346 TEXT-BOOK OF PHYSIOLOGY. 



the rubber armlet H, which is 3! inches wide. This is the widest armlet 

 that can be adjusted to the average-sized arm and presents distinct advan- 

 tages over the narrow armlet hitherto employed. This armlet is prevented 

 from expanding outward by a cuff, F, of double thick canvas with inserted 

 strips of tin, which is held in place by two straps which completely encircle 

 the cuff. On the rigidity of this depends to a large extent the transmission 

 of pulsation. The rubber armlet is connected by glass with a stiff-walled 

 rubber tube, G, which in turn connects with the manometer. The man- 

 ometer is perhaps the most important part of the apparatus. It is constructed 

 entirely of metal except for the glass tube containing the mercury column. 

 The chamber c communicates by means of a metal tube with the glass 

 column D, which is connected by a screw-thread at 3, the caliber of c being 

 approximately 100 times that of D. The cap of the chamber, which screws 

 on, is provided with a metal T which is connected at 2 with the rubber armlet 

 and at i with the bulb, used as an air-pump. At A is a stopcock shutting the 

 rubber bulb completely from the rest of the apparatus while at B is a screw- 

 valve which allows the air to escape from the closed system. When desired, 

 the manometer can be made portable (without removing the mercury) by 

 screwing the caps i and 2 into either end of the T at i and 2. The man- 

 ometer is then tilted away from the glass column D until all the mercury has 

 run into the chamber, the glass is then unscrewed and cap 3 screwed in. 

 Before removing cap 3 the manometer must always be tilted, else the mercury 

 will be lost. The rubber bulb is similar to those found on atomizers, with 

 the addition of a distensible reservoir to obliterate the air pulse. 



In using this apparatus the pressure is raised by the air-bulb forcing air 

 into the closed system distending the rubber armlet and with the same 

 degree of force displacing the mercury in c, driving it up the glass column D. 

 When the pulse is no longer felt, the bulb still being compressed, the arm of 

 valve A is turned until it is at right angles with the thumb and finger. The 

 valve B is now slowly unscrewed until the mercury column begins to fall. At 

 a given level it exhibits a considerable oscillation which may be mistaken 

 for the actual systolic pressure but which is probably due to the impact of the 

 blood against the upper edge of the rubber portion of the cuff. If the 

 column of mercury be still further lowered so that the pressure indicated is 

 a trifle lower than the systolic pressure the blood will be forced through the 

 compressed artery and give rise to a pulse wave, which may be felt at the 

 wrist. The highest excursion of the mercurial column noted by the eye at 

 the moment the pulse reappears is regarded as the systolic pressure. 



The pressure is then lowered 5 millimeters at a time and the oscillations of 

 the mercurial column noted. As the pressure is thus slowly lowered there 

 will come a moment when the oscillations will attain a maximal value and 

 beyond which the oscillations again diminish. The lowest level of the 

 mercury column at the time of the greatest oscillation is taken as the diastolic 

 pressure. 



Erlanger's sphygmomanometer is, also, a most valuable instrument for 

 obtaining both systolic and diastolic pressure. It possesses an advantage 

 in that it is provided, in addition to the mercurial manometer, with a tam- 

 bour and lever by which changes in pressure can also be recorded on a re- 

 volving cylinder (Fig. 162). A complete description of this apparatus, 



