THE PULSE. 183 



pulse-curves taken 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 show 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. For the total 

 expansion required to make room for the new quantity of blood is not pro- 

 vided by that portion alone of the aorta into which the blood is actually 

 received ; it is supplied by the whole arterial system ; the old quantity of 

 blood which is replaced by the new in this first portion has to find room for 

 itself in the rest of the arterial space. As the expansion travels onward, 

 however, the increase of pressure which each portion transmits to the suc- 

 ceeding portion will be less than that which it received from the preceding 

 portion. For the whole increase of pressure due to the systole of the ven- 

 tricle has to be distributed over the whole of the arterial system ; the general 

 mean arterial pressure is, as we have seen, maintained by repeated systoles, 

 and any one systole has to make its contribution to that mean pressure ; the 

 increase of pressure which starts from the ventricle must, therefore, leave 

 behind at each stage of its progress a fraction of itself; that is to say, the 

 expansion is continually growing less, as the pulse travels from the heart 

 to the capillaries. Moreover, while the expansion of the aorta next to the 

 heart is, so to speak, the direct effect of the systole of the ventricle, the expan- 

 sion of the more distant artery is the effect of the systole transmitted by the 

 help of the elastic reaction of the arterial tract between the heart and the 

 distant artery ; and since this elastic reaction is slower in development than 

 the actual systole, the expansion of the more distant artery is slower than 

 that of the aorta, the up-stroke of the pulse-curve is less sudden, and the 

 whole pulse-curve is more flattened. 



The object of the systole is to supply a contribution to the mean pressure, 

 and the pulse is an oscillation above and below that mean pressure an oscil- 

 lation which diminishes from the heart onwards, being damped by the elastic 

 walls of the arteries, and so, little by little, converted into mean pressure 

 until in the capillaries the mean pressure alone remains the oscillations 

 having disappeared. 



132. If in the model the points of the two levers at different distances 

 from the pump be placed exactly one under the other on the recording 

 surface, it is obvious that, the levers being alike except for their position 

 on the tube, any difference in time between the movements of the two 

 levers will be shown by an interval between the beginnings of the 

 curves they describe, the recording surface being made to travel suffi- 

 ciently rapidly. 



If the movements of the two levers be thus compared, it will be seen that 

 the far lever (Fig. 64, II.) commences later than the near one (Fig. 64, I.) ; 

 the further apart the two levers are, the greater is the interval in time 

 between their curves. Compare the series I. to VI. (Fig. 64). This means 

 that the wave of expansion, the pulse-wave, takes some time to travel along 

 the tube. In the same way it would be found that the rise of the near lever 



