as rapidly as when in a state of repose. 

 The same takes place when a hybernating animal is roused ; 

 and it is remarkable that even extreme cold will effect this 

 for a time ; but the animal, if exposed for too long a period 

 to a very low temperature, will not be able to resist its 

 influence, and will perish. 



414. Although the combustion of carbon and hydrogen 

 within the Animal body is undoubtedly the chief source of 

 the production of heat, yet it must not be left out of view that 

 there are other chemical changes in the system, which also con- 

 tribute to it, though in a minor degree ( 343). Of this kind 

 are the oxidation of the sulphur and phosphorus which enter 

 the body in the organic compounds used as food, and which, 

 being united by a combustive process with oxygen, pass out 

 of the system in the urine, in the form of sulphuric and phos- 

 phoric acids, combined with alkaline bases ( 367). 



415. Besides all these sources, it seems probable from 

 various considerations, that Heat may occasionally be generated, 

 like light and electricity, by the direct agency of the Nervous 

 system; as one of the modes of force into which nervous 

 power may be metamorphosed. Of course, in any such gene- 

 ration of heat, there must be the same consumption of nervous 

 tissue, as would occur if its equivalent of nerve-force had been 

 manifested. 



