BLOOD FLOW IN THE CAPILLARIES. 225 



with the blood flowing through them, comes into view 

 (Fig. 61). The arteries, a, are readily recognized by the 

 fact that the flow in them is Jastest and from larger to 

 smaller branches. The smallest are seen to end in capillar- 

 ies, which form networks, the channels of which are all . 

 nearly equal in size. In the veins arising from the capil- 

 laries the flow is from smaller to larger trunks, and slower 

 than in the arteries, but faster than in the capillaries. 



Why the Blood flows slowest in the Capillaries. The 

 reason of the slower flow of he capillaries is that their 

 united area is considerably greater than that of the arteries 

 supplying them, so that the same quantity of blood flowing 

 through them in a given time has a wider channel to flow 

 in and therefore moves more slowly. The area of the veins 

 is smaller than that of all the capillaries, but greater than 

 that of the arteries, and so the rate of movement in them is 

 intermediate. 



We may picture to ourselves the vascular system as a 

 double cone, widening from the ventricles to the capillaries, 

 and narrowing from the latter to the auricles. Just as 

 water forced in at a narrow end of this would flow quickest 

 there, slowest at the widest part, and quicker again where 

 it passed out the other narrow end, so the blood flows quick 

 in the aorta and hollow veins,* and slow in the capillaries, 



How may the arteries be recognized? In what are the smallest 

 arteries seen to end? Do the capillaries vary much in size? What 

 is the direction of flow in the veins? How does its rate differ 

 from that in the arteries? From that in the capillaries? 



Why does blood flow slowest through the capillaries? Why in the 

 veins quicker than in the capillaries, but slower than in the arteries? 



How may we picture the vascular system? Illustrate. How do 

 capillaries differ in size from the large arteries? 



* A good illustration taken from physical geography is afforded by the Lake of 

 Geneva, in Switzerland. This is supplied at one end by a river which derives its 

 water from the melting glaciers of some of the Alps. From its other end the 



