218 ANATOMY FOR NURSES [Chap. XII 



and more rigid, and become less well adapted for the unceasing 

 work they are called upon to perform. This condition is kncnvn 

 as arteriosclerosis. 



(3) The position and direction of the valves. — The position 

 and direction of the tricuspid and bicuspid valves permit the 

 ventricles to fill during their period of relaxation, and prevent the 

 backward flow when they are emptying themselves. The aortic 

 and pulmonary valves allow blood to pass out during each con- 

 traction of the ventricles, but not to return from either artery into 

 the heart when the resting stage ensues. The valves also show 

 signs of age as years advance, and even if not injured by disease, 

 do not adjust themselves so perfectly as in early life. 



(4) The large number and small bore of the capillaries. — The 

 total number of capillaries is so immense tiiat, e\en though each 

 individual one is small, their total capacity is several hundred 

 times greater than that of the aorta. This is due to the fact that 

 each time an artery divides, the total capacity of its branches is 

 greater than that of the parent artery, although each of the indi- 

 vidual branches is of smaller calibre. In this way the bed of the 

 blood stream is becoming greater until we reach the capillaries, 

 and then it is increased enormously. The eft'ect of this arrange- 

 ment is to make the blood flow rapidly through the larger arteries, 

 more slowly in the smaller and more numerous arteries, and 

 slowest of all in the capillaries. 



(5) The large size and non-contractile character of the walls of 

 the veins. — In the case of the veins the conditions we have 

 described are reversed. The size and capacity of a vein is always 

 less than the size and capacity of its tributaries, hence the total 

 bed of the blood stream is becoming continually smaller but never 

 so small as in the corresponding arteries. A vein is always twice 

 the size, often more than twice the size, of the corresponding artery. 

 On leaving the capillaries the blood flows faster because the bed 

 of the stream becomes narrower, but its speed in a vein is little 

 more than half that in a corresponding artery, because the bed is 

 twice as great. The veins contain very little elastic tissue in 

 their walls, hence they are not capable of distention and con- 

 traction as the arteries are. As the blood flows in a steady stream, 

 and the diameter of the veins is large, the non-contractile character 

 of the walls offers no resistance to the flow of blood toward the 



