ARTERIES AND VEINS 



375 



very strong, they are elastic. They expand as the blood 

 stream increases in size, and contract as it decreases. 

 Where arteries come near the surface of the body, the 



wave of blood sent out at each ^ 



contraction of the left ventricle 

 can be felt as the pulse. The 

 pulse reading is usually taken at 

 the wrist. Since the arteries 

 are elastic, they act as exten- 

 sions of the ventricles, keeping 

 the blood under pressure even 

 when the ventricles are not con- 

 tracted. They can do this be- 

 cause of the heart valves, which 

 prevent a flow of the blood back- 

 ward into the ventricles. 



Veins have thinner walls than 

 arteries, and are much less elas- 

 tic. Blood from the body circu- 

 lation enters the right auricle 

 through several large veins; 

 other veins bring blood from the 

 lungs to the left auricle. Blood 

 enters the auricles " by suction" 

 (cf. 42), much as lemonade 

 enters a straw when you remove 

 the air. When the auricle contracts, it forces its blood 

 into a ventricle. When the auricle is relaxed, its cavity 

 increases in volume, and the pressure in the cavity de- 

 creases. Blood cannot flow back into the auricle from 

 the ventricle, because the heart's valves prevent this. So 



FIG. 285. 



Heart, Lung, and Body Circula- 

 tion. Note the course taken by 

 the blood. 



