SOURCES OP 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. 
