46 AMERICAN FARMER'S HORSE BOOK. 



other side, if there was one. Before it can be sent forth 

 again to minister to the needs of the body, it must be puri- 

 fied. Not only must the particles of efiete, cast-off matter, 

 which it has accumulated from every part of the system, be 

 eliminated and thrown off; but the chyle also — the substance 

 into which the nutritive elements of the food have been pre- 

 viously converted by digestion — must be submitted to yet 

 another process before it becomes fully prepared to afford the 

 means of nutrition to the constantly disorganizing tissues of 

 all kinds. (The chyle, it must be understood, empties into 

 one of the vena cavce through the thoracic duct, and mingles 

 with the venous blood which is returned to the right auricle.) 



These objects are accomplished through the pulmonary 

 circulation already mentioned. The lungs, composed of two 

 lobes, are of a I^Dongy texture, and filled with innumerable 

 little air-cells. They ^re furnished with an exceedingly fine 

 net-work of capillary vessels, distributed on their walls, and 

 throughout the surface of all the air-cells also. The impure 

 venous blood, as it circulates through the capillaries, is sub- 

 mitted to the agency of atmospheric air under extremely 

 favorable circumstances. It absorbs the oxygen of the air, 

 and, at the same time, gives oft* large volumes of carbonic acid 

 gas, this being the form in which the accumulated impurities 

 of the blood now exist. The wonderful rapidity with which 

 this process is carried on may be understood, when it is con- 

 sidered that the extent of surface upon which the minute 

 capillaries ramify in the lungs is supposed to be ten or 

 twelve times that of the skin. 



Having been thus purified, the blood regains its bright red, 

 or scarlet, appearance, and is again propelled forward, through 

 the heart and arteries, upon the same excursion as before. 

 It reaches every part of the body, perfectly ramifies through- 

 out every organ, and permeates every muscle, tendon, liga- 

 ment, bone, and even to the skin itself, ^nd every hair upon 

 its surface. There is no part, however minute, remote, or 

 unimportant, to which it does not find its way, by means of 

 the divisions and innumerable subdivisions of the vessels 



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