THE CIRCULATION. 301 



tion a manometer with a column of mercury. This instrument is 

 more convenient to handle, and with it all of the scientific world is 

 acquainted to-day. 



The manometer with its column of mercury has undergone still 

 further modifications. Thus, Magendie has employed, under the 

 name of Ticemometer, an instrument composed of a mercury reservoir. 

 Upon this the blood-pressure is exerted, and it communicates with 

 a tube in which the metal rises. The height of the level of the mer- 

 cury in that single tube expresses the intensity of the pressure. 



Fig. 100. Blood-pressure Curve Recorded by the Mercurial 

 Manometer. ( YEO. ) 



c-x, Zero-line, y-y, Curve with large respiratory waves and small waves 

 of heart impulse. A scale is given to show height of pressure in millimeters 

 of mercury. 



By use of the mercury as a substance against which the blood 

 may expend its force, the inconvenience of handling the great column 

 of blood is overcome. 



One objection to the mercury is that columns of it, in their 

 oscillations, take on acquired momentum, which makes them pass 

 beyond the points which exactly express the maximum and the 

 minimum of blood-pressure. 



When such instruments are used, care must always be taken to 



