CIRCULATION OF THE BLOOD. 



course, by the length of the lever, and the number of pulsations in a given time. The 

 latter can 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. 



FIG. 22. Sphygmograph of Marey. 



The apparatus is securely fixed on the forearm, so that the spring under the screw V is directly over the radial artery. 

 The movements of the pulse are transmitted to the long and light wooden lever L and registered upon the sur- 

 face P, which is moved at a known rate by the clock-work H. The apparatus is so adjusted that the movements 

 of the vessel are accurately amplified and registered by the extreme point of the lever. 



FIG. 23 (K). Sphygmograph of Marey applied to the arm. 



FIG. 23 (B). Trace of Tier or dt. 



FIG. 23 (C). Trace of Marey. 

 Portions of four traces taken in different conditions of the pulse. 



The principle on which the instrument of Vierordt 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 



