THE PULSE / 61 



sponds to the jerky outflow of blood from a cut 

 artery. 



The pulse which is felt by the finger does not correspond 

 in time precisely with the beat of the heart, but takes 

 place a little after it, and the delay is longer, the greater 

 the distance of the artery from the heart. For example, 

 the pulse in the tibial artery on the inner side of the 

 ankle is a little later than the pulse in the temporal artery 

 in the temple. By suitable instruments the rate at which 

 the pulse travels along the arteries may be readily deter- 

 mined and is found to be about 30 feet per second. This 

 rate of progression of the pulse-wa\e must be carefully 

 distinguished from the rate at which the blood is flowing 

 along the artery. Even in the aorta, where the blood 

 flows most rapidly (p. 64), the velocity is not more than 

 about 15 inches per second. In fact "the pulse-wave 

 travels over the nioving blood somewhat as a rapidly 

 moving natural wave travels along a sluggishly flowing 

 river." 



Under ordinary circumstances, the pulse is no longer to 

 be detected in the capillaries, or in the veins. Sometimes 

 a backward pulse from the heart along the great venous 

 trunks may be observed ; but this is quite another matter, 

 and is the result of the movements of breathing. (See 

 Lesson IV.) This actual loss, or rather transformation of 

 the pulse is eflected hij means of the elasticity of the arterial 

 ivalls, called into i)la[i by the peripheral resistance, in the 

 following manner. 



In the first place it must be borne in mind that, owing 

 to the minute size of the small arteries and capillaries, the 

 amount of friction taking place in their channels when the 

 blood is passing through them is very great ; in other 

 words, they ofter a veiy great resistance to the passage of 

 the blood. The consequence of this is, that, in spite of 

 the fact that the total area of the capillaries is so much 

 greater than tliat of the aorta, the blood ha.s a difliculty in 

 getting through the capillaries into the veins as fast as it 



