CIKCULATORY SYSTEM. 47 



Ultimately most of the lymphatics empty their contents 

 into the thoracic duct, which leads into the left subclavian 

 vein. This is especially true of the lacteals or chyliferous 

 vessels, which are filled with the fibrin, the fatty and 

 other matters, the products of digestion, derived from the 

 small intestine (Fig. 55). 



It must be added, however, that other lymphatics empty 

 their contents by a common canal into the right subclavian 

 and right jugular veins. 



The blood of a living animal is in constant circulation, 

 nourishing and building up every part of the body. The 

 fact of the circulation of the blood was first demonstrated in 

 1619 by William Harvey, an English physician. 



In the Mammals and the same is true of Birds the or- 

 gans of the circulatory system, as already stated, are a heart 

 to propel the blood, tubes, or arteries, to carry the blood 

 from the heart to all parts of the body, and other tubes, or 

 veins, to bring the blood back to the heart, and minute ca- 

 pillary vessels to connect the terminations of the arteries 

 with the beginning of the veins. 



The arteries are deeply situated, and are composed of three tissues 

 the inner continuous with the inner membrane of the heart; the middle 

 fibrous and elastic, with the fibers arranged circularly; and the outer 

 cellular and also elastic. When empty the arteries do not readily col- 

 lapse; when cut they do not readily heal. 



The veins have thinner walls than the arteries, but are also com- 

 posed of three tissues, the middle tissue of which is made up of irreg- 

 ularly arranged fibers. When empty the veins readily collapse, when 

 cut they readily heal. The veins follow nearly the same course as the 

 arteries, but, in most cases, they are nearer the surface, and they are 

 more numerous than the latter. 



The blood flows in the arteries in a continued stream, and with con- 

 siderable force. This is owing to the action of the heart, and also to 

 the elasticity of the arteries themselves. 



The blood passes from the arteries into the veins by means of the 

 capillaries, as before indicated, and the impulse it receives from the 



