142 ADVANCE!) PHYSIOLOGY 



blood rushes into the ventricle from the auricle, the curtain* 

 like valves lie in line with the current, just as a flag flies out 

 in the current of the wind. Although the wind may be steady, 

 the flag "flaps" from side to side. So, in the heart, we can 

 imagine the mitral and tricuspid valves wavering in the cur- 

 rent of blood, even if it is steady. This is thought to be the 

 cause of the first sound. 



The second sound, following closely on the first, is sharper 

 and shorter. It is believed to be caused by the closing of the 

 semilunar valves in the large outgoing arteries. These, it is 

 thought, are thrown into their tense, filled condition so sud- 

 denly that they give rise to a distinct impact and noise, just 

 as doors do when they suddenly close in a current of wind, 

 even though they may be fitted with appliances for pre- 

 venting their actual "slamming." 



The Throb at the Breast, and the Pulse. When the heart 

 beats, the apex is turned distinctly forward with sufficient 

 force to lift the body wall between the fifth and sixth ribs, 

 and at this point a throb is easily felt. Every time the heart 

 beats a small amount of blood is forced into the arteries. 

 This fills those near the heart more full of blood than else- 

 where; consequently a wave of pressure travels rapidly along, 

 causing a slight swelling of the arteries on its way. If the 

 fingers be placed on the artery of the wrist, where it comes 

 near the surface, this wave can be felt, and is called the pulse. 

 The pulse is due to a temporary increase in the diameter of 

 the artery, and is not as it seems to be, a little jet of blood 

 flowing through the artery at the point where the pulse is 

 felt. The pulse can be felt in any of the arteries which come 

 near the surface; but as arteries are commonly deeply im- 

 bedded in the muscles, there are only a few places aside from 

 the wrist where it is evident, e. g. the neck, and about the 

 temple, back of the eye. By feeling the pulse a physician 

 can determine the rate of the heart beat as well as its 

 force. The normal rate is 72 per minute. 



