342 A YEAR IN SCIENCE 



they can not receive more blood. During this time 

 blood is entering the auricles which serve as reservoirs. 

 Blood and lymph vessels. When the heart beats, the 

 blood is forced with considerable pressure into vessels 

 which we have called arteries. In order to withstand 

 this pressure, arteries must have thick, elastic walls. 

 After the blood has passed through the arteries and 

 before the heart beats again, the arteries spring back 

 to their normal size. 



Vein. Artery 



Fig. 161. Cross section of an artery and of a vein. 



Arteries generally are not near the surface of the 

 body. In a few places, however, such as the wrist, the 

 temples, and on the under side of the knees arteries 

 are near enough the surface so that the expansion of 

 their walls following each heart beat can be felt. This 

 we call the pulse. If we trace the arteries away from 

 the heart, we find that each one divides and subdivides 

 a great many times so that gradually the branches become 

 smaller and smaller. At the same time the walls 

 become thinner. Finally very small tubes with very 

 thin walls are left, called capillaries. We can not prick 

 any spot on the skin with a needle without causing 



