ANIMALIA VERTEBRATA. 39 



of the nourishment of the parts is also carried into the veins by vess- 

 els analogous to these lacteals, and forming with them one same 

 system called the lymphatic system. 



The blood which has served to nourish the parts, and which has 

 just been renewed by the chyle and lymph, is returned to the heart 

 by the veins — but this blood is obliged, either wholly or in part, to 

 pass into the organ of respiration, in order to regain its arterial na- 

 ture, 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 collection of cells into which air penetrates. In Fishes only, 

 and in some Reptiles, while young, it consists of branchiee 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 vence 

 porta^ is again subdivided at the liver. 



Subdivision of ike 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 depends 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 respiratory organ, pre- 

 vious 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. 



'J'he latter is the case with Reptiles. The (juantity of tiieii respi- 



