270 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. xxi. 



In order to study the pulse more fully, it is necessary to obtain 

 a graphic record of the pulse-beat, and this is accomplished by 

 the use of an instrument called the sphygmograph. This 

 instrument consists of a series of levers, at one end of which is a 

 button placed over the artery ; the other end is provided with 

 a writing point to inscribe the magnified record of the arterial 

 movement on a travelling surface. 



The instruments most frequently used are those of Marey, one 

 of the numerous modifications of \vhich is represented in figures 

 260, 261, and 262, and of Dudgeon (fig. 263). 



Each instrument is provided with an arrangement by which 

 the pressure can be adjusted so as to obtain the best record. 

 The measurement of the pressure is, however, rough, and both 

 instruments have the disadvantage of giving oscillations of 

 their own to the sphygmogram; this is specially noticeable 



in Dudgeon's sphygmograph. 

 But these defects may be over- 

 come by the use of some form 

 of sphygmometer. (See later, 

 p. 295.) It is also important 

 to remember that the pad 

 or button placed upon the 

 artery rests partly on the vense 

 comites, so that not only 

 arterial tension, but any tur- 



gidity arising from venous COn- 

 Fig. 264. Diagram of pulse-tracing. A, up- o-pQtirvn will iflWt thp lipio-Vit 

 stroke; B, down-stroke ; c.pre-dicrotic & 6S "> ^ 1IJ 



wave ; D, dicrotic ; E, post-dicrotic wave. and form of the sphy gmographic 



record. 



Fig. 264 represents a typical sphygmographic tracing obtained 

 from the radial artery. It consists of an upstroke due to the 

 expansion of the artery, and a downstroke due to its retrac- 

 tion. The descent is more gradual than the upstroke, because 

 the elastic recoil acts more constantly and steadily than the 

 heart-beat. On the descent are several secondary (katacrotic) 

 elevations. 



A is the primary, or percussion wave ; C is the pre-dicrotic, or 

 tidal wave ; D is the dicrotic wave, and E the post-dicrotic wave, 

 and of these there may be several. In some cases there is a 

 secondary wave on the upstroke, which is called an anacrotic 

 wave (fig. 265). 



These various secondary waves have received different inter- 

 pretations, but the best way of explaining them is derived 

 from information obtained by taking simultaneous tracings of 



