Definition. A pulsating swelling on an artery, consisting of a sac filled with arterial blood.
Divisions. A true aneurism (aneurism by dilatation, arteriectasis) is a simple dilatation of the artery, the tumor being surrounded on all sides by the distended arterial walls. It is usually fusiform or cylindroid, but may have the form of a more or less rounded sac.
A false aneurism is where the wall of the artery has been lacerated and the blood is enclosed in an adjacent sac of condensed connective tissue and communicates with the interior of the vessel. The same name has been given to cases in which the inner coat only has given way, and the middle and outer coats constitute the walls of the sac. From its liability to extend and separate the tissues this is further known as a dissecting aneurism. Arterio-venous aneurism in which an intervening sac communicates with both artery and vein, has been found in the human subject.
Mixed aneurisms are those in which a dilatation of the artery is complicated by the presence of an outside pouch.
A distinction has also been made according to origin into traumatic and spontaneous. The former is of necessity false, whereas the latter may be false or true.
Causes. Apart from rupture of the arterial coat by direct violence, the common cause is a debility and loss of resistance in the walls. In horses a far larger proportion of aneurisms are deep-seated than in man, in whom forced muscular effort is less common. Yet even in horses the most common seat—the posterior aorta—is liable to overstretching and to inflammation and softening by reason of contiguity to dorsal sprains. The posterior aorta too, from its size and direction on leaving the heart, is in the direct line of the strongest blood current, and under long continued, forced and violent efforts (as in racing, hunting, and heavy uphill draughts), has to sustain an extraordinary blood pressure. Bouley claims as an additional cause the pressure of a loaded colon. This is also the point of all others where the vessels suffer from the presence of the larval strongyli. From whatever cause originating, congestion of the arterial coats leads to more or less attenuation, softening or lack of cohesion, and they tend to yield under the blood pressure. Similar conditions operate on the smaller vessels in different parts of the body, and thus overstretching, contiguous inflammation, and excessive blood pressure cause such lesions in the chest, trunk and limbs.
Another cause is embolism which by blocking an artery at once increases the tension in the vessel on the cardiac side of the obstruction, and develops inflammation in the arterial coats, robbing them of their cohesion and resisting power.
Eppinger has shown the importance of infectious microbes in weakening the arterial walls and predisposing to aneurism.
The larval strongylus armatus already referred to is the most potent factor in solipedes. They accumulate in the anterior mesenteric artery, leading to clotting of the blood, inflammation of the serous coat, and dilatation, so that in some verminous localities nearly every old horse shows a lesion of this vessel.
All forms of arteritis, and disease of the vascular walls which entail attenuation or weakening, predispose to aneurism.