CHAP, iv.] THE VASCULAR MECHANISM. 173 



1. Whatever the origin of the dicrotic wave, its features may 

 be modified by changes taking place in the peripheral (arterial) 

 districts without any alteration in the central (cardiac) events. 

 Thus dicrotism may become conspicuous in one artery while re- 

 maining indistinct in others. 



2. The prominence of the dicrotic wave, though favoured by 

 a sudden strong ventricular systole, is especially assisted by a 

 diminution of blood-pressure. Thus it is a marked characteristic 

 of the pulse in many cases of fever (Fig. 31) where blood-pressure 



, is low. So also it may be brought on at once in an artery in which 

 it was previously insignificant by sudden lowering of the blood- 

 pressure as is shewn in Fig. 38. It may similarly be induced by 



FIG. 38. TRACING FBOM RADIAL IN MAN ; 



shewing change in form of pulse-curve accompanying a sudden fall in the blood- 

 pressure. The pulse, at first not markedly dicrotic, rapidly becomes so, and then 

 passes on into the condition known as hyperdicrotism, where the dicrotic notch 

 reaches a level lower than that from which the primary rise started. 



section of the vaso-motor nerves belonging to the branches of the 

 artery; this, as we shall presently see, diminishes the peripheral 

 resistance, through an expansion of the minute arteries, and so 

 leads to a lowering of the blood-pressure in the main arteries. The 

 prominence of the dicrotic wave is further dependent on the amount 

 of extensibility and elastic reaction of the arterial walls. Hence 

 the dicrotic wave is not well marked in arteries which have become 

 rigid by disease or old age. 



We may add that an anacrotic pulse, in which a crest followed 

 by a notch is visible on the ascending portion of the curve, before 

 the maximum of expansion is reached, though it may sometimes 

 be produced temporarily in healthy persons, is generally associated 

 with diseased conditions, usually such in which the arteries are abnor- 

 mally rigid. It has been interpreted as due to the pressure in the 

 aorta rising even after the first rapid rush from the ventricle. Under 



