22(5 THE HUMAN BODY. 



which, though thousands of times smaller than the great 

 arteries and veins, are millions of times more numerous. 

 The channel through which the blood flows in them is, 

 therefore, when they are all taken together, very much 

 greater than that to which it is confined in the large arterial 

 and venous trunks. 



Why there is no Pulse in the Capillaries and Veins. 

 The heart sends blood into the arteries not steadily but 

 intermittently; each beat forces in some blood, and then 

 comes a pause before the next beat. Accordingly the flow 

 in the larger arteries is not even and continuous, but jerky, 

 as indicated by the pulse. 



But in the capillaries the flow is quite steady, and yet the 

 capillaries are supplied by the smaller arteries. We have to 

 inquire how this is brought about. 



The disappearance of the pulse is due to two things, (1) 

 the fact that in the tiny capillaries the blood meets with 

 considerable resistance to its flow, dependent on friction, 

 and (2) that the arteries are very elastic. 



On account of friction in the capillaries the arteries have 

 difficulty in passing on blood through them; blood there- 

 fore accumulates in the aorta and its large branches and 

 stretches their elastic walls. The stretched arteries press 

 all the time on the blood inside them, and constantly keep 

 squeezing it on into the small arteries and the capillaries; 



How in number? Is the total blood channel greater in arteries or 

 capillaries? In veins or capillaries? 



Why is the blood-flow in the great arteries not steady? Name 

 vessels in which it is steady. 



To what is the loss of pulse in the capillaries due? What results 

 from friction in the capillaries? What is done by the stretched 

 arteries? 



water is carried off by the river Rhone. In the comparatively narrow inflowing 

 and outflowing rivers the current is rapid ; in the wide bed of the lake it is much 

 slower. 



