THE CIRCULATION OF THE BLOOD 



361 



Erlanger's sphygmomanometer is a most valuable instrument for obtain- 

 ing both the systolic and the diastolic pressure. It possesses an advantage in 

 that it is provided, in addition to the mercurial manometer, with a tambour 

 and lever by which changes in pressure can also be recorded on a revolving 

 cylinder (Fig. 163). A complete description of this apparatus, the manner 



COPYRIGHT 1906, BY SCHNEIDER BROS., N.Y. 



FIG. 163. ERLANGER'S SPHYGMOMANOMETER. 



of using it and the results that can be obtained with it will be found in the 

 Johns Hopkins Hospital Reports, Vol. XII. 



With this apparatus, the lever often exhibits a considerable oscillation 

 even when the pressure exerted on the arm exceeds the systolic pressure. 

 It is difficult, therefore, to determine at times the moment at which the pres- 

 sure indicated by the mercurial column just falls below the systolic pressure 

 and allows the blood to pass through. A new criterion for this determination 



