470 



CIRCULATION OF BLOOD AND LYMPH. 



eter is necessary, the instrument corresponding with the Riva-Rocci appa- 

 ratus described above. By means of the pressure bag, c, the bag, a, upon the 

 arm is blown up until the pressure is above the systolic pressure and the radial 

 pulse below disappears. By turning stopcock i properly the system is allowed 

 to communicate with the air through a capillary opening, k. Consequently 

 the pressure upon the artery in the arm falls slowly, and by palpating the 

 radial artery one can determine the pressure, as measured by the mercury 

 manometer, at which the pulse just gets through. This pressure will measure 

 approximately the systolic pressure. The second method gives higher and 

 doubtless more accurate results. In this method the pressure is at first raised 

 above systolic pressure with stopcocks d and g open, a, e, and b are under 



Fig. 195. Erlanger apparatus. The collar for the arm is not shown. The parts may be 

 understood by reference to the schema given in Fig. 194. 



the same pressure. If stopcock g is now turned off, the pulsations in a are 

 transmitted to e and through it to the tambour, h, and the lever of the tam- 

 bour writes these pulsations on a kymographion. It should be explained that 

 pulsations are obtained even when the pressure on the arm is much more than 

 sufficient to completely obliterate the brachial artery. The reason for this 

 is that the pulsations of the central stump of the closed artery will be communi- 

 cated to bag a. When the pressure is suprasystolic these pulsations are small. 

 If now the pressure in the system is diminished slowly by turning stopcock i 

 so as to communicate with the capillary opening, k, it will be found that at 

 a certain point the pulsations suddenly increase in height. This point marks 



