THE MAIN FACTS OF THE CIRCULATION. 149 



is affixed, is raised slightly at each ventricular stroke, and falls slightly 

 between the strokes. 



Turning now to the venous manometer, Fig. 44, V 1 , we observe that each 

 stroke of the pump produces on this much less effect than it did before the 

 introduction of the increased peripheral resistance. The mercury, instead of 

 distinctly rising and falling at each stroke, now shows 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 manom- 

 eter. This mean " venous " pressure is a continuation of the mean arterial 

 pressure so obvious in the arterial manometer, but is much less than that be- 

 cause a large part of the arterial mean pressure has been expended in driv- 

 ing the fluid past the peripheral resistance. What remains is, however, 

 sufficient to drive the fluid along the wide venous tubing right to the open 

 end. 



Thus this artificial model may be made to illustrate how it comes about 

 that the blood flows in the arteries at a relatively high pressure, which at 

 each ventricular systole is raised slightly above and at each diastole falls 

 slightly below a certain mean level, and flows in the veins at a much lower 

 pressure, which does not show the immediate effects of each heart-beat. 



If two manometers, instead of one, were attached to the arterial system, 

 one near the pump and the other further off, close to the peripheral resist- 

 ance, the pressure shown by the near manometer would be found to be greater 

 than that shown by the far one. The pressure at the far point is less 

 because some of the pressure exerted at the near point has been used to drive 

 the fluid from the near point to the far one. Similarly on the venous side, a 

 manometer placed close to the peripheral region would show a higher pres- 

 sure than that shown by one further off, because it is the pressure still remain- 

 ing in the veins near the capillaries which, assisted, as we shall see, by other 

 events, drives the blood onward to the larger veins. The blood-pressure is 

 at its highest at the root of the aorta and at its lowest at the mouths of the 

 vena3 cavse, and is falling all the way from one point to the other, because 

 all the way it is being used up to move the blood from one point to the 

 other. The great drop of pressure is, as we have said, in the peripheral 

 region, because more work has to be done in driving the blood through this 

 region than in driving the blood from the heart to this region, or from this 

 region to the heart. 



The 'manometer on the arterial side of the model shows, as we have seen, 

 an oscillation of pressure, a pulse due to each heart-beat, and the same pulse 

 may be felt by placing a finger, or rendered visible by placing a light lever, 

 on the arterial tube. It may further be seen that this pulse is most marked 

 nearest the pump, and becomes fainter as we pass to the periphery ; but we 

 must reserve the features of the pulse for a special study. On the venous 

 side of the model no pulse can be detected by the manometer or by the 

 finger, provided that the peripheral resistance be adequate. If the periph- 

 eral resistance be diminished, as by unscrewing the clamps, then, as neces- 

 sarily follows from what has gone before, the pulse passes over on to the 

 venous side ; and, as we shall have occasion to point out later on, in the 

 living organism the peripheral resistance in particular areas may be at 

 times so much lessened that a distinct pulsation appears in the veins. 



If in the model, when the pump is in full swing, and arterial pressure well 

 established, the arterial tube be pricked or cut, or the small side tube a 

 be opened, the water will gush out in jets, as does blood from a cut artery in 

 the living body ; whereas, if the venous tube be similarly pricked or cut, or 



