CHAPTER X. 



THE VASCULAR SYSTEM CONTINUED: ARTERIAL DISTRIBUTION 

 AND VENOUS RETURN. 



The arteries. The arteries, which carry and regulate the 

 supply of blood from the heart to the capillaries, are distributed 

 throughout the body in a systematic manner, and before taking 

 up the circulation we must try to gain a general idea of this 

 system of distribution, in order that we may be able to locate 

 the position of these important vessels. The arteries usually 

 occupy protected situations, that they may be exposed as little 

 as possible to accidental injury. As they proceed in their 

 course they divide into branches, the division taking place in 

 different ways. An artery may at once resolve itself into two 

 or more branches, no one of which greatly exceeds the rest in 

 size ; or it may give off several branches in succession, and still 

 maintain its character as a trunk. An artery, after a branch 

 has gone off from it, is smaller than before, but usually con- 

 tinues uniform in diameter until the next secession. A branch 

 of an artery is less in diameter than the trunk from which it 

 springs, but the collective capacity of all the branches into 

 which an artery divides is greater than the parent vessel. Since 

 the area of the arterial system increases as its vessels divide, it 

 is evident that the collective capacity of the smaller vessels 

 and capillaries must be greater than the collective capacity of 

 the trunks from which they arise. As the same rule applies to 

 the veins, it follows that the arterial and venous systems may 

 be represented, as regards capacity, by two blunt cones whose 

 apices are at the heart, and whose bases are united in the cap- 

 illary system. The effect of this arrangement of the circulatory 

 vessels is to make the blood flow more slowly as it passes 

 through the more widely distributed vessels, and to accelerate 

 its speed in the larger and less numerous trunks, just as the 

 water of a river flows more rapidly through its narrow chan- 

 nels, and lingers in those that are broad. 



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