THE ARTERIAL PULSE. 303 



which passes rapidly along those vessels. This motion in the walls 

 of the vessel can be felt wherever the artery can be reached by 

 the finger, but best as in the case of the radial and temporal ar- 

 teries where the vessel is superficial and lies on some unyielding 

 structure such as bone. 



This motion of the vessel wall is called the arterial pulse. It 

 consists of a simultaneous widening and lengthening of the artery. 

 The arteries near the heart are much more affected by the pulse 

 wave than those more remote, the wave becoming fainter and 

 fainter as it travels along the branching arteries. In the smallest 

 arteries it is hardly recognizable, and under ordinary circum- 

 stances is quite absent in the capillaries and veins. 



The diminution in the pulse-wave in the smaller arteries chiefly 

 depends upon the fact that the force of the wave is used up in 

 distending the successive part of the arteries. In the small arte- 

 ries the extent of surface to which the pulse-wave is communicated 

 is enormous, and thereby the wave is much decreased. Moreover, 

 it is probable that reflected waves pass from the peripheral end 

 of the arterial tree, and meeting the pulse-wave in the small arte- 

 ries help to obliterate it. 



The pulse-wave can easily be shown to take some time to pass 

 along the vessels. Near the orifice of the aorta the arterial dis- 

 tension occurs practically at the same time as the ventricular 

 systole, but even with comparatively rough methods the radial 

 pulse can be observed to be a little later than the heart- beat. 

 The difference of time between the pulse in the facial and the 

 dorsal artery of the foot has been estimated to be one-sixth of a 

 second, and the difference in the distance of these vessels from the 

 heart is about 1500 mm., so that the rate at which the pulse-wave 

 travels is nearly 10 metres per second. The velocity of the wave 

 is said to be regulated by the degree of elasticity in the walls of 

 the vessels, and it would appear to be quicker in the lower than 

 in the upper extremity. 



The time that the wave takes to pass any given point must be 

 equal to the time taken to produce it, that is to say, the time the 

 ventricle occupies in sending a new charge of blood into the aorta, 

 which is about one-third of a second. Knowing the rate at which 



