The single ligature, when properly applied, is the most safe, and preferable to any other, for arresting permanently the flow of blood in a vessel. In its application, the artery must not be separated from its connexions farther than is barely sufficient for the passage of the armed needle beneath it; but the external incision ought to be free, in order that this may be readily effected, and that the operation may be easily and speedily performed. By the firm application of a single ligature, the vessel is rendered impervious; the internal and middle coats are divided, so that the ligature only encircles the outer or cellular one, which resists the influence of any moderate degree of force by which it may be tightened. The blood coagulates above the deligated point,—the coagulum is of greater or less extent, in proportion to the vicinity of a collateral branch, and is of a conical form, the apex of the cone pointing to the free portion of vessel. Incited action in the vessel takes place at the deligated point; the divided margins of the internal and middle coats secrete lymph, by which they adhere, and so obliterate the canal of the artery. Lymph is also effused on the external surface, and in this deposit the ligature becomes imbedded. The direct influx of blood into the aneurismal sac is thus intercepted, and time is allowed for coagulation of the blood which it contains; the artery for a considerable distance below the ligature becomes ultimately converted into a firm and impervious chord. The coats of the vessel above the ligature are much thickened, and the internal membrane is occupied with the transverse rugæ occasioned by projecting fasciculi of the fibres, which are always apparent after obstruction of an artery. If this operation be properly conducted, success must almost uniformly follow. Before determining on its performance, however, the state of the arterial system ought to be examined as carefully as possible; for not unfrequently the degeneration of the coats is almost universal, and therefore an artery, or even arteries, may be diseased at more points than one; and if this aneurismal diathesis exist, the patient may be found to labour under an internal aneurism of the aorta. In such a case, an operation could not with propriety be undertaken for the cure of the external aneurism; there might be no inconsiderable danger of the patient’s death being suddenly accelerated by the operation, the sac of the internal aneurism giving way perhaps during its performance: such a circumstance has actually occurred.