CHAP, iv.] THE VASCULAR MECHANISM. 261 



pressure exerted by the fluid in the tube, in order that the 

 tracing may be best marked. This is shewn in Fig. 47 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. 



142. It will be observed that in Fig. 45 curve I., which is 

 nearer the pump, rises higher, and rises more rapidly 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 resis- 

 tance, 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, as for instance 

 in Fig. 48 which is a tracing from 

 the dorsalis pedis artery, compared * 9 



with the tracing from the radial ~/\J>__/\^_/\. ^_T\ 

 artery Fig. 47, taken from the 



same individual with the same . 



instrument on the same occasion. Fm 48> PULSE TRACING FBOM DoR . 

 This feature is of course not ob- SALTS PEDIS TAKEN FBOM THE SAME 

 vious in all pulse-curves taken INDIVIDUAL AS Fm. 47. 

 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 



