=Ossification.= Ossification of the aorta, like that of the coronary and other arteries, consists rather in a process of _calcification_. The deposits are largely confined to the arch, and consist mainly of patches of calcareous matter of various sizes. We seldom find the whole circumference of the vessel involved, as in the case of smaller arteries. The aortic valves will generally be found more or less loaded with the same deposits.[17] =Aneurism.= This disease is said to occur more frequently in the aorta, than in any other artery. It may be developed in any portion of this vessel or its principal branches, but is more commonly found in the arch. The walls of the vessel being weakened by fatty degeneration, they become less and less able to resist the pressure of the contained blood, and gradually yielding to the systolic force of the heart, become more and more distended, until the complete aneurismal sac is formed. Aneurism may be either _true_, in which there is a dilatation of all the coats of the vessel, or _false_, where there is rupture of the inner, and perhaps also of the middle, and dilatation of the outer coat alone. The latter form, when developed upon the aorta, may become very large, and by pressure, cause absorption of the sternum, costal cartilages and ribs, and even of the clavicle. In some cases, the inner and middle coat having ruptured, the blood instead of being confined in a sac formed by the outer coat, becomes diffused between the middle and outer, or between the layers of the middle coat, thus constituting what is known as _dissecting_ aneurism. In these cases, the blood may extend the whole length of the aorta, and even upwards upon the carotids to their bifurcation. In an examination of an aneurismal sac, the _true_ aneurism will be recognized by the walls presenting all the coats of the artery, and generally by the indication of the presence of atheromatous and calcareous deposits, which are confined to the inner and middle coats. In these cases also, the communication between the sac and the aorta is large and free. If the aneurism be _false_, however, there will be an absence of those deposits, the opening into the artery will be comparatively small, and the inner and middle coats will terminate abruptly at its margin. The interior of aneurismal sacs will usually be found containing a quantity of fibrine deposited from the blood, and arranged in concentric layers. In color, these fibrinous layers are of a light buff, the outer layers being dry and firm, while the inner ones are softer and more moist, and the central portion, at the same time, filled with a dark mass of coagulated blood. A spontaneous cure will sometimes be effected by a complete blocking up of the sac with fibrinous deposits, thus preventing further dilatation or danger of rupture. The following case, which has been before reported,[18] shows a combination of both true and false aneurism, with spontaneous cure of the latter: