506 



CIRCULATION OF BLOOD AND LYMPH. 



pressure, according to the principle first suggested by Marey. This 

 principle assumes that when the pressure on the outside of an artery 

 is just equal to the diastolic pressure on the inside, that is to say, to 

 the pressure in between the pulse beats, the arterial wall is in a posi- 

 tion in which it will give the widest excursion for any given force of 

 heart beat. The correctness of this principle has been demonstrated 

 upon the exposed artery. In the living person a certain amount 

 of soft compressible tissue intervenes between the cuff and the 



Fig. 204. Erlanger apparatus. The collar for the arm is not shown. The parts may be 

 understood by reference to the schema given in Fig. 203. 



brachial artery, and a portion of the external pressure is used in 

 overcoming the resistance of this tissue. The readings as made 

 are, therefore, probably somewhat above the actual diastolic pres- 

 sure by an amount that has been estimated at from 6 to 10 mms. 

 of mercury. As these instruments are usually employed, measure- 

 ments are made upon the brachial artery at the level of the heart, 

 and consequently no correction need be made for a hydrostatic 



