132 



ELEMENTARY PHYSIOLOGY 



finger along one of these in a direction opposite to the course of 

 the blood we cause the blood to flow backward on the valves, thus 

 filling them out, stopping the vein, causing it to swell out and 

 present a knotted appearance. 



If we thus prevent the blood from passing through one of the 

 veins of the arm, we do not interfere with the general circulation, 

 as. the veins are connected with each other by numerous branches, 

 and thus the blood can readily take another course ; and, the 

 total capacity of the veins being much greater than that of the 



Fig. 124. Vein with Valves 

 open. 



Fig. 125. Vein with Valves closed ; 

 the stream of blood passing off by 

 a lateral branch. 



arteries, they are capable of conveying to the heart much more 

 blood in a given time than they receive. 



Some veins have no valves. Among these may be mentioned 

 the venae, cavce, the pulmonary "veins, the portal vein which supplies 

 the liver, the hepatic vein (Gr. hepar, the liver) which takes blood 

 from the liver, and the renal veins (Lat. renes, kidneys) which 

 lead from the kidneys to the vena cava inferior. 



Having noticed the chief characters of the different blood- 

 vessels, we now turn our attention to the action of the heart, 

 commencing for convenience at that period immediately following 

 the contraction of the ventricles, and during which the heart is 

 passive. The cavities now dilate, and the blood flows freely from 

 che venae cavce and the pulmonary veins into the right and left 



