456 



CIRCULATION OF BLOOD AND LYMPH. 



in overcoming this resistance. The amount of the resistance offered by the 

 tissues has been estimated differently by various authors, but probably lies 

 between 6 and 10 mms. of mercury, that is, the pressure as measured exceeds 

 the real diastolic pressure by this amount. Several instruments have been 

 devised, according to this principle, to measure diastolic pressures, but the 

 sphygmomanometer described by Erlanger* is probably the most complete 

 and the most convenient for actual use. This instrument is illustrated in 

 Figs. 191 and 192. 



It may be used to determine both systolic and diastolic pressures. 



The way in which the apparatus is used may be understood from the sche- 

 matic Fig. 191 . a is the rubber bag which is buckled upon the arm by a leather 

 strap. This bag communicates with the mercury manometer, b, with a pres- 

 sure bag, c, through the two-way stopcock, i, and through the stopcock d with 

 a rubber bag, e, contained in a glass chamber, /. This glass chamber com- 

 municates above with a sensitive tambour, h, and by means of the stopcock 



Fig. 191. 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 in a to fall slowly) ; e, 

 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. 



g can be placed in communication with the outside air. The systolic pressure 

 may be determined in two ways : By one method only the mercury manom- 

 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 



* "American Journal of Physiology," "Proceedings of the American 

 Phvsiological Society," 6, xxii, 1902; and "Johns Hopkins Hospital Reports," 

 12, 53, 1904. 



