CHAP, iv.] 



THE VASCULAR MECHANISM. 



165 



return continues after the flow has ceased, and the aortic valves 

 soon becoming closed, the elastic force thus brought into play serves 

 to drive the blood onward. The elastic recoil being slower than the 

 initial expansion, the down-stroke of the pulse-curve is more gradual 

 than the up-stroke. Of this portion of the aorta, which actually 

 receives the blood ejected from the heart, the part immediately 

 adjacent to the semilunar valves begins to expand first, and the ex- 

 pansion travels thence on to the end of this portion. In the same 

 way it 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 cannot be 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, and a fraction 

 of it must therefore be left behind at each stage of its progress ; 

 that is to say, the expansion is continually growing less, as the 

 pulse travels from the heart to the capillaries ; hence the diminished 

 height of the pulse-curve in the more distant arteries, and its 

 disappearance in the capillaries. 



Secondary Waves and Dicrotism. In nearly all pulse tracings, 

 the curve of the expansion and contraction of the artery is broken 



A/VVWl/WWWWV 



FlG. 28. PULSE-TBACING FEOM CAROTID ARTERY OF HEALTHY MAN 1 (from Moens). 



x, commencement of expansion of the artery. A, summit of the first rise. 



C , dicrotic secondary wave. B, predicrotic secondary wave, p notch preceding this. 



D, succeeding secondary wave. The curve above is that of a tuning-fork with ten 

 double vibrations in a second. 



1 It will be understood that in the case of this and the succeeding sphygmo- 

 graphic tracings (for the latter I am indebted to Dr Galabin and Dr Roy) 

 comparisons between the several curves can only be made in a limited 

 manner and with precautions, since the tracings are taken with different 

 amplifications, pressures, &c. and are some from man, others from animals. They 

 are introduced simply to illustrate points treated of successively in the text. 



