VELOCITY AND PRESSURE OF BLOOD-FLOW. 



457 



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 



Fig. 192. Erlanger apparatus. The collar for the arm is not shown. The parts may be 

 understood by reference to the schema given in Fig. 191. 



the moment when the pulse wave is first able to break through the brachial 

 artery, and it gives, therefore, the systolic pressure. After finding the sys- 

 tolic pressure the diastolic pressure is obtained by allowing the pressure to 

 drop still further. The pulsations increase in height to a maximum size and 

 then decrease. The pressure at which the maximum pulse wave is obtained 

 marks the diastolic pressure. It is better perhaps in dropping the pressure 

 for this last purpose to manipulate stopcock i so as to drop the pressure 5 

 mms. at a time, recording the pulse wave at each pressure. In this way a 

 record is obtained such as is given in Fig. 190. It should be added, also, that 

 in order to keep the lever of the tambour horizontal while the pressure in the 



