524 



Messrs. M. Flack, L. Hill, and J. McQueen. 



For let us consider the arm when it is compressed. The veins and 

 capillaries under the armlet are first flattened and some are emptied, the 

 pressure rises pari passu with the compression in all the remaining patent 

 blood-vessels enclosed by the armlet. This must be so, for their outlet is 

 obstructed. In the forearm, peripheral to the armlet, the venous pressure 

 will steadily rise, and the veins become more and more swollen and tense if 

 the compression is maintained just below the arterial pressure. Under these 

 conditions the arterial blood can flow into the forearm, so long as capillary 

 fields, hitherto empty, open out and the veins swell, the forearm becoming 

 more and more congested. 



If a second armlet be put just below the first, and the pressure raised 

 equally in the two armlets, the conditions are made the same as in Experi- 

 ment II, in so far as the peripheral resistance in the arm is concerned. But, 

 be it noted, as the blood has other arterial pathways open to it in the rest of 

 the body, the diastolic pressure in the arteries enclosed by the upper armlet 

 is not raised nearly up to the systolic pressure. To make the conditions in 

 that the same as in Experiment II, the resistance to outflow in all the 

 arteries would have to increase pari passu with the compression of the arm. 



In the case of the vessels enclosed by the lower armlet under these condi- 

 tions the diastolic pressure is raised nearly up to the systolic pressure. Now, 

 we have determined experimentally that the reading of systolic pressure 

 taken with an armlet round the calf is raised by placing a second armlet 

 round the thigh and raising the pressure therein to, and keeping it at, say, 

 50 mm. Hg. This correspondingly increases the diastolic pressure in the 

 veins and arteries of the leg, and the arteries, being made more rigid thereby, 

 conserve better the crest of the systolic wave in its passage from thigh to 

 calf. Similarly, the pulse in the radial artery increases in amplitude at first 

 when the compression is raised in an armlet placed round the upper arm, 

 because the compression by obstructing the outflow and making tenser the 

 arteries aids the conduction of the systolic wave. 



Experiment IV. 



A single length of artery was compressed and the outflow measured. The 

 compression chamber was connected with the manometer. ■ When the com- 

 pression reached 25 cm. H 2 the artery began to flatten. At 34 cm. H 2 

 the pulse became maximal, and the water then issued in strong pulses ; 

 212 c.c. flowed out in one minute. At 47 cm. H2O the water issued in shorter 

 pulses, for the artery remained deformed for a longer period during each 

 diastole ; the outflow was reduced to 166 c.c. At 70 cm. H 2 the outflow 

 was reduced to feeble spurts synchronous with the systoles ; while at 77 cm. 



