Cuvierian System of Zoology, 131 



In all vertebrated animals the blood which supplies the liver 

 with the matter that forms the bile, is veinous blood which 

 has circulated in the intestines, and which, after reuniting in 

 one trunk, called the vena porta, is spread again through the 

 liver. All vertebrated animals have a particular secretion 

 from two large glands attached to the sides of the backbone 

 called the kidneys ; this liquor, denominated urine, generally 

 remains sometime in a reservoir called the bladder. 



The sexes are always distinctly separated ; but the mode of 

 impregnation varies very much. The eggs of some reptiles, 

 and of almost all fishes, are impregnated by the male after they 

 are spawned. 



We may perceive in the above characters, how far all ver- 

 tebrated animals resemble each other : nevertheless they admit 

 of four grand subdivisions or classes, characterised by the 

 kind or strength of their motions, which also depend on the 

 quantity of their respiration ; since it is from the respiration 

 that the muscular fibres derive their energy, and their irri- 

 tability. 



The quantity of respiration depends on two conditions : the 

 first is the relative proportion of blood which is presented to 

 the respiratory organs in a given time ; the second, the relative 

 proportion of oxygen which enters into the composition of the 

 fluid in which the animal lives, whether water or air. 



The quantity of blood which is acted on by respiration, de- 

 pends on the structure and disposition of the organs of respir- 

 ation and circulation. The organs of circulation may be double, 

 so that all the blood which is returned by the veins is obliged 

 to circulate through the respiratory organs, before it is carried 

 again to different parts by the arteries ; or these organs may 

 be simple, so that only a portion of the blood returned from the 

 body to the heart is obliged to pass through the respiratory 

 organs, and the rest circulates again through the body, with- 

 out having been subjected to the effects of respiration. The 

 latter is the case with reptiles ; their quantity of respiration, 

 and all the qualities that depend on it, vary according to the 

 proportion of blood which enters the lungs at each pulsation. 

 From these characters Cuvier forms the four subdivisions or 

 classes of vertebrated animals, which are, — 



Class 1. Mammiferous Animals, which, bring forth their young 

 alive and suckle them, being provided with teats (Lat. mam" 

 mce), whence the name is derived. 



Class 2. Birds, 



Class 3. Reptiles, 



Class 4. Fisk, 



