THE ARTERIAL PULSE 217 



muscular folds (cf. page 167), a negative wave starts from the root of the 

 aorta running toward the periphery, and a portion of the aortic blood flows 

 back toward the heart. When this returning blood strikes against the semi- 

 lunar valves, which have just been closed, a positive centrifugal wave starts at 

 the root of the aorta, producing the dicrotic elevation (Marey, Grashey et al). 

 We cannot discuss here the principles underlying these two explanations. 

 Both have eminent advocates and an agreement is not yet in sight. 



However we conceive the dicrotic character to be produced, it is certain 

 that waves reflected from the periphery of the arterial system have a very mate- 

 rial influence on the form of the pulse curve. The reflected waves interfere with 

 the direct ones ; they spread to neighboring arteries where again they are propa- 

 gated as direct waves, which in turn interfere with both the direct and reflected 

 waves proper to these arteries, and so on. Thus a great variety of pulse tracings 

 with a varying number of secondary and tertiary elevations on the pulse curve 

 may be obtained. 



Attempts have frequently been made to draw conclusions as to the activity 

 of the heart, the condition of the blood vessels, and the blood pressure from 

 the character of the pulse curve. This is possible to a certain extent, but 

 must be done with great caution. 



The pulse curve may give some approximate information as to the tem- 

 poral course of events in the heart. The beginning of the pulse curve corre- 

 sponds to the entrance of the blood wave into the arteries, and the beginning 

 of the dicrotic elevation is synchronous with the beginning of a corresponding 

 elevation on the cardiogram. Since this point appears at a slight interval 

 after the closure of the semilunar valves (cf. page 175), the time elapsing 

 between the beginning of the pulse curve and the dicrotic elevation is slightly 

 greater than the time during which the left ventricle and the arteries are in 

 open communication with each other. 



It might be supposed also that a pulse curve of large amplitude would 

 indicate a large pulse volume; but this is true only in a very limited sense, 

 for the pulse curve of a given artery depends to so great a degree upon the 

 tonus of this artery and upon the resistance in its peripheral branches that 

 no definite relation between volume and size can be laid down. 



Moreover, a large amplitude of the pulse curve is by no means significant 

 of a high blood pressure, but at best signifies only that the fluctuation of 

 pressure is great. But since we know that the variation of systolic pressure 

 is within certain limits less with a high than with a low pressure, we might 

 possibly say that under circumstances otherwise the same the greater ampli- 

 tude means a lower blood pressure. But even this is not invariable. The de- 

 gree of constriction of the artery under investigation has much to do with it. 



Again the height of the dicrotic elevation has often been regarded as an 

 index of the pressure, a greater elevation being produced with a low than 

 with a high pressure. In many cases this is true. But there are exceptions, 

 since the degree of dilatation of a particular arterial region may influence 

 the production of any particular dicrotic pulse. 



All of this shows how careful one must be in drawing conclusions from 

 the pulse curve regarding the condition of the vascular system. 



