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MANUAL OF PHYSIOLOGY. 



blood leaving the vessels tends to empty them and to reduce the 

 pressure in the arteries ; besides the coagulation of the blood soon 

 stops the experiment. We now employ the mercurial manometer, 

 which consists of a column of mercury in a U-shaped tube. To 

 prevent coagulation, the tube between the mercury and blood is 

 filled with a solution of sodium carbonate, the pressure being regu- 

 lated to equalize as nearly as possible that of the blood. A rod 

 is made to float upon the mercury, in the open side of the tube, 



FIG. 132. 



The ordinary modern form of rotating blackened cylinder (R), which is 

 moved by the clockwork in the box (A) by means of the disk (D) pressing 

 upon the wheel (n), which can be raised or lowered by the screw (L), so as 

 to rub on a part of the disk more or less near the centre, and thus rotate at 

 different rates. The cylinder can be raised by the screw (v), which is turned 

 by the handle (u). (Hermann.) 



and to the upper extremity of this a writing apparatus can be 

 attached, so that by the movements of the mercury, a graphic 

 record of the blood pressure and its variation can be traced on a 

 regularly moving surface. This instrument, known as Ludwig's 



