THE CIRCULATION" OF THE BLOOD. 207 



blood exerts upon the walls of the contained artery, equals the pres- 

 sure of a column of blood of a certain height; in the case of the rab- 

 bit's carotid it is equal to 3 feet of blood, or rather more than 3 feet of 

 water. In the case of the vein, if a similar experiment be performed, 

 blood will rise in the tube for an inch or two only. 



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 contain- 



Fig. 173. Diagram of mercurial kymograph. A, revolving cylinder, worKed by a clock-work 

 arrangement contained in the box (B), the speed being regulated by a fan above the box; cylinder 

 supported by an upright (&), and capable of being raised or lowered by a screw (a), by a handle 

 attached to it; D, c, E, represent mercurial manometer, a somewhat different form of which is 

 shown in next figure. 



ing mercury, or a mercurial manometer is employed, and the artery 

 is made to communicate with it by means of a small canula which is 

 inserted into the vessel, and a connecting tube, an arrangement being 

 made whereby the canula, tubes, etc., are filled with a saturated saline 

 solution to prevent the clotting of blood when it is allowed to pass from 

 the artery into the apparatus. The passage of blood is prevented during 

 the arrangement of the details of the experiment by a pair of clamp or 

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

 very fine glass piston, the bulbous end of which floats upon the surface 

 of the mercury, rising with its rise and oscillating with its oscillations. 



