70 CLASSES OP THE VERTEBRATED ANIMALS. 



The organs of circulation may be double, so that 

 all the blood transported by the veins from the dif- 

 ferent parts, must undergo a process of circulation 

 in the respiratory organ before it can be returned 

 by the arteries ; or they may be simple, in which 

 latter case a portion of the blood only which returns 

 to the body passes through the organ of respiration. 



This last is the case of reptiles. The quantity 

 of their respiration, and all the qualities depending 

 on it, vary with the relative proportion of blood re- 

 turned at each pulsation into the respiratory organ. 



Fishes have a double circulation, but as they re- 

 spire through the medium of water, and their blood 

 consequently receives only the portion of oxygen 

 which is mingled with their circumambient element, 

 the quantity of their respiration is, perhaps, still less 

 than that of reptiles. 



In the mammalia the circulation is double, and 

 the respiratory process simple, that is, it is per- 

 formed by the lungs alone. The quantity of their 

 respiration is superior to that of reptiles, by reason 

 of the form of their circulating organ, and to that of 

 fishes from the nature of the surrounding element 

 which they respire. 



But the quantity of respiration in birds is still 

 greater than that of quadrupeds, because they pos- 

 sess not only a double circulation and an aerial re- 

 spiration, but also because they respire by various 

 other cavities as well as the lungs. The air pene- 

 trates through their whole body and acts upon the 



