170 ARTIFICIAL MODEL. [BooK i. 



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 shew the immediate effect of each heart beat. 



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

 arterial system, one near the pump and the other farther off, close 

 to the peripheral resistance, the pressure shewn by the near 

 manometer would be found to be greater than that shewn 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 closed to the peripheral region would shew 

 a higher pressure than that shewn by one farther off, because it is 

 the pressure still remaining 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 venae 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 shews, 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 mano- 

 meter or by the finger, provided that the peripheral resistance be 

 adequate. If the peripheral resistance be diminished, as by 

 unscrewing the clamps, then, as necessarily 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 



