326 THE ANIMAL KINGDOM. 



which, in turn, sends it through the pulmonary arteries, 

 into the lungs; while there, it is exposed in the network 

 of vessels which ramify on the walls of the air-cells; 

 another process of endosmose takes place, and the car- 

 bonic acid passes through the walls of the vessels into the 

 air-tubes, and escapes by the breath ; at the same time that 

 this carbonic acid is cast off, an equal bulk of oxygen is 

 absorbed from the air, so that the air which enters the 

 lungs, and that which passes out, are the same in bulk plus 

 watery vapour although what enters is pure air, and what 

 is expelled, is loaded with carbonic acid. The blood, having 

 exchanged carbonic acid for oxygen, is now altered from a 

 black to a bright scarlet colour, and is in a condition to be 

 circulated through the system. On leaving the lungs by 

 the " pulmonary veins," it enters the left auricle of the 

 heart, and passes from thence to the left ventricle, which 

 is very powerful, and forces it by compression upwards 

 into the " aorta" (the first great artery), and thence 

 through the other arteries, which divide, and become 

 smaller and smaller until it arrives at every part of the 

 body, forming a network of fine vessels which are called 

 capillaries ; so perfectly are these vessels distributed, that 

 it is almost impossible to cut or prick any part of the body 

 without wounding one of them, and thereby drawing blood. 

 "When these capillaries have supplied this pure blood to 

 repair and renovate every part of the system, and received 

 for the new material, that which is worn out or spoiled, 

 they urge it onwards to where they unite into small 

 veins, and, as these continue to unite, they form large 

 trunks, and pour the blood, now black and impure, into 

 the right auricle of the heart, together with the fresh 

 material (chyle) derived from the food, again to be sent 

 through the lungs and purified for fresh circulation. This 

 description applies as well to the other Mammalia as it 

 does to Man, for these classes do not clifter from Man in 

 their physical structure, except in form and size, and the 

 alteration in the proportion of the various parts to adapt 

 each to its peculiar purpose. 



