er causes. Aneurism of the anterior aorta may be situated very
closely to the heart or in the arch, and it is very seldom that we can
distinguish it from disease of the heart. The tumor may encroach upon
the windpipe and produce difficulty in breathing, or it may produce
pressure upon the vena cava or the thoracic duct, obstructing the flow
of blood and lymph. In fact, whatever parts the aneurism may reach or
subject to its pressure, may have their functions suspended or
disturbed. When the tumor in the chest is large, we generally find much
irregularity in the action of the heart; the superficial veins of the
neck are distended, and there is usually dropsical swelling under the
breast and of the limbs. There may be a very troublesome cough without
any evidence of lung affection. Sometimes pulsation of the tumor may be
felt at the lower part of the neck where it joins the chest. When the
aneurism occurs in the posterior aorta no diagnostic symptoms are
appreciable; when it occurs in the internal iliac arteries an
examination per rectum will reveal it.

There is one form of aneurism which is not infrequently overlooked,
affecting the anterior mesenteric artery, primarily induced by a
worm--_Strongylus vulgaris_. This worm produces an arteritis, with
atheroma, degeneration, and dilatation of the mesenteric arteries,
associated with thrombus and aneurism. The aneurism gives rise to colic,
which appears periodically in a very violent and often persistent type.
Ordinary colic remedies have no effect, and after a time the animal
succumbs to the disease. In all cases of animals which are habitually
subject to colicky attacks, parasitic aneurism of the anterior
mesenteric artery may be suspected. (See p. 92.)

_Pathology._--Aneurisms may be diffuse or sacculated. The diffuse
consists in a uniform dilatation of all the coats of an artery, so that
it assumes the shape of a cylindrical swelling. The wall of the aneurism
is atheromatous, or calcified; the middle coat may be atrophied. The
sacculated, or circumscribed, aneurism consists either in a dilatation
of the entire circumference of an artery over a short portion of its
length, or in a dilatation of only a small portion of one side of the
wall. Aneurism may become very large; as it increases in size it presses
upon and causes the destruction of neighboring tissues. The cavity of
the aneurismal sac is filled with fluid or clotted blood or with layers
of fibrin which adhere cl