THE CIRCULATORY OR VASCULAR SYSTEM I 19 



There is no pulsation in veins, as there is in the arteries, 

 hence the circulation in them depends upon a different 

 mechanism. They are supplied with numerous valves 

 (Fig. 60), which prevent any reflux of blood, and close in 

 such a manner that the blood can flow only toward the 

 heart. 



Physiologists are not yet in accord regarding all the theo- 

 ries that have been proposed to account for the grand 

 principle of venous circulation, or the power of sustaining 

 it. The valvular arrangement just mentioned at once 

 suggests that an alternate 

 contraction and relaxa- 

 tion is exercised upon 

 them; such action, we 

 can readily perceive, is 

 afforded to some extent 

 by the movements of the 

 muscles, but this action 

 is not sufficiently regular 

 to account for the regu- 

 larity of the venous cir- 

 culation. Another action, 

 however, that is more 

 regular and universal, is 

 that of pulsation of the 

 arteries. The swell of the 



arteries in the diastolic motion produces great pressure 

 in every part of the system. We have a very striking 

 example of this in the throbbing pain in the head in 

 some forms of fever, or in cases of inflammation, especi- 

 ally when the parts are much swollen. 



Thus the impulse of the heart and arteries not only 

 propels the blood through the arteries, but tends indi- 

 rectly also to force it forward in the veins. 



There is, however, another physical principle which 



FlG. 60. Diagram of the valves of 

 veins (Morrow). 



