THE CIRCULATION OF THE BLOOD AND LYMPH 



Various instruments have been devised for determining the blood 

 pressure from the changes in the oscillations communicated to the arm- 

 let by the artery as the pressure in the armlet is allowed to fall or caused 

 to rise. The sphygmomanometer of Erlanger (Fig. 45) is arranged to 



obtain graphic records of the pulse 

 from which both the maximum 

 and the minimum blood-pressures 

 may be deduced. The mean pres- 

 sure cannot be directly measured, 

 but must lie much nearer to the 

 minimum than to the maximum, 

 since the line of mean pressure bi- 

 sects the area enclosed by the 

 pulse curve, and this area is broad 

 at the base and narrow at the apex. 

 The rubber bag is applied in the 

 form of a cuff or armlet to the arm 

 above the elbow over the brachial 

 artery. It communicates with a 

 mercury manometer, which gives 

 the pressure exerted upon the arm. 

 It is also connected with a rubber 

 bulb, B, enclosed in 

 a glass bulb, G, and 

 through a stopcock 

 with a syringe bulb, 

 V, .provided with a 

 valve. ^The space 

 between" B and G 

 communicates (i) 

 with the tambour ; 

 (2) with the exterior 

 through the stopcock 

 by the tube E, and 

 also through a pin- 

 point opening in the 

 membrane of the 

 tambour. While the 

 armlet is 

 being ad- 

 justed the 

 stopcock is 

 * turned so 

 that the rubber bag is 

 in communication with 

 the external air through 

 F. The same is true of 

 the space TS in the 

 glass -bulb. The tam- 

 bour is thus protected 

 against undue strain during adjustment* The stopcock is now rotated 

 so as to cut off the armlet from the exterior and to permit the entrance 

 of air through F from V, which is used as a pump to raise the pressure, 

 the space TS and the tambour being still in communication with the 

 exterior. When the desired pressure has been reached, the stopcock 

 is turned into an intermediate position, which cuts off both the armlet 

 and the space TS from the exterior, and the pulse is then transmitted to 

 the tambour and recorded on the drum. By certain adjustments of the 



Fig. 45. Sphygmomanometer of Erlanger. 



