304 



THE MAINTENANCE OF THE INDIVIDUAL 



comes into contact with the tissues and serves as the go-between for 

 the blood and cells. Lymph gradually flows from the lymph spaces 

 into lymph capillaries, which in turn unite to form larger and larger 

 lymph vessels, interspersed with numerous lymph glands and lymph 



nodes. Finally the lymph vessels unite into a 

 large thoracic duct emptying into the jugular 

 vein in the neck region. 



The Conduits — Arteries, Veins, and 

 Capillaries 



Having considered the "stuff" that blood is 

 made of, we can now turn to a consideration 

 of the vessels through which it passes. The 

 chief function of the capillaries centers about 

 the exchange of the products of metabolism 

 with the lymph. Some of the plasma of the 

 blood actually transudes through the walls of 

 the capillaries, while certain types of leuco- 

 cytes also pass through the walls, which are 

 composed of nothing more or less than a 

 single-celled layer of epithehal cells, called 

 endothelium. 



Distinct structural differences exist between 

 the capillaries and the arteries and veins of all 

 vertebrates. Both arteries and veins are cov- 

 ered externally by a rough protective coat of 

 connective tissue. Between this and the inner 

 endothelial lining lies a layer of elastic mus- 

 cular fibers. -In veins, this layer is relatively 

 thin, while in the arteries, it is quite well de- 



Principal lymph chan- 

 nels of man. Note the 

 abundance of lymph ves- 

 sels in the region of the 

 intestine. What function 

 do they serve ? Find the 

 thoracic duct emptying 

 into the jugular vein. 



veloped, probably being correlated with the 

 greater pressure to which arteries are subjected as evidenced by the 

 periodic spurting of blood whenever an artery is cut. 



In the veins blood is prevented from flowing back away from the 

 heart by a series of cuplike valves that open in the direction of the 

 blood-flow toward the heart but which close when the reversed move- 

 ment is attempted. They are quite similar to the semilunar valves 

 of the heart (page 308). 



Veins collapse when cut while arteries do not. This fact proved 

 a stumbling block to the proper interpretation of the anatomy and 



