CHAP, iv.] THE VASCULAR MECHANISM. 267 



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 leads 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 it 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 consequently 

 begins again to expand. This secondary expansion (taking for 

 simplicity sake a pulse-curve in which the so-called predicrotic 

 wave, B, is absent or inconspicuous) causes the secondary rise 

 of the lever up to (7, that is the dicrotic rise. In thus expanding 

 again the aorta tends to draw back towards 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 expanded 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, 

 i. e. when the atmospheric pressure is equal inside and outside, 

 so that when it shrinks too much, it expands again in striving to 

 regain its natural calibre. This we have seen to be a character- 

 istic of the arteries. A collapsible tube of thin membrane will 

 not shew 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 semilunar valves ; we shall have to speak of these 



