The aorta is more frequently the seat of aneurism than any other vessel, and the parts most commonly affected by aneurismal formations, are the ascending aorta, and the arch.

The aneurisms most common on the trunk of the aorta are the cylindrical and spindle-shaped aneurism, the saccular form affecting only one side of the vessel, the pedicled, and even the cirsoid aneurism, which is occasionally observed along the entire length of the tube of the aorta.

Saccular expansions very frequently occur in the ascending aorta at the sinuses, and especially at those two which correspond to the convex wall of this portion of the aorta. These aneurisms very frequently burst at an early stage into the cavity of the pericardium, and occasionally into the right auricle. - Pericarditis in some instances gives rise to the rupture of aneurisms projecting into the cavity of the pericardium.

Aneurisms are incomparably more frequent on the convex than on the concave side of the ascending aorta.

The same is the case in reference to aneurism of the arch of the aorta.

On the aorta descendens, aneurisms within the thorax appear most frequently to proceed from the posterior wall and the sides of the vessel, so that they very commonly implicate the vertebral column and the adjacent thoracic wall.

Aneurisms of the abdominal aorta are usually spindle-shaped and saccular, and are most frequently developed from the anterior and lateral portions.

An extensive series of observations has afforded the following particulars in reference to the remarkable peculiarities presented by these aneurisms.

1. Those aneurisms which arise from the convexity of the ascending aorta, and from the anterior and upper wall of the arch of the aorta in general attain a very considerable size, inclining in such a direction that they touch the right half of the sternum, the costal cartilages, and the ribs of the right side from the first to the fifth or sixth, or even extend to the sterno-clavicular articulation and the right clavicle, finally destroying the parts by detritus, and coming to view externally in the corresponding region of the thorax. It is important to remember that such is their course, from which there are very few exceptions, because an aortic aneurism occurring at the sterno-clavicular articulation and at the right clavicle is very commonly mistaken by the bedside for a subclavian aneurism, which is in general erroneously supposed to be of great frequency.

2. Aneurisms, proceeding from the concavity of the ascending aorta, extend in the direction of the pulmonary artery, or are seated in front of it, towards the wall of the left auricle, and open into one or other of these parts.

Those aneurisms which proceed from the concavity and posterior portion of the arch of the aorta, abut upon the trachea and the bronchi, and in general open into them at an early period, and long before they have attained any considerable volume.

3. Aneurisms of the thoracic aorta commonly first implicate the vertebral column at the part corresponding to the above-described points of origin, and destroy it to various extents, and in rare cases, to such a degree that they- come in contact with the dura mater of the spinal chord, and even burst into the canal. They moreover diffuse themselves over the posterior wall of the left side of the thorax, and occasionally open freely into its cavity, or, in some rare cases, so completely destroy the thoracic wall as to come to view externally on the back. They very often implicate the left bronchus, make their way into the pulmonary parenchyma, and open into it, or into one of the larger bronchial tubes within the lung. - When they occur on the right side of the vessel, they are situated in the mediastinum and on the oesophagus into which they open.

4. In the very rare cases in which aneurisms of the abdominal aorta burst, their contents are usually effused into the cavity of the peritoneal sac.