21G HANDBOOK OF PHYSIOLOGY. 



The distention of each artery increases both its length and its diam- 

 eter. In their elongation, the arteries change their form, the straight 

 ones becoming slightly curved, and those already curved becoming more 

 so; but they recover 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 distin- 

 guish the sensation produced by the dilatation from that produced by 

 the elongation and curving; that which it perceives most plainly, how- 

 ever, is the dilatation, or return, more or less, to the cylindrical form, of 

 the artery which has been partially flattened by the finger. 



Fig. 179. Marey's Sphygmograph, modified by Mahomed. 



The pulse due to any given beat of the heart is not perceptible 

 at the same moment in all the arteries of the body. Thus, it can be 

 felt in the carotid a very short time before it is perceptible in the radial 

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

 the foot. The delay in the beat is in proportion to the distance of the 

 artery from the heart, but the difference in time between the beat of 

 any two arteries probably never exceeds -J- to -J of a second. 



A distinction must be carefully made between the passage of the 

 wave along the arteries and the arterial flow itself. Both wave and 

 current are present; but the rates at which they travel are very different, 

 that of 'the wave 1G.5 to 33 feet per second (5 to 10 metres), being twenty 

 or thirty times as great as that of the current. 



The Sphygmograph. Much light has been thrown on what may 

 be called the form of the pulse wave by the Sphygmograph (figs. 179 

 and 182). The principle on which it acts will be seen on reference to 

 fig?. 



The small button replaces the finger in the act of taking the pulse, 



