THE ARTERIAL PULSE. 309 



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, i. e., the contracted arterioles, and meeting 

 the pulse wave in the small arteries help to obliterate it. So 

 long as the arterioles remain contracted to the normal degree no 

 pulsation is communicated to the capillaries, because the wave is 

 reflected from the arterioles. 



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- 

 tention 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 

 the wave travels (10 m. per sec.) and the time it takes to pass 

 any given point (i sec.), its length may be calculated to be about 

 three metres, or about twice as long as the longest artery. Thus 

 the pulse wave reaches the most distant artery in one-sixth of a 

 second, or about the middle of the ventricular systole, and when 

 the wave has quite passed from the arch of the aorta, the summit 

 of it has only just reached the arterioles. 



Numerous instruments have been in vented for the demonstration 

 and graphic representation of the pulse in the human being. Of 

 these the one most commonly used is Marey's Sphygmograph 



