THE PULSE. 187 



the ventricle, and would travel thence along the elastic arteries. The facts 

 that each beat is rapidly succeeded by another, and that the flow which sud- 

 denly ceases is also, by the nature of the ventricular stroke, suddenly gen- 

 erated, may render the waves more complicated, but will not change their 

 essential nature. 



The exact interpretation of the generation of these waves is perhaps not 

 without difficulty, but two factors seem of especial importance. In the first 

 place, as we have already more than once said, when a rapid flow is suddenly 

 stopped a negative pressure makes its appearance behind the column of fluid. 

 In a rigid tube this simply tends to a reflux of fluid. In an elastic tube its 

 effects are complicated by the second factor, the elastic action and inertia of 

 the walls of the tube. Upon the sudden cessation of the flow, the expansion 

 of the tube, or as we may at once say, of the aorta, ceases, the vessel begins to 

 shrink, and the lever placed on its walls falls, as from A onward in the pulse- 

 curve. This shrinking is in part due to the elastic reaction of the walls of 

 the aorta, but is increased by the " suction " action of the negative pressure 

 spoken of above. In thus shrinking, however, under these combined causes, 

 the aorta, through the inertia of its walls, overshoots the mark, it is carried 

 beyond its natural calibre i. e., the diameter it would possess if left to itself 

 with the pressure inside and outside equal ; it shrinks too much and conse- 

 quently begins again to expand. This secondary expansion (taking for 

 simplicity sake a pulse-curve in which the so-called pre-dicrotic wave, B, is 

 absent or inconspicuous) causes the secondary rise of the lever up to C that 

 is, the dicrotic rise. In thus expanding again the aorta tends to draw back 

 toward the heart the column of blood which by loss of momentum had come 

 to rest, or, indeed, under the influence of the negative pressure spoken of 

 above, was already undergoing a reflux. In this secondary expansion, more- 

 over, the aorta is by the inertia of its walls, aided by that of the blood, again 

 carried, so to speak, beyond its mark, so that no sooner has it become ex- 

 panded and filled with fluid to a certain extent than it again begins to shrink 

 as from C onward. And this shrinking may in a similar manner to the first 

 be followed by a further expansion and shrinking, giving rise to a post- 

 dicrotic wave, or it may be to post-dicrotic waves. And the successive 

 changes thus inaugurated at the root of the aorta travel as so many waves 

 along the arterial system, diminishing as they go. It will be observed that 

 for the development of these waves a certain quality in the walls of the tubing 

 is necessary. The tube must be such as possesses when at rest an open lumen ; 

 the walls must be of such a kind that the tube remains open when empty 

 " e. y when the atmospheric pressure is equal inside and outside so that when 

 it shrinks too much it expands again in striving to retain its natural calibre. 



This we have seen to be a characteristic of the arteries. A collapsible 

 ube of thin membrane will not show the phenomena ; such a tube when the 

 stop-cock is turned collapses and empties itself, continuing to be collapsed 

 without any effort to expand again. 



In the above explanation no mention has been made of the closing of the 

 semiluuar valves ; we shall have to speak of these a little later on in refer- 

 ing to the pre-dicrotic wave, and shall see that, under the view we have 

 just given, the closing of the semilunar valves is to be regarded rather as 

 the effect than the cause of the dicrotic wave. Many authors, however, give 

 in interpretation of the dicrotic wave different from that detailed above, 

 "'hus, it is held that the primary shrinking from A onward, being brought 

 bear on the column of blood already come to rest, in face of the great 

 >ressure in front, drives the blood back against the semilunar valves, thus 

 closing them, and that the impact of the column of blood against the valves 

 irts a new wave of expansion, which reinforcing the natural tendency of 



