142 



THE HUMAN MECHANISM 



called in hydraulics) the valves prevent, and the ventricles 

 again fill through the only open channel, that is, the one 

 leading from the great veins and the auricles. Thus by 

 contractions rhythmically repeated the heart continues to 

 spout or deliver blood from the two sets of great veins into 

 the two sets of great arteries. It is plainly a double force 

 pump or, better, a pair of force pumps lying and working 

 side by side. 



9. The arterial and the venous reservoirs. To understand 

 the exact nature and result of the work of the heart we must 



now consider the rela- 

 tion of this living pump 

 to the pipe system (ar- 

 teries, capillaries, and 

 veins) with which it 

 is connected. The stu- 

 dent should first trace 

 the general course of 

 the circulation in the 

 simple diagrammatic 

 representation given in 

 Fig. 70. This shows 

 that the blood which 

 enters the aorta from 

 the left ventricle must 

 return to the right side of the heart and pass through the 

 lungs before it can again reach the aorta. As the physical 

 principles of the circulation are the same for the systemic 

 and the pulmonary vessels, we shall confine our attention 

 to the former. 



In the first place, we may observe that the heart pumps the 

 blood into what is practically a large reservoir (the larger 

 arteries) and that the blood flows from this reservoir to a sec- 

 ond reservoir (the larger veins) by various routes ; for the ves- 

 sels of the different organs represent many alternative courses 



FIG. 69. The force-pump action of a ventricle 

 of the heart 



On the left is shown the condition during dias- 

 tole; on the right, during systole 



