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HAND-BOOK OF PHYSIOLOGY. 



simple (see Fig. 124). The small button replaces the finger in the act of 

 taking the pulse, and is made to rest lightly on the artery, the pulsations 

 of which it is desired to investigate. The up-and-down movement of the 

 button is communicated to the lever, to the hinder end of which is at- 

 tached a slight spring, which allows the lever to move up, at the same 

 time that it is just strong enough to resist its making any sudden jerk, 



Fia. 125. The Sphygmograph applied to the arm. 



and in the interval of the beats also to assist in bringing it back to its 

 original position. For ordinary purposes the instrument is bound on the 

 wrist (Fig. 125). 



It is evident that the beating of the pulse with the reaction of the 

 spring w r ill cause an up-and-down movement of the lever, the pen of which 

 will write the effect on a smoked card, which is made to move by clock- 

 work in the direction of the arrow. Thus a tracing of the pulse is ob- 

 tained, and in this way much more delicate effects can be seen, than can 

 be felt on the application of the finger. 



The pulse-tracing differs somewhat according to the artery upon which 

 the sphygmograph is applied, but its general characters are much the 

 same in all cases. It consists of: A sudden upstroke (Fig. 126, A), which 



is somewhat higher and more abrupt in 

 the pulse of the carotid and of other 

 arteries near the heart than in the radial 

 and other arteries more remote; and a 

 gradual decline (B), less abrupt, and 

 therefore taking a longer time than (A). 

 It is seldom, however, that the decline is 

 an uninterrupted fall: it is usually marked 

 about half-way by a distinct notch (c), 

 called the dicrotic notch, which is caused 

 by a second more or less marked ascent 

 of the lever at that point by a second wave 

 called the dicrotic wave (D); not unfrequently (in which case the tracing 

 is said to have a double apex) there is also soon after the commencement 

 of the descent a slight ascent previous to the dicrotic notch, this is called 

 the predicrotic wave (c), and in addition there may be one or more 

 slight ascents after the dicrotic, called post dicrotic (E). 



FIG. 126. Diagram of pulse tracing. 

 A, upstroke; B, down-stroke; c, predi- 

 crotic wave ; D, dicrotic ; E, post dicrotic 

 wave. 



