The Blood and its Circulation 



155 



for example, on the radial artery near the wrist, there is 

 felt an intermittent pressure, throbbing with every beat of 

 the heart. This movement, frequently visible to the eye, 

 is the result of the alternate expansion of the arterial walls 

 by the wave of blood, and the recoil by their elasticity. In 

 other words, it is the wave produced by throwing a mass of 

 blood into the arteries already full. The blood wave strikes 

 upon the elastic walls of the 

 arteries, causing an increased 

 distention, followed at once 

 by contraction. 



This intermittent rise and 

 fall of the arterial wall felt 

 just under the ringers, which 

 corresponds to the successive 

 heart beats, is known as the 

 pulse. 



This wave of distention, 

 or the pulse, may be easily 

 felt at the wrist, the temple, 

 and the inner side of the ankle. The throb of the two 

 carotid arteries may be plainly felt by pressing the thumb 

 and ringer backwards on each side of the larynx. 



235. The Pulse Wave. The progress of the pulse wave 

 must not be confused with the actual current of the blood 

 itself. For instance, the pulse wave travels at the rate of 

 about thirty feet a second, and takes about one-tenth of a 

 second to reach the wrist, while the blood itself is from 

 three to five seconds in reaching the same place. The 

 pulse wave may be compared to the wave produced by a 

 stiff breeze on the surface of a slowly moving stream. 



236. How the Pulse Rate may be modified. The rate of 

 the pulse is modified by age, fatigue, posture, exercise, 



FIG. 81. Capillary Blood Vessels 

 in the Web of a Frog's Foot, as 

 seen with the Microscope. 



