226 THE CIRCULATION OF THE BLOOD 



THE PULSE. 



The most characteristic feature of the arterial pressure and blood flow 

 is its intermittency, and this intermittent flow is seen or felt as waves of change 

 in diameter of the arteries, known as the Pulse. 



The pulse is generally described as a wave-like expansion of the artery 

 produced by the injection of blood at each ventricular systole into the already 

 full aorta. The force of the left ventricle is expended in pressing the blood 

 forward and in dilating the aorta. With the injection of each new quantity 

 of blood into the aorta there is a wave of dilatation which passes on, expand- 

 ing the arteries as it goes, running as, it were, over the more slowly traveling 

 blood contained in them, and producing the pulse as it proceeds. A sharp 

 distinction must be made between the passage of the pulse wave along an 

 artery and the rate of flow of the blood in the vessel. The pulse produced by 

 any given beat of the heart is not felt at the same moment in all parts of the 

 body. Thus, it can be felt in the carotid a short time before it is perceptible 

 in the radial artery, and in this vessel before it occurs in the dorsal artery of 

 the foot. Careful measurements of the intervals between the time of the 

 pulse at the carotid and at the wrist shows that the delay in the beat is in 

 proportion to the distance of the artery from the heart. The difference in 

 time between the pulse of any two arteries probably never exceeds one-sixth to 

 one-eighth of a second. The rate at which the pulse travels in the arteries 

 is from five to ten meters per second. 



The distention of each artery increases both its length and its diameter. 

 In their elongation the arteries change their form, the straight ones becoming 

 slightly curved, and those already curved becoming more so; but they re- 

 cover their previous form as well as their diameter when the ventricular 

 contraction ceases, and their elastic walls recoil. The increase of their 

 curves which accompanies the distention of arteries, and the succeeding 

 recoil, may be well seen in the prominent temporal artery of an old person. 

 In feeling the pulse, the finger cannot distinguish the sensation produced 

 by the dilatation from that produced by the elongation and curving. That 

 which it perceives most plainly, however, is the dilatation and return more 

 or less to the cylindrical form, of the artery which has been partially flattened 

 by the finger. 



The Sphygmograph. Much light has been thrown on what may 

 be called the form of the pulse wave by an instrument called the sphygmo- 

 graph, figures 195 and 196. The principle on which it acts will be seen 

 on reference to the figures. 



A small button replaces the finger in the act of taking the pulse. This 

 button is made to rest lightly on the artery the pulsations of which it is de- 

 sired to investigate. The up-and-down movement of the button is com- 

 municated to the lever, to the hinder end of which is attached a light spring. 



