36 ANIMALIA VERTEBRATA. 



Subdivision of the Vertebrata into Four Classes. 



We have just seen how far vertebrated animals resemble 

 each other; they present, however, four great subdivisions or 

 classes, characterised by the kind or power of their motions, 

 which depend themselves on the quantity of their respiration, 

 inasmuch as it is from this respiration that the muscular fibres 

 derive the strength of their irritability. 



The quantity of respiration depends upon two agents : the 

 first is the relative amount of blood which is poured into the 

 respiratory organ in a given instant of time ; the second is the 

 relative amount of oxygen which enters into the composition 

 of the surrounding fluid. The quantity of the former de- 

 pends upon the disposition of the organs of circulation and 

 respiration. 



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

 blood which is brought back from the various parts of the 

 body by the veins, is forced to circulate through the respi- 

 ratory organ, previous to resuming its former course through 

 the arteries; or they may be simple, so that a part only of 

 the blood is obliged to pass through that organ, the remainder 

 returning directly to the body. 



The latter is the case with reptiles. The quantity of their 

 respiration, and all their qualities which depend on it, vary 

 with the amount of blood thrown into the lungs at each pul- 

 sation. 



Fishes have a double circulation, but their organ of respi- 

 ration is formed to execute its function through the medium 

 of water ; and their blood is only acted on by the portion of 

 oxygen it contains, so that the quantity of their respiration is 

 perhaps less than that of reptiles. 



In the mammalia the circulation is double, and the aerial 

 respiration simple, that is, it is performed in the lungs only ; 

 their quantity of respiration is, consequently, superior to that 

 of reptiles, on account of the form of their respiratory organ, 

 and to that of fishes from the nature of their surrounding ele- 

 ment. 



