208 HANDBOOK OF PHYSIOLOGY. 



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 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 the upper part of the mercurial column; and the 

 depth to which the latter sinks, added to the height to which it rises in 

 the other, will give the height of the mercurial column which the blood- 

 pressure balances; the weight of the saline solution being subtracted. 

 For the estimation of the amount of blood pressure at any given mo- 

 ment, no further apparatus than this, which is called Poiseuilles's hce~ 



Fig. 174. Ludwig's Kymograph. The manometer is shown in fig. 173, D. C. E. The mercury 

 which partially fills the tube supports a float in 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 

 tube fixed to the wire; the pressure is communicated to the mercury by means of a flexible metal 

 tube filled with fluid. 



madynamometeTfis necessary; but for noting the variations of pressure 

 in the arterial system, as well as its absolute amount, the instrument is 

 usually combined with a recording apparatus, in this form called a 

 kymograph (fig. 173). 



The recording apparatus consists of a revolving cylinder (fig. 173, 

 A), which is moved by clockwork, and the speed of which is capable of 

 regulation. The cylinder is covered with glazed paper blackened in the 

 flame of a lamp, and the mercurial manometer is so fixed (fig. 173, 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 fig. 174 is seen a form, which is known as Ludwig's Kymograph. 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 



