A text-book of physiology, for medical students and physicians . ed measures, according to the principle stated above,the diastolic pressure within the artery. That the principle is correct has 4ss CIRCULATION OF BLOOD AND LYMPH. been shown by direct experiments upon the exposed artery of a dog, in whichthe pressure was measured by the method outlined above and also directlyby a manometer connected with the interior of the artery.* In such experi-ments upon man, however, one condition is present which detracts from theabsolute value of the results obtained, although, since it is substantiallya
A text-book of physiology, for medical students and physicians . ed measures, according to the principle stated above,the diastolic pressure within the artery. That the principle is correct has 4ss CIRCULATION OF BLOOD AND LYMPH. been shown by direct experiments upon the exposed artery of a dog, in whichthe pressure was measured by the method outlined above and also directlyby a manometer connected with the interior of the artery.* In such experi-ments upon man, however, one condition is present which detracts from theabsolute value of the results obtained, although, since it is substantiallya constant factor, it does not seriously interfere with relative results, thatis, 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 indirectlythrough the intervening tissues. These tissues interpose a certain resistanceto the pressure exerted from without, and some of this pressure must be spent. Fig. Erlanger apparatus, The collar for the arm is not shown. The parts maybeunderstood by reference to the Bchema given in Fig. 200. in overcoming this resistance. The amount of the resistance offered by thetissues lias been estimated differently by various authors, but probably liesbetween •» and 10 nuns, of mercury,—that is, the pressure as measured exceedsthe real diastolic pressure by this amount. Several instruments have beendevised, according to this principle, to measure diastolic pressures, but thesphygmomanometer described by Erlangerf is probably the most complete * Howell and Brush, Proceedings of the Massachusetts Medical Society,1901. ?(?American Journal of Physiology, Proceedings of the AmericanPhysiological Society, 6, xxii., 1902; and Johns Hopkins Hospital Reports,12,53, 1 «.J04. VELOCITY AND PRESSURE OF BLOOD-FLOW. 489 and the most convenient for actual use. This instrument is illustrated i
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