CIRCULATION 263 



prevent the development of a venous pulse. But the inflow 

 is not intermittent. With each beat of the heart the blood 

 does not pass freely from the arteries into the capillaries and 

 veins, but it only slowly escapes, just as much passing out 

 between the beats as during the beats. Hence the most important 

 factor in causing a pulse, an intermittent inflow, is absent. 



With no sudden intermittent inflow, and with no resist- 

 ance to outflow, the development of a pulse is impossible. 



In certain abnormal conditions, where, from the extreme 

 dilatation of the arterioles, the inflow into the veins is very 

 free, and where the outflow from the part of the body is not 

 so free, a local venous pulse may develop. 



Characters of the Pulse Wave. If a finger be placed on 

 the carotid artery and another upon the radial artery it will 

 be felt that the artery near the heart expands (pulses) before 

 that further from the heart. 



The pulse develops first in the arteries near the heart 

 and passes outwards towards the periphery. The reason for 

 this is obvious. The arteries are always overfilled with blood. 

 The ventricle drives its contents into this overfilled aorta, 

 and to accommodate this the aortic wall expands. But since 

 the aorta communicates with the other arteries this increased 

 pressure passes outwards along them, expanding their wall as 

 it goes. 



The pulse wave may thus be compared to a wave at sea, 

 which is also a wave of increased pressure, the only differ- 

 ence being that, while the waves at sea travel freely over the 

 surface, the pulse wave is confined in the column of blood, 

 and manifests itself by expanding the walls of the arteries. 



It greatly simplifies the study of the pulse to regard it in 

 this light, and to study it just as we should study a wave 

 at sea. 



1. Velocity. To determine how fast a wave is travelling 

 we might select two points at a known distance from one 

 another, and with a watch note how long the wave takes to 

 pass from one to the other. So with the pulse wave, two 

 points on an artery at a known distance from one another 

 may be taken and the time which the wave takes to pass 

 between them may be measured. 



It is thus found that the pulse wave travels at about 9 or 



