VERTEBRATED ANIMALS. 21 



are always proportioned to the quantum of respiration ; for up- 

 on the perfection of this function, in a great measure, depend 

 the irritability of the muscular fibre, and the energy of the mus- 

 cular action. The quantity of respiration depends upon the re- 

 lative portion of blood contained at every given instant of time 

 in the lungs, and the amount of oxygen which enters into the 

 composition of the fluid. The quantity of blood is altogether 

 determined by the peculiar disposition of the organs of respi- 

 ration and circulation. 



The organs of circulation may be double, so that all the blood 

 conveyed by the veins from the different parts must undergo 

 a process of circulation before it can be returned by the arteries ; 

 or they may be simple, in which case only a portion of the blood 

 which returns to the body passes through the lungs. This last 

 is the case with reptiles. The quantity of their respiration, and 

 the qualities depending on it, vary with the relative proportion 

 ef blood returned at each pulsation into the lungs. 



Fishes have a double circulation, but as they respire through 

 the medium of water, and their blood only receives the portion 

 of oxygen in that medium, their quantity of respiration is per- 

 haps less than that of reptiles. 



In the Mammalia the circulation is double, and the respira- 

 tory process simple. The quantity of their respiration is supe- 

 rior to that of reptiles and fishes. But the quantity of respira- 

 tion in birds is still greater than that of quadrupeds, because 

 they also respire by various other cavities as well as the lungs. 

 The air penetrates through their whole body, and acts upon 

 the branches of the aorta with the same efficiency as upon those 

 of the pulmonary artery. 



From these circumstances result four different kinds of motion 

 among vertebrated animals. Quadrupeds, in which the quantity 

 of respiration is moderate, are formed for walking and running, 

 and their predominant characteristic is vigour. Birds, whose 

 respiratory system is more extensive, possess the lightness and 

 strength of muscles necessary to support them in their flight. 

 Reptiles, which respire more feebly, creep upon the earth, and 

 many of them pass more or less of their existence in a state of 

 torpor. And fishes, which move in a fluid almost as specifical- 

 ly heavy as themselves, are enabled to execute their movements 



