CHAPTEE X. 



THE HEART AND CIRCULATION. 



Preliminary Considerations. The main organ of circulation 

 is the heart ; the circulating fluids are the blood and the lymph ; 

 the channels of circulation are the blood-vessels and lymphatics. 

 The principal vessels which convey blood from the heart are the 

 arteries ; when these are much reduced in size, through oft- 

 repeated branching, they are called arterioles ; the arterioles 

 end in an anastomosing network of very fine tubes, which ramify 

 throughout the organs of the body these are the capillaries. 

 The blood returning from the capillaries collects into venules ; 

 by oft-repeated unions with each other the venules form veins, 

 which convey the blood back to the heart from which it started. 



The tissues may be said to be irrigated by the blood which 

 flows within the capillaries, through the delicate. walls of which 

 some of the plasma of the blood exudes and collects in the 

 crevices and in the spaces between the connective tissue strands. 

 The fluid that thus exudes, which contains less albumen than 

 blood plasma and some products of the waste of the tissues, 

 forms the lymph, and the crevices are the so-called lymph- 

 rootlets. From the lymph-rootlets the lymph passes into fine 

 tubes with endothelial (epithelioid) lining called lymph capil- 

 laries ; the larger vessels which receive the lymph from these 

 capillaries are the lymphatics. In the course of the lymphatics 

 are the lymphatic glands. The lymphatics in the lower verte- 

 brates are not infrequently in connection with large lymph-spaces, 



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