4 8 



THE VITAL PROCESSES 



FIG. 19. Ar- 

 tery dissected to 

 show the coats. 



tissue and is also much thicker and stronger 

 in the arteries than in the veins. 



Marked differences exist between the 

 arteries and the veins, and these vessels 

 are readily distinguished from each other. 

 The walls of the arteries are much thicker 

 and heavier than those of the veins (Fig. 

 19). As a result these tubes stand open 

 when empty, whereas the veins collapse. 

 The arteries also are highly elastic, while 

 the veins are but slightly elastic. On the 

 other hand, many of the veins contain 

 valves, formed by folds in the inner coat 

 (Fig. 20), while the arteries have no 

 valves. The blood flows more rapidly through the arte- 

 ries than through the veins, the difference being due to 

 the fact that the system of veins has 

 a greater capacity than the system of 

 arteries. 



Why the Arteries are Elastic. The 

 elasticity of the arteries serves a twofold 

 purpose. It keeps the arteries from 

 bursting when the blood is forced into 

 them from the ventricles, and it is a 

 means of supplying pressure to the blood 

 while the ventricles are in a condition of 

 relaxation. The latter purpose is accom- 

 plished as follows : 



Contraction of the ventricles fills the FIG. 20. Vein 

 arteries overfull, causing them to swell *P 1!t open to show 



, , the valves. 



out and make room for the excess of 



blood. Then while the ventricles are resting and filling, 



the stretched arteries press upon the blood to keep it 



