yphilis plays an important part in the production of aneurysm, probably
by predisposing the patient to arterio-sclerosis and atheroma, and
inducing an increase in the vascular tension in the peripheral vessels,
from loss of elasticity of the vessel wall and narrowing of the lumen as
a result of syphilitic arteritis. It is a striking fact that aneurysm is
seldom met with in women who have not suffered from syphilis.

#Varieties--Fusiform Aneurysm.#--When the _whole circumference_ of an
artery has been weakened, the tension of the blood causes the walls to
dilate uniformly, so that a fusiform or tubular aneurysm results. All
the coats of the vessel are stretched and form the sac of the aneurysm,
and the affected portion is not only dilated but is also increased in
length. This form is chiefly met with in the arch of the aorta, but may
occur in any of the main arterial trunks. As the sac of the aneurysm
includes all three coats, and as the inner and outer coats are usually
thickened by the deposit in them of connective tissue, this variety
increases in size slowly and seldom gives rise to urgent symptoms.

As a rule a fusiform aneurysm contains fluid blood, but when the intima
is roughened by disease, especially in the form of calcareous plates,
shreds of clot may adhere to it.

It has little tendency to natural cure, although this is occasionally
effected by the emerging artery becoming occluded by a clot; it has also
little tendency to rupture.

#Sacculated Aneurysm.#--When a _limited area_ of the vessel wall is
weakened--for example by atheroma or by other form of arteritis--this
portion yields before the pressure of the blood, and a sacculated
aneurysm results. The internal and middle coats being already damaged,
or, it may be, destroyed, by the primary disease, the stress falls on
the external coat, which in the majority of cases constitutes the sac.
To withstand the pressure the external coat becomes thickened, and as
the aneurysm increases in size it forms adhesions to surrounding
tissues, so that fasciae, tendons, nerves, and other structures may be
found matted together in its wall. The wall is further strengthened by
the deposit on its inner aspect of blood-clot, which may eventually
become organised.

The contents of the sac consist of fluid blood and a varying amount of
clot which is deposited in concentric layers on the inner aspect of the
sac, where it forms a pale, striated, firm mass, which constitutes