PHYSIOLOGY OF THE CIRCULATION. 71 



would be obtained were the arterial blood really to 

 flow from the wide to the contracted portion of a 

 tube. 



The arrangement of the system of arterial tubes is 

 therefore such as to present greater facilities for the 

 entrance of blood, at one end, from the heart into 

 the aorta, than for its discharge at the other, from 

 the small arteries and capillaries into the veins. 



And consequently, if the ventricle invariably ex- 

 pelled the whole of its contents at each contraction, 

 there would, during an excited action of the heart, 

 be so great a disproportion between the rates of 

 influx and efflux of the arterial blood, as to render a 

 rupture of some of the containing vessels almost ine- 

 vitable. But it appears to me that this evil is to a 

 certain extent guarded against by the following sim- 

 ple provision viz. that the opposition to the entrance 

 of fresh fluid from the left ventricle into the aorta 

 increases in a direct ratio to the amount of dispro- 

 portion between the rates of influx and efflux of the 

 arterial blood. This opposition to the flow of blood 

 from the heart arises from the backward pressure of 

 the mass of aortic blood, which, after distending the 

 arterial walls, is by their reaction driven back upon 

 the aortic orifice of the ventricle. And as the extent 

 to which the aortic walls are distended is, cteteris 

 paribus, proportioned to the quantity of blood de- 

 tained in it at the time, and as the quantity of this 

 blood is again wholly dependent upon the prepon- 

 derance of the influx over the efflux of arterial 

 blood, so it follows that whatever increases the dis- 

 proportion between the rates of influx and efflux of 



p 4 



