An aneurysm exerts pressure on the surrounding structures, which are usually thickened and adherent to it and to one another.  Adjacent veins may be so compressed that congestion and oedema of the parts beyond are produced.  Pain, disturbances of sensation, and muscular paralyses may result from pressure on nerves.  Such bones as the sternum and vertebrae undergo erosion and are absorbed by the gradually increasing pressure of the aneurysm.  Cartilage, on the other hand, being elastic, yields before the pressure, so that the intervertebral discs or the costal cartilages may escape while the adjacent bones are destroyed (Fig. 71).  The skin over the tumour becomes thinned and stretched, until finally a slough forms, and when it separates haemorrhage takes place.

[Illustration:  FIG. 72.—­Sacculated Aneurysm of Abdominal Aorta nearly filled with laminated clot.  Note greater density of clot towards periphery.]

In the progress of an aneurysm towards rupture, timely clotting may avert death for the moment, but while extension in one direction has been arrested there is apt to be extension in another, with imminence of rupture, or it may be again postponed.

#Differential Diagnosis.#—­The diagnosis is to be made from other pulsatile swellings.  Pulsation is sometimes transmitted from a large artery to a tumour, a mass of enlarged lymph glands, or an inflammatory swelling which lies in its vicinity, but the pulsation is not expansile—­a most important point in differential diagnosis.  Such swellings may, by appropriate manipulation, be moved from the artery and the pulsation ceases, and compression of the artery on the cardiac side of the swelling, although it arrests the pulsation, does not produce any diminution in the size or tension of the swelling, and when the pressure is removed the pulsation is restored immediately.

Fluid swellings overlying an artery, such as cysts, abscesses, or enlarged bursae, may closely simulate aneurysm.  An apparent expansion may accompany the pulsation, but careful examination usually enables this to be distinguished from the true expansion of an aneurysm.  Compression of the artery makes no difference in the size or tension of the swelling.

Vascular tumours, such as sarcoma and goitre, may yield an expansile pulsation and a soft, whifling bruit, but they differ from an aneurysm in that they are not diminished in size by compression of the main artery, nor can they be emptied by pressure.

The exaggerated pulsation sometimes observed in the abdominal aorta, the “pulsating aorta” seen in women, should not be mistaken for aneurysm.

#Prognosis.#—­When natural cure occurs it is usually brought about by the formation of laminated clot, which gradually increases in amount till it fills the sac.  Sometimes a portion of the clot in the sac is separated and becomes impacted as an embolus in the artery beyond, leading to thrombosis which first occludes the artery and then extends into the sac.