Aneurism (Gr. a widening or extension), a term used in surgery to signify a vascular tumor or enlargement, arising from the morbid distention of an artery It is much more common in some arteries than others, but any artery of the body is liable to it. The corresponding disease and enlargement of a vein is termed varix. An artery is composed of three coats or membranes which form the walls of a strong, elastic, and distensible tube. In a healthy state, the tube maintains a certain diameter under the ordinary impulse of the blood; but when the walls of an artery become diseased, they yield before the constant pressure of the circulating fluid, causing the diseased part to form a bag or tumor. This bag enlarges as the diseased walls distend; and the inner and middle coats, being less resisting than the outer, especially in a diseased state, give way and burst, leaving the outer coat alone to form the walls of the aneurismal tumor. This admits of a considerable amount of distention; but it eventually bursts, and then the unrestrained current gushes out with violence, and the patient dies from loss of blood.

Sometimes, however, the dense cellular sheath of the artery, though very distensible, is strong enough to retain the blood for a time after the rupture of the proper Avails of the artery, and the aneurismal sac may thus be very much enlarged, and not give rise to fatal haemorrhage. At times, in fact, this external sac allows the ruptured walls within to partially collapse; the current flows as usual; the blood in the external sac coagulates and forms a clot around the ruptured part beneath; the clot increases from its stagnant state outside the current, and eventually plugs up even the ruptured parts, extending into the diseased artery and plugging up the tube, thus forcing the current to flow more abundantly through collateral channels, enlarging the walls of neighboring arteries, and forming a spontaneous cure for the original disease. This is a very rare occurrence. The tumor usually enlarges by degrees, pressing upon the nerves and tissues near it, and causing pain as well as absorption of the soft or bony structures against which it presses. An aneurismal tumor always pulsates strongly, and can generally be distinguished from all other tumors by this characteristic feature.

It sometimes happens, however, that an artery pulsating beneath an abscess or an ordinary tumor causes the latter to simulate to some extent this pulsating character, and hence arise at times errors of diagnosis of a serious character; real aneurisms have been mistaken for abscesses lying upon a pulsating artery, and when opened under this impression to let out pus, the blood has gushed out from an aneurismal tumor, and the error has proved fatal. Ruysch relates that a friend of his opened a tumor near the heel, not suspecting it to be an aneurism, and the haemorrhage, though stopped at last, placed the life of the patient in great danger. Boerhaave was consulted by a patient on a swelling of the knee, and, suspecting it to be an aneurism, cautioned him against having it opened; but it was opened by another person, and the man died on the spot. It is said that Ferrand, the head surgeon of the Hotel Dieu in Paris, mistook an axillary aneurism for an ab-scess, plunged his bistoury into the swelling, and killed the patient.

Such mistakes, however, can hardly happen now, as all the best works on surgery give ample instructions on the means of diagnosis in this and other important diseases. - The cure of aneurism consists in the obliteration of the diseased portion of the artery, by passing a ligature around the sound portion of the vessel at some distance above the locality of the tumor. The merit of this method of cure is due to the celebrated John Hunter, who, observing that the old practice of passing the ligature upon the artery immediately above the tumor often failed, was led to think that the arterial walls, being diseased near the tumor, could not sustain the process of inflammation necessary to cause the tissues to adhere; and consequently he undertook to tie the femoral artery in a case of popliteal aneurism, and was perfectly successful. Since then his method has been universally adopted. Recently many attempts have been made, some of them with considerable success, to produce a similar result, either by continuous pressure over the artery kept up for a day or two, or by a ligature applied temporarily to the artery and withdrawn as soon as coagulation has taken place in the aneurism and the neighboring portion of the artery.

By whatever means this is accomplished, the flow of blood is stopped in the large vessels below the ligature; but the secondary vessels communicate with each other so abundantly in all parts of the limb, by what is called anastomosis, that the blood soon finds its way through these smaller channels, and enlarges them by slow degrees to suit the wants of nutrition.