The Resonance of the Tissues. 



259 



registered on a plethysmograph curve. When the bag of the pocket 

 sphgymometer is apphed to the artery, either at position I or at position II, 

 the pulsations in the air of the bag are at a minimum, because the tissues 

 lying under the bag are comparatively pulseless. In the case of the armlet, 

 with its wider embrace of pulsing tissues, the air shows pulsations more 

 or less synchronous to the pulse in the artery, the arterial pulse is thereby 

 strengthened and enabled to resist the damping-down effect of the armlet. 

 Consequently the pressure applied to the arterial wall may be increased from 

 60 to 100 or 110 mm. of Hg, as the case may be, and yet the pulse persists, 

 provided the medium through which the pressure is applied is itself in a 

 condition of bike periodic vibration. Of course, the vibrations must be 

 of such a period as will strengthen the pulse of the artery and not 

 oppose it. 



It is on this fundamental experiment that the hypothesis of the resonance 

 of the tissues is grounded. By this hypothesis we can explain the various 

 readings obtained by the same instrument {e.g. bag of pocket syhygmometer) 

 at positions I, II, III and IV. Position II is obviously similar to position I. 



In position III the radial artery lies as we trace it centrally, first on the 

 pronator quadratus, and then on the flexor longus pollicis. When the bag is 

 applied to the artery in position III, there are beneath it fleshy tissues with 

 numerous arteries in them. Consequently, the tissues below the bag are 

 throbbing more or less synchronously with the pulse in the radial artery at 

 position III. The air of the bag is then in a state of periodic vibration, as in 

 the case of the air of the armlet. Accordingly, the reading becomes the high 

 one of 130-135 mm. of Hg. The damping-down effect of the bag on the 

 pulse has been compensated for by the resonance of the tissues beneath it. 

 Headings with the bag of the pocket sphygmometer placed at position IV 

 have been noted to vary from 60 to 100 mm. of Hg. In taking these 

 readings, the recurrent ulnar pulsation can be damped down at position I, 

 and the pulse felt at position II. Such variable readings do not occur 

 haphazardly ; it can be demonstrated that they depend on the varying 

 anatomical condition of the areas below the bag. Such areas may be 

 classified into areas of high resonance and areas of low resonance. 



If a diagonal line is drawn through the centre of the superficial 

 brachial artery at position IV (see Diagram), the bag of the sphygmometer 

 can be so placed that 1/3 of the bag lies to the radial side of the artery and 

 2/3 on the ulnar side, or, the same length of artery being covered as before 

 by the bag, 2/3 of the bag can lie to the radial side of the artery and 1/3 to 

 the ulnar side. 



These positions are indicated by the circle (a) and the circle (I). 



