68 THE VERTEBRATED ANIMALS. 



mentary canal, the part of it adapted to the pur- 

 poses of nutrition, and termed the chyle, is ab- 

 sorbed by the lacteal vessels and carried into the 

 veins. After each part has received its proper de- 

 gree of nutriment, the residue is carried back into 

 the veins by means of a set of vessels analogous to 

 the lacteal, and which together form what is usually 

 denominated the lymphatic system. 



The veins carry back to the heart the blood 

 which has served the purposes of nutrition, and 

 which has been repaired by the chyle and lymph. 

 This blood, however, must pass either totally, or 

 partially, into the organ of respiration, for the 

 purpose of resuming its arterial character, before 

 it is carried back by the arteries to the different 

 parts of the body. In the three first classes of 

 vertebrated animals, the organ of respiration con- 

 sists of lungs; which are an assemblage of small 

 cells penetrable by the external air. In fishes 

 alone respiration is performed by gills, or a series 

 of laminae, between which the water passes. 



In all the vertebrated animals the blood which 

 furnishes to the liver the materials of the bile is 

 supplied from that venous blood which has cir- 

 culated in the intestines, and which, after being 

 re-united in a trunk called the vena porta, is again 

 sub-divided at the liver. 



All these animals have likewise a peculiar se- 

 cretion called the urine, which is formed by two 

 large glands attached to the sides of the spine, 

 called kidneys. The fluid secreted by these glands 



