Mastication, Digestion a/nd Assimilation, 

 Com/position of chyle. 



21 



36. The circulatory system. — The arteries are the tubes or 

 canals which carry the blood from the heart. As these tubes 

 pass throughout the body they branch again and again, gradually 

 growing smaller, until they end in minute divisions called capil- 

 laries. The ends of the capillaries connect in turn with the 

 veins, which carry the blood back to the heart. The purified 

 blood forced outward from the heart through the arteries reaches 

 the capillai'ies, thi-ough which portions escape to nourish the 

 tissues of the body. That which is not lost through the capilla- 

 ries, together with tte impurities gathered on the way, returns to 

 the heart, whence it is forced to the lungs for purification. Prom 

 these it returns to the heart, to be again forced over the body. 



37. The lymphatics. — Besides the circulatory system there is 

 a second one which drains the whole animal body toward the 

 heart only, called the "Lymphatic system." The broken-down 

 tissues of the body are removed through tlie lymphatic system, 

 and the material thus drained is called lymph. Lymph varies 

 in composition according to the stage of activity of the organs 

 contributing it and the locality from wMch it is taken. Lymph 

 and chyle, the one worn-out material from the tissues of the body, 

 the other fresh material from the digestive tract, are poured 

 through the lymphatics into the venous system and on to the 

 heart, where they mingle with the blood current. It is estimated 

 that for every 220 pounds body weight there are 7.5 pounds of 

 lymph and 6 pounds of chyle formed daily. 



