ANIMALIA VERTEBRATA. 29 



the organ of respiration, in order to regain its arterial nature, previous 

 to being again sent through the system by the arteries. In the three 

 first classes this respiratory organ consists of lungs, that is, a collec- 

 tion of cells into which air penetrates. In Fishes only, and in some 

 Reptiles, while young, it consists of branchiae or a series of laminae, 

 between which water passes. 



In all the Vertebrata, the blood which furnishes the liver with the 

 materials of the bile is venous blood, which has circulated partly in the 

 parietes of the intestines, and partly in a peculiar body called the 

 spleen, and which, after being united in a trunk called the venae porta, 

 is again subdivided at the liver. 



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 quan- 

 tity of the former depends upon the disposition of the organs of circu- 

 lation 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 respiratory organ, previous to resum- 

 ing its former course through the arteries j 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 respi- 

 ration, and all their qualities which depend on it, vary with the amount 

 of blood thrown into the lungs at each pulsation. 



Fishes have a double circulation, but their organ of respiration 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 thelungs only; their quantity of 

 respiration is, consequently, superior to that of Reptiles, on account of 



