2 



Scientific Proceedings (69). 



Dr. Dawson and myself) made the observation 1 that after partial 

 occlusion of the thoracic aorta the maximum pressure may be 

 permanently lowered as much as 46 mm. Hg, and the minimum 

 pressure actually increased distal to the constricting band of metal. 



The dilatation of the artery observed in arterio-venous fistula, 

 might, it seemed to me, have a bearing on the interpretation of the 

 aneurisms in cases of cervical rib. Might not both phenomena, 

 I asked myself, be due to degenerative changes in the arterial 

 wall consequent upon lowered pressure — in the case of the cervical- 

 rib-aneurisms, upon lowering of the pulse pressure. 



Now, inasmuch as dilatation of the subclavian artery has rel- 

 atively so seldom been observed with cervical rib (perhaps 24 times 

 in about 400 cases) it seemed to me that if it were due merely to 

 the lowered pulse pressure then only a very definite absolute or 

 relative amount of reduction of the systolic pressure would suffice 

 to produce it. 



In June, 1914, I observed, in a dog, for the first time an un- 

 questionable dilatation of the three arteries below the constricting 

 band which had been placed just above the aortic trifurcation. 

 The constriction exercised by the band was sufficient to greatly 

 lessen, if not, indeed, to obliterate the palpable thrill produced by 

 the constriction, but not enough to shut off the palpable pulse. 

 With this observation as fresh incentive, Dr. Reid and I have con- 

 tinued the experiments for the past year and a half with encourag- 

 ing results: in only one additional instance, however, was there a 

 very striking dilatation. In this, as in the one of the preceding 

 year, the occlusion of the aorta by the band was almost total. 



If the occlusion must be so nearly complete in order to effect 

 a pronounced dilatation it will assist to explain not only the dif- 

 ficulty we have had in producing it in dogs, but also the fact that 

 it has been observed relatively so seldom in the human subject 

 from compression of the subclavian artery by a cervical rib. For 

 when in dogs the aortic pulse is occluded beyond the stage of 



1 Dog 96. Partial Occlusion of Thoracic Aorta. 

 Operation, 22/5/1906. Sacrificed 7 months later. 



Maximum Mean Minimum 



Pressure. Pressure. Pretsure. 



Pulse 

 Pretsure. 



Femoral 

 Carotid . 



116 93 88 



160 113 83 



28 

 77 



