In some rare cases the deposition of fibrin has gone on gradually accumulating, filled completely the aneurismal cavity, and thus effected a spontaneous cure, the remaining solid tumour imperceptibly diminishing by the action of the absorbents. After obliteration of the aneurismal cavity, the fibrin is generally deposited in so great quantity as to occupy the calibre of the vessel above and below the tumour, obstructing the progress of the blood, causing it to flow by the smaller and collateral branches, and effecting a spontaneous cure, somewhat similar to that produced by the artificial application of a ligature. Coagula are seldom formed in the dilated vessel, to whatever size it may be enlarged, unless there is fissure of the internal coat; for in no other way can a portion of the blood readily become stagnant, while the calibre of the vessel remains pervious. There is in my collection a preparation of dilated aorta, to the coats of which adheres a large firm coagulum. Occasionally, though rarely, a dilatation of the internal coats is met with accompanied by thinning of the external ones. Of this sort of diverticulum, there is also a good specimen in the collection here alluded to. A spontaneous cure may also be accomplished from the original aneurism being compressed by one of a more recent origin, causing ultimate obliteration of the canal. Of this I recollect one remarkable instance; the patient was afflicted with an aneurism of the axillary artery, which had attained a large size, and the cure for the disease in this situation being then unknown or unattempted, the patient was considered as lost; but some time after the tumour began to diminish, and disappeared. The patient died; and the cause of death was found to be the giving way of an aneurismal tumour of the arteria anonyma, which was situated so closely to the aneurism of the subclavian as to have acted as a mechanical compress, causing obliteration of the vessel at that point. When a cure has been effected, the vessel is found to be converted into a dense and impervious cord at the site of the tumour. The canal above is dilated; the coats are thickened, especially the middle; and from the thickening and increased action of the fibres, the internal coat becomes somewhat rugous, the rugæ being in a transverse direction.