144 



THE CIRCULATION OF THE BLOOD 



The veins continuously increase in size as they diminish 

 in number, until finally there are but two large tubes 

 which receive the blood from all the others and pour 

 it into the right auricle of the heart. This second system 

 is known as the venous system. 



Blood flow in the veins. In structure, the veins 

 closely resemble the arteries, except that their walls are 

 much thinner, since they do not have to stand any con- 

 siderable pressure. The pressure which in the arteries 

 served to push the blood forward, is not ordinarily trans- 

 mitted to the veins, because it is largely blocked by the 

 contraction of the arterioles. If it were transmitted, the 

 delicate walls of the capillaries would be overstretched 



or even burst by the 

 high pressure of the ar- 

 terial system. The pro- 

 pulsion of the blood back 

 to the heart through the 

 veins must therefore be 

 accomplished by a dif- 

 ferent mechanism from 

 that which pushed it 

 from the heart through 

 the capillaries. The veins 

 have numerous valves 

 which are readily pushed 

 open by the blood flow- 

 ing from the capillaries 

 toward the heart but are 



closed against its return toward the capillaries. The 

 veins are thus converted into a series of short sections 

 separated by valves. Each section is emptied by the ex- 

 ternal pressure exerted by the muscles, the skin, and 

 the pulsations of the arteries which lie close to it. As a 



White 



corpuscle 

 Red corpuscle 



Red corpuscle 

 / |r caught at branch 



'lo. 85. Diagram showing the bending of 

 red blood corpuscles and the rolling of 

 white blood corpuscles in the capillaries 

 of a frog. 



