CHAP, iv.] THE VASCULAR MECHANISM. 227 



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 proper characters of the pulse ; these are seen more 

 distinctly in the middle curve with a medium pressure. 



124. It will be observed that in Fig. 59, curve I., which is 

 nearer the pump, rises more rapidly and rises higher than curve II., 

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

 farther 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 resist- 

 ance, 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 neighbourhood 

 of the artificial capillaries, there was hardly any trace of it left. 

 In other words, we might trace out step by step the gradual 

 disappearance of the pulse. 



The same changes, the same gradual lowering and flattening 

 of the curve, may be seen in natural pulse tracings ; compare, for 

 instance, Fig. 62, which is a trac- 

 ing from the dorsalis pedis artery, 

 with the tracing from the radial 

 artery Fig. 61, taken from the 

 same individual with the same . 

 instrument on the same occasion. 

 This feature is, of course, not ob- FIG. 62. PULSE TRACING FROM DOR- 



vious in all pulse-curves taken ^^l^ 3 

 from different individuals with 



different instruments and under varied circumstances ; but if 

 a series of curves from different arteries were carefully taken 

 under the same conditions, it would be found that the aortic 

 tracing is higher and more sudden than the carotid tracing, 

 which again is higher and more sudden than the radial tracing, 

 the tibial tracing being in turn still lower and more flattened. 

 The pulse-curve dies out by becoming lower and lower, and more 

 and more flattened out. 



And a little consideration will shew us that this must be so. 

 The systole of the ventricle drives' a quantity of blood into the 

 already full aorta. The sudden injection of this quantity of blood 

 expands the portion of the aorta next to the heart, the part 

 immediately adjacent to the semilunar valves beginning to expand 

 first, and the expansion travelling thence on to the end of this 

 portion. In the same way the expansion travels on from this 

 portion through all the succeeding portions of the arterial system. 



