FOEM OF THE PULSE. 255 



alone. This fact has been appreciated by physiologists ; and 

 within the last few years, in order to accurately study this 

 important subject, instruments for registering the impulse 

 felt by the arterial system have been constructed, to enable 

 us to accurately analyze the dilatation or movements of the 

 vessels. The idea of such an instrument was probably sug- 

 gested by the following simple observation : When the legs 

 are crossed, with one knee over the other, the beating of the 

 popliteal artery will produce a marked movement in the foot. 

 If we could apply to an artery a lever provided with a mark- 

 ing point in contact with a slip of paper moving at a definite 

 rate, this point would register the movements of the vessel, 

 and its changes in caliber. The first physiologist who put 

 this in practice was Yierordt, who constructed quite a com- 

 plex instrument, so arranged that the impulse from an acces- 

 sible artery, like the radial, was conveyed to a lever, which 

 marked the movement upon a revolving cylinder of paper. 

 This instrument was called a " sphygmograph." The traces 

 made by it were perfectly regular, and simply marked the 

 extremes of dilatation, exaggerated, of course, by the length 

 of the lever, and the number of pulsations in a given time. 

 The latter can, of course, be easily estimated by more simple 

 means ; and as the former did not convey any very definite 

 physiological idea, the apparatus was regarded rather as a 

 curiosity than an instrument for accurate research. 



The principle on which the instrument of Yierordt was 

 constructed was correct, and it only remained to construct 

 one which would be easy of application, and produce a 

 trace representing the shades of dilatation and contraction 

 of the vessels, in order to lead to important practical results. 

 These indispensable conditions are fully realized in the 

 splrygmogragh of M. Marey, to whose researches on the cir- 

 culation we have repeatedly referred. The instrument sim- 

 ply amplifies the changes in the caliber of the vessel, without 

 deforming them ; and though its application is, perhaps, not so 

 easy as to make it generally useful in practice, in the hands 



