228 CIRCULATION OF THE BLOOD. 



lever (A A'). The purpose of the screw (T) is to vary at will 

 the distance between the wooden lever and the upper surface 

 of the spring, without interfering with the mechanism by which 

 the movement is transmitted. As the distance between the 

 steel points (c) and the knife-edge (D) is much less than the 

 length of the lever, the oscillations of the extremity of the 

 lever (A') are much more extensive than the vertical move- 

 ments of the spring. The lever ends in a metal point, which 

 writes on a glass plate blackened by passing it rapidly back- 

 ward and forward through the flame of a spirit-lamp trimmed 

 with paraffin. 



When this instrument is applied in the proper manner to 

 the wrist, the radial artery is compressed between the surface 

 of the radius and a spring, the bearing of which is in a fixed 

 position in relation to that surface. This being the case, the 

 spring performs movements which are more or less conform- 

 able with the variations of the diameter of the artery. These 

 movements are transferred in a magnified, but otherwise little 

 altered, form to the lever. As regards the relative and actual 

 duration of the movements, the correspondence is exact; but 

 as regards their extent, this is true only in so far as the lever 

 follows the movements of the spring with precision, 1 and as 

 the strength of the spring, i. e., the pressure exercised by it on 

 the artery, is adapted to the antagonistic pressure exerted by 

 the blood stream on the internal surface, and to the extent of 

 the movements it is intended to measure. 



The relation between the pressure of the spring and its effect 

 on the artery is a complex one, and need only be considered 

 here in so far as is necessary for the interpretation of sphyg- 

 mographic results. To facilitate our understanding of it, let 

 us call the position which the spring takes when left to itself 

 its equilibrium position ; and as regards the artery, let us de- 

 signate a plane parallel to the surface of the skin, and touch- 

 ing the surface of the artery, when most dilated, the plane of 

 expansion; and a plane in similar relation to it, when least 

 expanded, the plane of collapse ; and to simplify the problem, 

 let us suppose that the artery is not covered by skin. It is 

 evident that, if the sphygmograph accomplished its professed 

 end complete!}', the under surface of its spring would coincide 

 with one of these planes at the moment of the pulse, and with 

 the other during the interval. The question is, How ought 

 the spring to be set, in order to obtain a movement which shall 

 approach this standard of perfection as nearly as possible ? 

 We may proceed one step towards answering this question 

 without difficulty. It should be set so that if the spring were 

 in the equilibrium position its under surface would lie within 



1 See note on p. 235. 



