AN EXPERIMENTAL STUDY OF CIRCUMSCRIBED DILATION OF AN ARTERY IMMEDIATELY DISTAL TO A PARTIALLY OCCLUDING BAND, AND ITS BEARING ON THE DILATION OF THE SUBCLAVIAN ARTERY OBSERVED IN CERTAIN CASES OF CERVICAL RIB.

1. A partially occluded artery may dilate distal to the site of constriction. 2. The dilation is circumscribed. 3. When the constriction has been either slight in amount or complete, dilation has not been observed. 4. The dilation was greatest when the lumen of the artery (the aorta) was reduced to one-third or perhaps one-fourth of its original size (Figs. 8 and 11). 5. Dilation or aneurysm of the subclavian artery has been observed twenty-seven or more times in cases of cervical rib. 6. The dilation of the subclavian is circumscribed, is distal to the point of constriction, and strikingly resembles the dilation which we have produced experimentally. 7. The genesis of the experimental dilation and of the subclavian dilation occurring with cervical rib is probably the same. 8. When the lumen of the aorta is considerably constricted the systolic pressure may be permanently so lowered and the diastolic pressure so increased that the pulse pressure is greatly diminished. 9. The experimentally produced dilations and the aneurysms of the subclavian artery in cases of cervical rib are probably not due to vasomotor paralysis, trauma, or sudden variations in blood pressure. 10. The abnormal, whirlpool-like play of the blood in the relatively dead pocket just below the site of the constriction, and the lowered pulse pressure may be the chief factors concerned in the production of the dilations. 11. Intimal surfaces brought, however gently, in contact by bands or ligatures do not, in our experience, unite by first intention, for the force necessary to occlude the artery is sufficient to cause necrosis of the arterial wall. 12. Bands, rolled ever so tightly, do not rupture the intima. 13. The death of the arterial wall having been brought about by the pressure of the band, a gradual substitution of the necrotic tissue takes place, the new vessels penetrating it from both ends. It is, I believe, in this manner that an artery becomes occluded, and it is thus that a fibrous cord forms within the constricting band.