VELOCITY AND PRESSURE OF BLOOD-FLOW. 



469 



by a manometer connected with the interior of the artery.* In such experi- 

 ments upon man, however, one condition is present which detracts from the 

 absolute value of the results obtained, although, since it is substantially 

 a constant factor, it does not seriously interfere with relative results, that 

 is, with observations upon the variations of pressure under different condi- 

 tions. This source of error lies in the fact that in the living person the out- 

 side pressure can not be applied directly to the arteries, but only indirectly 

 through the intervening tissues. These tissues interpose a certain resistance 

 to the pressure exerted from without, and some of this pressure must be spent 



Fig. 194. 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 ; 6, 

 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. 



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 

 sphygmomano meter described by Erlanger f is probably the most complete 

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

 Figs. 194 and 195. 



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. 194. 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 

 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- 



* Howell and Brush. " Proceedings of the Massachusetts Medical Society," 

 1901. 



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

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

 12, 53, 1904. 



