122 OF RESPIRATION. 



celeration of the circulation. The quantity of air consumed 

 varies, therefore, with the proportion of the blood which ia 

 sent to the lungs. 



256. The proper tennperature 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 temperature 

 of about 108° Fahrenheit ; and in a large proportion of mam- 

 mals it is generally from 95° to 105°. These bear the 

 general designation of warm-blooded 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 

 often, however, 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-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 respiratory organs, are among them, (243.) 



258. The production of animal heat is obviously connected 

 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 assiinilation ^ by which 

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

 maintenance of the proper temperature of animals, and the 



