THE BLOOD AND ITS CIRCULATION 



375 



a, Function and Structure of the Veins. — If the arteries are supply 

 pipes which convey fluid food to the tissues, then the veins may 

 be likened to drain pipes which carry away waste 

 material from the tissues. Extremely numerous in 

 the extremities and in the muscles and among other 

 tissues of the body, they, like the branches of a tree, 

 become larger and unite with each other as they 

 approach the heart. 



If the wall of a vein is carefully examined, it will 

 be found to be neither so thick nor so tough as an 

 artery wall. When empty, a vein collapses; the 

 wall of an artery holds its shape. If you hold your 

 hand downward for a little time and then examine 

 it, you will find that the veins, which are relatively 

 much nearer the surface than are the arteries, appear 

 to be very much knotted. This appearance is due 

 to the presence of tiny valves within. These valves 

 open in the direction of the blood current, but would 

 close if the direction of the blood flow should be re- 

 versed (as in case a deep cut severed a vein). As 

 the pressure of blood in the veins is much less than 

 in the arteries, the valves thus aid in keeping the 

 flow of blood in the veins toward the heart. The 

 higher pressure in arteries and the suction in the \'eins (caused by 

 the enlargement of the chest ca\'ity in breathing) are the chief 

 factors which cause a steady flow of blood through the veins in 

 the body. 



Valves in a 

 vein. No- 

 tice the thin 

 walls of the 

 vein. 



Problem L. S(jiii.c rlbaii^es in the composition of the blood- 

 {Laboratory Manual, Prob. L.) 



[ Function of Lymph. — Different tissues and organs of the body 

 are traversed by a network of tubes which carry the blood. Inside 

 these tubes is the blood proper, consisting of a fluid plasma, the 

 colorless corpuscles, and the red corpuscles. Outside the blood 

 tubes, in spaces between the cells which form tissues, is found 

 another fluid, which is in chemical composition very much like 

 plasma of the blood. This is the hnnph. It is, in fact, fluid food 

 in which some colorless amoeboid corpuscles are found. Blood gives 



