ANEURYSM, or ANEURISM (from Gr. [Greek: aneurisma], a dilatation), a
cavity or sac which communicates with the interior of an artery and
contains blood. The walls of the cavity are formed either of the
dilated artery or of the tissues around that vessel. The dilatation
of the artery is due to a local weakness, the result of disease or
injury. The commonest cause is chronic inflammation of the inner coats
of the artery. The breaking of a bottle or glass in the hand is apt to
cut through the outermost coat of the artery at the wrist (radial)
and thus to cause a local weakening of the tube which is gradually
followed by dilatation. Also when an artery is wounded and the wound
in the skin and superficial structures heals, the blood may escape in
to the tissues, displacing them, and by its pressure causing them to
condense and form the sac-wall. The coats of an artery, when diseased,
may be torn by a severe strain, the blood escaping into the condensed
tissues which thus form the aneurysmal sac.

The division, of aneurysms into two classes, _true_ and _false_, is
unsatisfactory. On the face of it, an aneurysm which is false is not
an aneurysm, any more than a false bank-note is legal tender. A better
classification is into _spontaneous_ and _traumatic_. The man who has
chronic inflammation of a large artery, the result, for instance, of
gout, arduous, straining work, or kidney-disease, and whose artery
yields under cardiac pressure, has a _spontaneous_ aneurysm; the
barman or window-cleaner who has cut his radial artery, the soldier
whose brachial or femoral artery has been bruised by a rifle bullet
or grazed by a bayonet, and the boy whose naked foot is pierced by
a sharp nail, are apt to be the subjects of _traumatic_ aneurysm.
In those aneurysms which are a _saccular_ bulging on one side of the
artery the blood may be induced to coagulate, or may of itself deposit
layer upon layer of pale clot, until the sac is obliterated. This
laminar coagulation by constant additions gradually fills the
aneurysmal cavity and the pulsation in the sac then ceases;
contraction of the sac and its contents gradually takes place and
the aneurysm is cured. But in those aneurysms which are _fusiform_
dilatations of the vessel there is but slight chance of such cure, for
the blood sweeps evenly through it without staying to deposit clot or
laminated fibrine.

In the treatment of aneurysm the aim is generally to lower the blood
pressure b