dal wave are diminished.

[Illustration: FIG. 71.--Radiogram of Aneurysm of Aorta, showing
laminated clot and erosion of bodies of vertebrae. The intervertebral
discs are intact.]

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
puls