In the progress of an aneurysm towards rupture, timely clotting may avert death for the moment, but while extension in one direction has been arrested there is apt to be extension in another, with imminence of rupture, or it may be again postponed. #Differential Diagnosis.#--The diagnosis is to be made from other pulsatile swellings. Pulsation is sometimes transmitted from a large artery to a tumour, a mass of enlarged lymph glands, or an inflammatory swelling which lies in its vicinity, but the pulsation is not expansile--a most important point in differential diagnosis. Such swellings may, by appropriate manipulation, be moved from the artery and the pulsation ceases, and compression of the artery on the cardiac side of the swelling, although it arrests the pulsation, does not produce any diminution in the size or tension of the swelling, and when the pressure is removed the pulsation is restored immediately. Fluid swellings overlying an artery, such as cysts, abscesses, or enlarged bursæ, may closely simulate aneurysm. An apparent expansion may accompany the pulsation, but careful examination usually enables this to be distinguished from the true expansion of an aneurysm. Compression of the artery makes no difference in the size or tension of the swelling. Vascular tumours, such as sarcoma and goitre, may yield an expansile pulsation and a soft, whifling bruit, but they differ from an aneurysm in that they are not diminished in size by compression of the main artery, nor can they be emptied by pressure. The exaggerated pulsation sometimes observed in the abdominal aorta, the "pulsating aorta" seen in women, should not be mistaken for aneurysm. #Prognosis.#--When _natural cure_ occurs it is usually brought about by the formation of laminated clot, which gradually increases in amount till it fills the sac. Sometimes a portion of the clot in the sac is separated and becomes impacted as an embolus in the artery beyond, leading to thrombosis which first occludes the artery and then extends into the sac. The progress of natural cure is indicated by the aneurysm becoming smaller, firmer, less expansile, and less compressible; the murmur and thrill diminish and the pressure effects become less marked. When the cure is complete the expansile pulsation is lost, and there remains a firm swelling attached to the vessel (_consolidated aneurysm_). While these changes are taking place the collateral arteries become enlarged, and an anastomotic circulation is established. An aneurysm may prove _fatal_ by exerting pressure on important structures, by causing syncope, by rupture, or from the occurrence of suppuration. _Pressure_ symptoms are usually most serious from aneurysms situated in the neck, thorax, or skull. Sudden fatal _syncope_ is not infrequent in cases of aneurysm of the thoracic aorta.