182 



THE VASCULAR MECHANISM. 



FIG. 



ABC 



Pulse-tracings from the same Ra- 

 dial Artery under Different Pressures 

 of the Lever. (The letters are ex- 

 plained in a later part of the text.) 



after the expanding power of the pump has ceased, is more gradual in its 

 action. 



These features, the suddenness of the rise or up-stroke, and the more 

 gradual slope of the fall or down-stroke, are seen also in natural pulse-curves 



taken from living arteries (Figs. 62, 66, etc.). 

 Indeed, the difference between the up-stroke 

 and the down-stroke is even more marked 

 in the latter than in the former, the delivery 

 of blood from the ventricle being more rapid 

 than the issue of water from a pump as ordi- 

 narily worked. 



It may here be noted that the actual size 

 of the curve, that is, the amount of excursion 

 of the lever, depends in part (as does also to 

 a great extent the form of the curve) on the 

 amount of pressure exerted by the lever on 

 the tube. If the lever only just touches the 

 tube in its expanded state, the rise will be 

 insignificant. If, on the other hand, the 

 lever be pressed down too firmly, the tube 

 beneath will not be able to expand as it 

 otherwise would, and the rise of the lever 

 will be proportionately diminished. There 

 is a certain pressure which must be exerted 

 by the lever on the tube, the exact amount depending on the expansive 

 power of the tubing and on the pressure exerted by the fluid in the tube, 

 in order that the tracing may be best marked. This is shown in Fig. 66 r in 

 which are given three tracings taken from the same radial artery with the 

 same instrument; in the lower curve the pressure of the lever is too great, 

 in the upper curve too small, to bring out the characters seen most distinctly 

 in the middle curve with a medium pressure. 



131. It will be observed that in Fig. 64, curve I., which is nearer the 

 pump, rises higher, and rises more rapidly than curve II., which is further 

 away from the pump ; that is to say, at the lever further away from the 

 pump the expansion is less and takes place more slowly than at the lever 

 nearer the pump. Similarly in curve IV. the rise is still less, and takes 

 place still less rapidly than in II., and the same change is seen still more 

 marked in V. as compared with IV. In fact, if a number of levers were 

 placed at equal distances along the arterial tubing of the model and the 

 model were working properly, with an adequate peripheral resistance, we 

 might trace out step by step how the expansion, as it travelled along the 

 tube, got less and less in amount and at the same time became more gradual 

 in its development, the curve becoming lower and more flattened out, until 

 in the neighborhood of the artificial capillaries there was hardly any trace 

 of it left. In other words, we might 

 trace out step by step the gradual dis- 

 appearance of the pulse. 



The same changes, the same gradual 

 lowering and flattening of the curve, 

 may be seen in natural pulse-tracings, 

 as for instance in Fig. 67, which is a 

 tracing from the dorsalis pedis artery, 

 compared with the tracing from the ra- 

 dial artery, Fig. 66, taken from the same individual with the same instru- 

 ment on the same occasion. This feature is, of course, not obvious in all 



FIG. 67. 



Pulse-tracing from Dorsalis Pedis, taken 

 from the same individual as Fig. 66. 



