THE CIRCULATION. 167 



most important function. Except for this reservoir in 

 which the venous blood may temporarily accumulate there 

 would have been quite a marked stagnation interfering 

 materially with the progress of the venous stream, but by 

 this means the blood flows on into the heart uninterruptedly 

 while the ventricles are beating, and by the time the auricles 

 are filled the ventricles have finished their beat, they relax 

 and the accumulated blood of the auricles drops at once 

 into the emptied ventricles. 



The Contraction of the Ventricles. Sounds of the Heart. 



At the beginning and again at the close of the ventricu- 

 lar systole the familiar sounds of the heart occur. The 

 first sound is somewhat dull and represented by the syllable 

 ( ' lubb." It was at first supposed to be due to the closing 

 of the auriculo- ventricular valves, but this is not true. 

 These valves are closed before the systole begins, and in 

 experiments these valves have actually been pinned back, 

 and yet the sound occurred. Still better evidence that the 

 first sound is not due to the closing of these valves lies in 

 the fact that when an emptied heart is made to contract the 

 sound still occurs. While not fully understood, there seems 

 no doubt but that this sound is partially due to the sudden 

 tension of the muscular walls of the ventricles. The ven- 

 tricular walls becoming suddenly very much stretched and 

 taut, produce a series of vibrations which we recognize as 

 sound. A not very close analogy may be cited in the case 

 of an ordinary guitar string. If such a string at first hang- 

 ing loosely be suddenly as with a jerk tightened, it will be 

 set into vibration and sound just as if it had been pulled by 

 the fingers. That this sound is due to the tension of the 

 muscles seems corroborated by the fact that the sound lasts 

 during the entire systole. That is, it continues as long as 

 the ventricular walls are taut. 



Towards the close of the systole is heard the second 

 sound. This is shorter and louder, and is plainest in the 

 regions of the aorta and pulmonary artery. It has been 



