210 THE CIRCULATION OF THE BLOOD 



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 therefore 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; and the depth to which the latter sinks, added to the 

 height to which it rises in the other limb, the weight of the saline solution 

 being subtracted, will give the height of the mercurial column which the 

 blood pressure balances. For the estimation of the amount of blood pres- 

 sure at any given moment no further apparatus than this is necessary; but 



FIG. 184. Ludwig's Mercury Manometer. The manometer is shown in figure 183, 

 D, C, E. The mercury which partially fills the tube supports a float in the form of a piston, 

 nearly filling the tube;'a wire is fixed to the float, and the writing style or pen is guided by 

 passing through the brass cap of the manometer tube; the. pressure is communicated to 

 the mercury by means of a flexible metal tube filled with fluid. 



for accurately noting the variations of pressure in the arterial system, as well 

 as its absolute amount, the instrument is usually combined with a recording 

 apparatus, called a kymograph, figure 183, and permanent records are made 

 of the observations. 



The recording apparatus consists of a revolving cylinder, figure 183, A, 

 which is moved by clock-work, and the speed of which is capable of regula- 

 tion. The cylinder is covered with glazed paper, blackened in the flame 

 of a lamp, and the mercurial manometer is so fixed, figure 183, D, that its 

 float, provided with a style, writes on the cylinder as it revolves. There are 

 many ways in which the mercurial manometer may be varied; in figure 184 

 is seen a form which is known as Ludwig's. In order to obviate the necessity 

 of a large quantity of blood entering the tube of the apparatus, it is usual to 



