238 KESPIEATIOX. 



the respiratory and circulating systems, and is in direct pro- 

 portion to it. In many animals the heat is maintained at a 

 uniform temperature, whatever may be the variations of the 

 surrounding medium. Thus birds maintain a temperature of 

 about 108 Fahrenheit ; and in a large proportion of mammals 

 it is generally from 95 to 105. These bear the general de- 

 signation of warm-blooded animals. 



400. Reptiles, fishes, and most of the invertebrate animals, 

 have not this power of maintaining a uniform temperature. 

 The heat of their body is always as low as from 35 to 50, 

 but varies perceptibly with the surrounding medium, being, 

 however, often a little above it when the external temperature 

 is very low, though some may be frozen without the loss of life. 

 For this reason they are denominated cold-blooded animals; 

 and all animals which have such a structure of the heart, that 

 only a part of the blood which enters it is sent to the respira- 

 tory organs ( 366), are among them. 



401. The production of animal heat is obviously con- 

 nected with the respiratory process. The oxygen of the 

 respired air is diminished, and carbonic acid takes its place. 

 The carbonic acid is formed in the body by the combination 

 of the oxygen of the air with the carbon of the blood. The 

 chemical combination attending this function is, therefore, 

 essentially the same as that of combustion. It is thus easy 

 to understand how the natural heat of an animal is greater, 

 in proportion as respiration is more active. How far nutri- 

 tion in general, and more particularly assimilation, by which 

 the liquid parts are fixed and solidified, is connected with the 

 maintenance of the proper temperature of animals, and the 

 uniform distribution through the body, has not yet been satis- 

 factorily ascertained. 



402. Some of the higher warm-blooded animals do not 

 maintain their elevated temperature during the whole year ; 

 but pass the winter in a sort of lethargy, called HIBEENATIOK, 

 or the hibernating sleep. The marmot, the bear, the bat, the 

 crocodile, and most reptiles, furnish examples. During this 

 state the animal takes no food ; and as it respires only after 

 very prolonged intervals, its heat is diminished, and its vital 

 functions generally are much reduced. The structural cause 

 of hibernation is not ascertained ; but the phenomena at- 

 tending it fully illustrate the lavs already stated (397 401). 



