The Measurement of Arterial Pressure in Man. 517 



encloses, by compressing the capillaries and obstructing the peripheral exits. 

 It thus converts the compressed area into a resonating mass ; the pulse is not 

 damped down in the labile arteries, but strikes the blood which fills to 

 distension not only the main artery, but every patent arteriole throughout 

 the mass, and causes the whole tense mass to vibrate. 



Thus we find in the case of the living subject, if the systolic pressure 

 be 115 mm. Hg and the armlet pressure be kept at 110 mm. Hg, the 

 venous pressure in the limb rises distal to the armlet. If another armlet 

 be put below the first, and the pressure raised within this, the pulse at 

 the radial will not be obliterated until the pressure in this armlet reaches 

 115 mm. Hg. If the conditions found in the schema of MacWilliam and 

 Melvin held good in the arm, a far lower pressure in this lower armlet 

 would suffice to obliterate the pulse, for in their schema, under similar 

 conditions, the distal manometers show a great diminution in pressure. 

 In the case of the arm, as the pressure is raised in the upper armlet 

 the venous outflow becomes obstructed, and the pulse then strikes a mass 

 of blood congested within the vessels which permeate the tissues ; no 

 pressure less than systolic in the lower armlet can prevent the vibration of 

 the mass reaching the radial artery. It is true that the pulse felt in the 

 radial becomes feeble as the pressure is raised in the upper armlet to 

 110 mm., but the pressure in the radial does not sink, because the blood still 

 flows in and cannot escape from the veins. The pulse in the radial is 

 enfeebled by the resistance which arises from the deformation of the 

 brachial artery brought about by a pressure in the upper armlet of 110 mm. 

 Its force is partly spent in the labile artery above this armlet. The range of 

 pulse pressure below the upper armlet is greatly diminished too, because the 

 diastolic pressure is raised owing to the venous obstruction. There is in 

 consequence a much smaller swing, but this swing cannot be stopped until 

 the pressure in the lower armlet is raised to the full systolic pressure, 

 115 mm. Hg. 



The facts we have detailed above show that the simple schema, in which an 

 artery is compressed in a chamber full of water, does not represent the 

 conditions which pertain in the arm. 



We have attempted to imitate these conditions in the schema represented 

 in fig. 1. 



Two glass compression chambers are filled with water and connected with 

 each other and to a compression bottle. In one is placed a piece of human 

 carotid artery, in the other a schematic representation of the tissue vessels. 

 This consists of a condom (thin-walled, wide rubber tube) filled to distension 

 with chopped rubber sponge. The expansion of the condom is limited by 



2 s 2 



