180 MAIN FEATURES OF CIRCULATION. [BOOK i. 



thrown into the aorta by the ventricular stroke. We have seen 

 further that in the large arteries at each stroke the pressure 

 rises arid falls a little above and below the mean, thus constituting 

 the pulse, but that this extra distension with its subsequent recoil 

 diminishes along the arterial tract and finally vanishes ; it dimin- 

 ishes and vanishes because it, too, like the whole force of the 

 ventricular stroke, of a fraction of which it is the expression, is used 

 up in establishing the mean pressure ; we shall, however, consider 

 again later on the special features of this pulse. We nave seen 

 further that the task of driving the blood through the peripheral 

 resistance of the small arteries and capillaries consumes much of 

 this mean pressure, which consequently is much less in the small 

 veins than in the corresponding small arteries, but that sufficient 

 remains to drive the blood, even without the help of the auxiliary 

 agents which are generally in action, from the small veins right 

 back to the auricle. Lastly, we have seen that while the above 

 is the cause of the flow from ventricle to auricle, the changing 

 rate of the flow, the diminishing swiftness in the arteries, the 

 sluggish crawl through the capillaries, the increasing quickness 

 through the veins are determined by the changing width of the 

 vascular ' bed.' 



Before we proceed to consider any further details as to the 

 phenomena of the flow through the vessels, we must turn aside to 

 study the heart 



