Under certain conditions arteries are liable to undergo dilatation in certain 'parts, so that the cavity of the vessel is more or less enlarged. Such a state is known as an aneurism. This form of disease is more especially- seen in foals and young horses, as a result of the presence of parasites in the large vessels which supply blood to the bowels. It may also be brought about by sprain or any injury which may induce inflammation and consequent softening of the vessel. Aneurism is sometimes seen in the aorta, as it courses beneath the spine, and more rarely in some other of the smaller vessels. In some instances the presence of the tumour does not seem to give rise to any disturbance in the circulation which can be recognized during life, and is therefore only discovered after death. Where, however, the dilatation is considerable, it may very seriously incommode the circulation, and by causing obstruction lead on to hypertrophy, with dilatation of the walls of the heart and embarrassment in the breathing.

In the horse, aneurism most frequently occurs in the anterior mesenteric artery as the result of injury done to the walls of the vessel by parasites (Strongylus armatus). How these creatures gain access to the vessel is not exactly decided, but most likely by boring their way through the tissues of the bone. When in the vessel they excite irritation in the walls, resulting in inflammatory softening, and the pressure of the blood from within acting upon a yielding surface results in dilatation or aneurism. At the same time the existing endarteritis provokes fibrinous deposit on the inner surface of the vessel, tending to its more or less complete closure. As a result of this, the intestines fail to receive their proper amount of blood, on account of which diarrhoea is induced and prostration follows.

Aneurism of the Aorta.

Fig. 196. - Aneurism of the Aorta.