x CIRCULATION OF THE BLOOD 139 



10. The blood flows from the right ventricle to the lungs, 

 where it is oxygenated ; then it returns to the left heart, from 

 which it is forced out into the aorta that distributes it by means 

 of its numerous branches to all the capillaries of the body. 

 From the capillaries it enters the veins and is brought back to 

 the starting point. 



11. The blood completes one entire circulation in about 

 twenty-two seconds. The rate of flow is fastest in the aorta 

 and arteries, slower in the veins, and slowest in the capillaries. 



12. The chief function of the heart is to pump blood into 

 the aorta under an immense pressure that causes the walls of 

 the aorta to bulge out greatly. These elastic walls contract 

 and force the blood onward in its course. 



13. The heart beats about seventy- two times per minute in 

 the adult. The two auricles work together and then the two 

 ventricles. In this way the heart rests a little more than half 

 the time when beating at the normal rate. 



14. The first sound of the heart is long and dull, and occurs 

 when the ventricles contract. The second is short and sharp ; 

 it occurs when the semilunar valves close. 



15. The walls of the arteries are elastic, like rubber tubes, 

 and distend when blood is forced into them by the ventricles. 



1 6. Alcohol acts on the red corpuscles, causing them to 

 shrink. It acts upon the walls of the arteries and the heart, 

 causing fatty degeneration, and in this way weakening them. 

 Alcohol and tobacco both have a -specific action upon the 

 nerves that control the size of the blood vessels and the action 

 of the heart. 



