CIRCULATION- OF THE NVTBISHTTS 117 



that their walls have far less muscular and elastic tissue than 

 have the walls of arteries. Veins, however, are provided with 

 valves shaped much hke the valves at the mouth of the large 

 arteries leading from the heart. The blood can flow toward 

 the heart, but as soon as it begins to pass in the opposite 

 direction, these valves are immediately filled and thus the 

 passage is obstructed (Fig. 37). 



V. Circulation of the Blood 



167. Course of the blood through the body. — Having 

 completed our survey of the structure and action of the heart 

 and the blood vessels, we are ready to study the blood system 

 as a whole and to learn how the blood goes to, through, and 

 from, the organs of the body. Let us now follow the course 

 of the blood from the time it leaves the left ventricle until 

 it again returns to this chamber of the heart. When the 

 left ventricle contracts, the blood is forced out into the largest 

 artery of the body, which is known as the aorta. This blood 

 vessel forms an arch (Fig. 38) from the upper portion of 

 which branches extend to the head and the arms. The aorta 

 then continues downward through the chest and abdominal 

 cavities, supplying on its way tte various organs in these 

 regions. It then divides into two arteries that continue down 

 the legs. Each of these larger arteries that we have men- 

 tioned divides again and again, until finally the blood is forced 

 through a network of very fine capillaries in the various 

 organs to which the arteries extend. 



From iihese capillaries blood passes into tiny veins which 

 carry all the blood into two large veins, one from the 

 upper part of the body, the other from the lower part of 

 the body ; and these two veins finally empty into the right 

 auricle of the heart. Thence the blood passes into the right 

 ventricle. 



