An aneurysm is a circumscribed dilatation of an artery. It may be (I) true or (2) false. True, an aneurysm in which the sac is formed by the arterial walls, one of which at least is unbroken. False, one in which all the coats are ruptured, the blood being retained by the surrounding tissues.

Endarteritis Productiva (Dürck).

Fig. 137. - Endarteritis Productiva (Dürck).

Aneurysms may be single or multiple, and may vary greatly in size. They result either from injury or disease of the vessel or from increased arterial blood-pressure.

They are classified according to their form. They may be:

1. Saccular, in which case there is a hemispherical dilatation extending from one side of the artery. As a result of the dilatation the middle coat soon atrophies, the inner coat is generally destroyed by an endarteritis so that the outer coat alone usually forms the wall of the aneurysm.

Aneurysm of the Arch of the Aorta (Bollinger).

Fig. 138. - Aneurysm of the Arch of the Aorta (Bollinger).

2. Fusiform, a cylindric dilatation extending for some distance along the artery. Are most common in the aorta.

3. Dissecting. In this variety the blood passes through an opening in the diseased intima and makes its way between it and the media, or between the media and adventitia. Occasionally the blood may re-enter the lumen through a lesion further along in the course of the vessel. Is most common in the large arteries.

4. False. Following an injury to an artery, a wall of fibrous tissue may form around the escaped blood, while the wound in the vessel remains open, so that there is an aneurysmal sac through which the blood is constantly flowing.

In the brain there is quite frequently found a condition known as multiple miliary aneurysms. In the course of the small vessels, particularly branches of the lenticulo-striate, numerous very small dilatations may be found. They are probably the result of degenerative changes in the media.

After an aneurysm has once been formed it may be rendered harmless by means of the deposition of layers of fibrin within the cavity. This may go on till there is complete obliteration; this, however, does not frequently occur as the deposit of fibrin cannot keep step with the dilatation of the vessel. Although an aneurysm is a soft structure there may be extreme destruction of surrounding tissues, particularly of bone. The pressure exerted by the aneurysm shuts off the periosteal blood-supply and more or less erosion of the bone follows.

Ultimately rupture of the sac generally occurs. The walls have become so thinned and weakened that they are no longer able to resist the pressure. Rupture commonly takes place when there has been some unusual muscular exertion. It may occur, though, when the individual is quiescent.