96 OF RESPIRATION. 



256. The proper temperature of an animal, or what is 

 termed ANIMAL HEAT, depends on the combined activity of 

 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 tempera- 

 ture of about 108 Fahrenheit ; and in a large proportion 

 of mammals it is generally from 95 to 105. These bear 

 the general designation of warm-Hooded animals. 



257. Reptiles, fishes, and most of the still lower 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 tempera- 

 ture is very low, though some may be frozen without the 

 loss of life. For this reason they are denominated cold- 

 Hooded animals ; and all of them have such a structure of 

 the heart, that only a part of the blood which enters it is 

 sent to the respiratory organs (243). 



258. 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 func- 

 tion 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 nutrition in general, and more particularly assimilation, 

 by which the liquid parts are fixed and solidified, is con- 

 nected with the maintenance of the proper temperature of 

 animals, and its uniform distribution through the body, has 

 not yet been satisfactorily ascertained. 



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



