ANEURISM, a circumscribed tumor of an artery composed of a sac, the cavity of which communicates with the artery and contains liquid or coagulated blood. So-called Itrue" aneurisms are everywhere composed of all of the coats of the artery, and are rare and always small. Aneurisms are usually of the class an atomically designated as °false," in which only one of the coats of the artery takes part in the formation of the walls of the sac. Con densed adjoining tissues may also enter into the formation of the wall. A more common classi fication divides aneurisms into (1) common or encysted aneurisms, (2) arteriovenous aneu risms, (3) circoid aneurisms and (4) dissecting aneurisms. The first division includes aneu rismal dilatation or fusiform dilatation which extends for some distance, with the three arte rial coats preserved; true aneurism, mentioned above, limited to one point; false aneurism in which one coat has disappeared ; and consecutive or diffused aneurism in which the condensed adjoining tissues take part. Arteriovenous an eurism denotes an abnormal communication be tween artery and vein with or without an inter mediate sac. Circoid aneurism means the gen eral dilatation of an artery and its branches and dissecting aneurism is formed by the effusion of blood between the coats of an artery. In
ternal aneurisms are those situated within the thoracic or abdominal cavities, external aneu risms those in other parts of the body.
Aneurisms are also known as spontaneous or traumatic according to their origin. The latter occurs in consequence of sudden division or injury of the walls of an artery through some external agent or a splinter from a fractured bone. A spontaneous aneurism is due to a de generative change in the wall of the artery by which its elasticity and resistance to strain are diminished. The degenerative change may be due to a lodgment of an embolus, over stretching, pressure of a ligature or some other mechanical cause. Anatomical factors in the production of an aneurism are those changes in the artery, change of direction, enlargement, bifurcation, attendant upon its course through the body. Occupations of exertion,, alcoholic excess, the gouty or rheumatic diathesis, syphi lis, are also causative factors. An aneurism may terminate spontaneously by coagulation of the blood within it. Absolute rest and low diet are the most essential medical means of treat ment, but the treatment is largely surgical, its details depending upon the variety and situa tion of the aneurismal swelling.