166 



EXPERIMENTAL PHYSIOLOGY 



Experiment 4. — Take another tracing of the movements of the radial 

 artery by means of Dudgeon's Sphygmograph (fig. 129). The form repre- 

 sented in this figure is a modification of the original pattern due to Richard- 

 son. A strip of paper blackened on its upper surface is carried under the 

 writing lever by two revolving rollers, the upper one of which is furnished 

 with a series of sharp edges which record lines 2 mm. apart upon the 

 blackened paper. These edges are interrupted so that the horizontal lines 



Fig. 129. — Kichardson's Modification of Dudgeon's Sphygmogeaph. 



are broken, and the clockwork is so made that each break follows the preced- 

 ing break after a distance of 2 mm. 



The arrangement of the levers in Dudgeon's sphygmograph is diagram- 



matically represented in fig. 130. p is a 

 little metal pad which rests upon the artery 

 and which follows the changes of its dia- 

 meter. It is kept in contact with the skin 

 by aid of a small weight, w, which slides 

 along a rod, k l, pivoted at k. By varying the 

 position of the weight the pressure exerted 

 upon the artery can be altered at pleasure. 

 A. bent brass lever, c b a, rotates round an 

 axis at b. Its short arm b a passes through 

 a hole bored in the upright of the pad p. 

 Fig. 130.-Plan of the Levees Any movement of p is therefore communi- 

 in Dudgeon's Sphygmogeaph. cated to ba, and therefore to c, where it is 



magnified about five times, because the long 

 arm b c of the bent lever is about five times that of the short one. A second 

 lever, dcf, rotates round an axis at d and has attached to it at f a writing 

 style, f G. It passes through a ring terminal at c and is kept in contact with 

 c by the counterpoise e. The movement at c is therefore communicated to 

 d f and magnified at f about five times because d f is about five times d c. 

 The movement of the writing point g is practically that of f. Thus the total 

 magnification of the movement of p is twenty-five times, m is the writing 

 surface. 



