1'6 PHYSIOLOGT 'AND HEALTH. 



the whole of these classes is in the apparatus of respiration. 

 The warm-blooded animals breathe more and purer air than 

 the cold-blooded. Fishes breathe only .by gills, and receive 

 only the little air that is in the water, and they are cold ; 

 we breathe with full lungs, and receive a more plentiful sup- 

 ply of air, and are heated to 98 ; while birds have the largest 

 means of respiration, and breathe the purest air, and are 

 consequently from 2 to 13 warmer than even man. Insects 

 have generally larger means of respiration, and a higher 

 temperature. 



412. There is a remarkable difference, in this respect, 

 among the inhabitants of the sea. Fishes such as the pike, 

 cod, haddock, sturgeon, smelt, &c. which breathe by gills 

 are dependent solely upon the air in the water. They can 

 therefore neither obtain nor consume more than a very small 

 portion of air, and consequently they are cold. On the con- 

 trary, whales, porpoises, and dolphins breathe by lungs. 

 They rise to the surface of the water, and inhale the free air 

 above it. They find this abundant, and consume it plenti- 

 fully, and consequently their temperature is about 100, and 

 independent of the heat of the water. It is neither depressed 

 in winter nor raised in summer. They are therefore classed 

 with the warm-blooded animals. 



413. In order to maintain this heat within the animal 

 body, constantly and independently of the influence of sur- 

 rounding and contiguous matters, two conditions are neces- 

 sary : 1st, each animal must possess some internal appara- 

 tus for generating or creating this heat ; 2d, the skin, or the 

 external covering, must be endowed with such a power of 

 regulating the transmission of heat, that it may prevent its 

 too rapid passage out in winter, or when the air is colder 

 than the body, and also its passage into the body in summer, 

 or when the air is warmer than the body. 



