228 DISAPPEARANCE OF PULSE. [BOOK i. 



For the total expansion required to make room foi the new 

 quantity of blood is not provided 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 succeeding 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 ventricle 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 there- 

 fore 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 expansion 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 oscillation 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 dis- 

 appeared. 



125. 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 shewn by an 

 interval between the beginnings of the curves they describe, the 

 recording surface being made to travel sufficiently rapidly. 



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

 seen that the far lever (Fig. 59, II.) commences later than the near 

 one (Fig. 59, I.) ; the farther apart the two levers are, the greater 

 is the interval in time between their curves. Compare the series 

 I. to VI. (Fig. 59). In the Same way it would be found that the 

 rise of the near lever began some fraction of a second after the 

 stroke of the pump. This means that the wave of expansion, the 

 pulse-wave, takes some time to travel along the tube. 



