METHODS OF MEASURING ARTERIAL BLOOD PRESSURE 189 



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 substracted, will 

 give the height of the mercurial column which the blood pressure balances. 

 For the estimation of the amount of blood pressure at any given moment, 

 no further apparatus than this is necessary; but 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 



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. 



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 

 have some arrangement by means of which the mercury may be made to 

 rise in the tube of the manometer to the level corresponding to approxi- 

 mately the mean pressure of the artery experimented with, so that the writing 

 style simply records the variations of the blood pressure above and below 

 the mean pressure. This is done by causing the saline solution, generally 

 a saturated solution of sodium carbonate or a 10 per cent magnesium sul- 



