CHAP, iv.] THE VASCULAR MECHANISM. 



217 



resistance and drives the fluid more and more rapidly through 

 the peripheral region. At last the arterial system is so distended, 



V 1 



FIG. 31*. TRACINGS TAKEN FROM AN ARTIFICIAL SCHEME WITH THE PERIPHERAL 

 RESISTANCE CONSIDERABLE. 



A', Arterial, F', Venous Manometer. 



and the force of the elastic reaction so great, that during the stroke 

 and the succeeding interval just as much fluid passes through the 

 peripheral region as enters the arteries at the stroke. In other 

 words, the repeated strokes have established a mean arterial pres- 

 sure which at the point where the manometer is affixed is raised 

 slightly at each ventricular stroke and falls slightly between the 

 strokes. 



Turning now to the venous manometer, Fig. 31* V, we ob- 

 serve that each stroke of the pump produces on this much less 

 effect than it did before the introduction of the increased peri- 

 pheral resistance. The mercury, instead of distinctly rising and 

 falling at each stroke, now shews nothing more than very gentle 

 undulations ; it feels to a very slight degree only the direct effect 

 of the ventricular stroke; it is simply raised slightly above the 

 base line and remains fairly steady at this level. The slight rise 

 marks the mean pressure exerted by the fluid at the place of 

 attachment of the manometer. This mean ' venous ' pressure is a 

 continuation of the mean arterial pressure so obvious in the arterial 

 manometer, but is much less than that because a large part of the 

 arterial mean pressure has been expended in driving the fluid past 



