Aneurism (Gr. aneurysma, 'a dilatation') is a 'tumour containing blood, and communicating with the cavity of an artery' (Holmes's System of Surgery, 3d ed.). The sac of an aneurism may be formed in the first instance by one or more of the tunics of the vessel, generally the outer one, the two inner having given way. This is called a true aneurism, in contradistinction to the false, in which the sac is formed of cellular tissue condensed by the blood flowing into it after a wound has been inflicted on the artery from without. Should the sac give way, and the blood escape among the tissues, the aneurism is said to be diffused. True aneurism is almost always a result of Atheroma (see ARTERIES, Diseases of), and is most common in middle life. Aneurisms prove fatal by their pressure on some important part, or by bursting and allowing a sudden escape of blood. They are cured by the deposit, within the sac, of fibrin from the blood—a result the surgeon can promote by obstructing the artery above the aneurism by compression or by ligature; applying the latter close to the sac, if the aneurism is of the 'false' variety, but at a distance if it is the result of disease, or by electrolysis by means of needles introduced into the sac, or by passing into it a coil of fine wire. Internal aneurisms are treated by those remedies which moderate the heart's action, especially iodide of potassium, in conjunction with an abstemious and tranquil mode of life.
Very minute (or military) aneurisms are sometimes found in the arteries of the brain, a condition which may lead to apoplexy.