INFLUENCE OF HEAT ON ANIMALS. 93 



who have become habituated to this kind of exposure, can (with proper 

 precautions) sustain a temperature of from 850 to 500. In all such 

 cases, however, the real heat of the body undergoes very little eleva- 

 tion ; for, by means of the copious evaporation from its surface, the ex- 

 ternal heat is prevented from acting upon it. But if this evaporation 

 be prevented, either by an insufficiency in the supply of fluid from 

 within, or by the saturation of the surrounding air with moisture, the 

 temperature of the body begins to rise ; and it is then found, that it 

 cannot undergo an elevation of more than a few degrees, without fatal 

 consequences. Thus in several experiments which have been tried on 

 different species of warm-blooded animals, for the purpose of ascertain- 

 ing the highest temperature to which the body could be raised without 

 the destruction of life, it was found that as soon as the heat of the body 

 had been increased, by continued immersion in a limited quantity of hot 

 air (which would soon become charged with moisture), to from 9 13 

 above the natural standard, the animals died. In general Mammals 

 die, when the temperature of their bodies is raised to about 111; the 

 heat which is natural to the bodies of Birds. The latter are killed by 

 an equal amount of elevation of bodily heat above their natural standard. 



139. Hence we see that the actual range of temperature, within 

 which vital activity can be maintained in warm-blooded animals, is ex- 

 tremely limited ; a temporary elevation of the bodily heat to 13 above 

 the natural standard, or a depression to 30 below it, being positively 

 inconsistent, not merely with the continuance of vital operations, but 

 also with the preservation of vital properties : and a continued departure 

 from that standard, to the extent of only a very few degrees above or 

 below it, being very injurious. The provisions with which these animals 

 are endowed, for generating heat in their interior, so as to supply the 

 external deficiency, and for generating cold (so to speak), when the ex- 

 ternal temperature is too high, are therefore in no respect superfluous : 

 but are positively necessary for the maintenance of the life of such ani- 

 mals, in any climate, save one whose mean should be conformable to 

 their standard, and whose extremes should never vary more than a very 

 few degrees above or below it. Such a climate does not exist on the 

 surface of the earth. 



140. The range of external temperature, within which cold-blooded 

 animals can sustain their activity, is much more limited, as well in regard 

 to its highest as to its lowest point ; notwithstanding that the range of 

 bodily heat, which is consistent with the maintenance of their life, is so 

 much greater. In those which, like the Frog, have a soft moist skin, 

 which permits a copious evaporation from the surface, a considerable 

 amount of heat may be resisted, provided the air be dry, and the supply 

 of fluid from within be maintained.* But immersion in water of the 

 temperature of 108, is almost immediately fatal. In many other cold- 

 blooded animals, elevation of the temperature induces a state of tor- 



* The Frog has a remarkable provision for this purpose ; in a bladder, which is 

 structurally analogous to our Urinary bladder, but which has for its chief function to 

 contain a store of fluids for the exhaling process. It has been noticed that, when this 

 store is exhausted by continued exposure of the animal to a warm dry atmosphere, the 

 bladder becomes full again, when the animal is placed in a moist situation, even though 

 it take in no liquid by its mouth. 



