THE TRANSPORTING SYSTEM OF THE BODY 



263 



arteries approach the surface. The normal rate at which 

 the heart beats may be affected in various ways. It quickens 

 with excitement and exercise and slows down during sleep or 

 even when one lies down. 



224. Circulation of the Blood. The heart is practically two 

 pumps in one, the right half being concerned with pumping 

 the blood to the lungs and 

 the left half sending it on its 

 tour through the body. The 

 blood returns from the body 

 through two veins, the vence 

 cavce, and enters the right 

 upper chamber, the right 

 auricle. From this it de- 

 scends into another chamber, 

 the right ventricle, which con- 

 tracts much like the bulb of 

 an atomizer and forces the 

 blood through the lungs. 

 From the lungs it flows back 

 to the heart, entering the left 

 auricle and flowing into the 

 left ventricle whence it is again 

 forced out through the body. 

 At the point where the aorta 

 leaves the heart, a set of valves 

 prevents the blood from flow- 

 ing back to the heart. There are no other valves in the 

 arteries. The walls of the arteries, being elastic, dilate as each 

 successive heart beat adds more blood to their store and, while 

 the heart is resting for another beat, contract and force the 

 blood along to the capillaries. In old age and in certain 

 diseases, the arteries lose some of their elasticity and thus fail to 

 expand as more blood is forced into them, causing the blood 



FIG. 91. The heart. 



VEINS 



(After Bundy.} 



