22O THE CIRCULATION OF THE BLOOD 



are first filled with a half -saturated solution of magnesium sulphate or other 

 saline to prevent the clotting of blood when it is allowed 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 for- 

 ceps. 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 

 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 near limb of the 

 column of mercury; 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 column of mercury which the 

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

 one can make direct readings at any given moment and no further apparatus 



FIG. 184. FIG. 185. 



FIG. 184. Arterial Cannula. T-form for convenience in washing out clots. 

 FIG. 185. Ludwig's Mercury Manometer. 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. 



than this is necessary. But in the more accurate study of the variations 

 of pressure in the arterial system, as well as its absolute amount, the instru- 

 ment is usually combined with a recording apparatus, called a kymograph. 

 Numerous forms of recording kymographs are to be had in the market. 

 These instruments, while all constructed on the same principle, vary chiefly 

 in the accuracy of their construction and convenience of their adjustments. 



