188 THE CIRCULATION OF THE BLOOD 



The usual method of estimating the amount of blood pressure differs 

 somewhat from the foregoing simple experiment. Instead of a simple straight 

 tube of glass inserted into the vessel, a U-shaped tube containing mercury, 

 the mercurial manometer, is employed. The artery is connected with the 

 manometer by means of a small cannula which is inserted into the vessel, 

 an arrangement being made whereby the cannula, tubes, etc., are filled with 

 a saturated saline solution to prevent the clotting of blood when it is allowed 



FIG. 183. Diagram of Ludwig's Kymograph and Mercurial Manometer. A, Revolving cylin- 

 der, worked by a clock-work arrangement contained in the box (B), the speed being regwlated by a 

 fan above the box; cylinder supported by an upright (b), and capable of being raised or lowered 

 by a screw (a), by a handle attached to it; D, C, E, represent a mercurial manometer, a somewhat 

 different form of which is shown in the next figure. 



to pass from the artery into the apparatus. The loss of blood is prevented 

 during the preparation of the details of the experiment by a clamp or bull- 

 dog forceps. The free end of the U-tube of mercury contains a very fine 

 glass or metal rod with a bulb which floats upon the surface of the mercury 

 and oscillates with the oscillations of the mercury. As soon as there is free 

 communication between the artery and the tube of mercury, the blood rushes 

 out and pushes before it the column of mercury. The mercury will there- 

 fore rise in the free limb of the tube, and will continue to do so until a point 

 is reached which corresponds to the mean pressure of the blood-vessel used. 

 The blood pressure is thus communicated to one limb of the mercurial column; 



