500 



CIRCULATION OF BLOOD AND LYMPH. 



the closed area are recorded through the tambour system connected 

 with the cuff. By means of a suitable stop-cock the pressure in 

 the cuff is dropped, 5 mms. at a time, and a record of the pulse is 

 taken at each level. The point of systoUc pressure is indicated 

 either by a sudden increase in the size of the pulse or by a sudden 

 spreading of the limbs of the pulse wave. As the pressure is 



Fig. 203. — Schema showing the construction of the Erlanger apparatus: a. Rubber 

 bag of the arm piece ; c, bulb for blowing up this bag and putting pressure on the arm ; b, 

 the manometer for measuring the pressure; i, two-way stopcock (when turned so as to 

 communicate with the capillary opening, k, it allows the pressure m a to fall slowly); c, 

 a rubber bag in a glass chamber, /; e communicates with a when stopcock d is open and 

 the pulse waves from a are transmitted to e; the pulsations of e in turn are transmitted 

 to the delicate tambour, h, and are thus recorded. 



dropped in steps below this point the size of the pulse wave in- 

 creases to a maximum, and then again dechnes (Fig. 202). The 

 pressure at which the maximum pulse waves are obtained or (Er- 

 langer) the first decrease in the size of the waves indicates diastolic 

 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 

 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- 



