THE SPHYGMOGRAPH 



205 



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- 



FIG. 195. Diagram of the Lever of the Sphygmograph. 



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. 

 The spring is adjusted to the proper tension to follow the movements of the 

 artery wall during the pulse wave. The Sphygmograph is bound on the 

 wrist while taking a record. 



It is evident that the beating of the pulse will cause an up-and-down 

 movement of the lever, the pen of which will write the effect on a smoked 

 card moved by the clock-work of the instrument. 



