AORTA.
189
the chest well formed, and the heart and the
arch of the aorta free from disease, the origin
of the aorta is opposite the sternal articulation
of the cartilage of the fourth rib of the left
side in the male, and the intercostal space
above it in the female ; the ascending limb of
the arch, which is behind the middle bone
of the sternum in the greater part of its length,
may be felt pulsating on the right side of the
sternum in the second intercostal space ; the
highest part of the transverse portion of the
arch is on a plane with the centre of the sternal
extremities of the first pair of ribs, and about
an inch below the upper margin of the ster¬
num : the arch of the aorta terminates oppo¬
site the lower edge of the cartilage of the
second rib of the left side.
The thoracic aorta descends in the posterior
mediastinum, and advances from the left side
to the front of the thoracic portion of the spine,
crossing in its course the left intercostal veins,
and the left vena azygos when that vein exists ; in
front it is covered by the left bronchus, the pos¬
terior surface of the pericardium, the lower ex¬
tremity of the oesophagus, and the left stomachic
cord of the par vagum ; on the right side it is
bounded by the oesophagus, thoracic duct, and
vena azygos ; on the left side it is covered by
the pleura, and in contact with the internal
surface of the left lung, and at its lower extremity
the left splanchnic nerve comes into contact
with it, and most frequently accompanies it
through the diaphragm.
The abdominal aorta, which enters the abdo¬
men between the crura of the diaphragm, des¬
cends along the front of the abdominal ver¬
tebrae and the left lumbar veins ; it is covered
in front by the solar plexus of nerves, the
stomach, pancreas, transverse portion of the
duodenum, the splenic and left renal veins, the
small intestine, and the root of the mesentery ;
on the right side it is bounded by the abdomi¬
nal vena cava, and the commencement of the
thoracic duct, and on the left it is covered by
the peritoneum going to form the left layer
of the mesentery. The termination of the aorta
in the common iliacs and the middle sacral
arteries is a little below the level of the um¬
bilicus.
A remarkable deviation from the cylindrical
form, which is one of the general characteristics
of the arterial system, is observable in two parts
of the arch of the aorta ; the first of these occurs at
the commencement of this vessel in form of three
dilatations corresponding to the semilunar flaps
already described ; they were first pointed out
by Valsalva, and have received the name of the
lesser sinuses of the aorta ; they exist at all
periods of life, and increase in size with years ;
the other deviation from the cylindrical form is
a dilatation on the right side of the ascending
limb of the arch at its junction with the trans¬
verse portion ; this dilatation, which does not
exist in the foetus, grows larger as life advances,
and appears to be produced by the impulse
of the blood striking against this part of the
aorta at each successive systole of the left
ventricle. The aorta in the succeeding part of
its course gradually grows smaller in a degree
proportionate to the size of the branches it
gives off.
The thickness of the aorta is proportionally
less than that of its branches ; it is thinner at its
commencement than in the arch, in which part,
according to Haller, it is thicker by an eighth
on the convex than on the concave side; it
gradually diminishes in thickness as it descends
through the thorax and abdomen, but its power
of resisting distention instead of being dimi¬
nished in an equal degree was found by Win-
tringham to be greater at its lower part than
near the heart.*
The structure of the aorta is the same as
that of the rest of the arterial system in general ;
its external tunic, however, is slighter than that
of all other arteries except those of the brain,
it is weaker the nearer it is examined to the
origin of the aorta ; it is strengthened near the
heart by the covering which the serous layer of
the pericardium gives to the aorta, and by an
expansion from the fibrous layer of that mem¬
brane, which is lost on the transverse portion of
the arch. The cellular sheath of the aorta in
which the soft fat around its origin is deposited,
becomes so fine where the vessel is passing out
of the pericardium as to lead some anatomists
to deny its existence in this situation ; it becomes
more evident in the course of the descending
aorta through the mediastinum, and is still
more considerable around the abdominal aorta,
where it is usually loaded with a considerable
quantity of adipose substance.
The branches which arise immediately from
the aorta may be divided into orders, according
to the degree of remoteness or the relative size
and importance of the parts which they supply
with blood ; first, the branches which convey
blood to the two extremities of the trunk and
the limbs attached to them ; these arteries,
which are of considerable size, are the arteria
innominata, the leftc arotid and left subclavian,
which, arising from the transverse portion of
the arch, are distributed to the head, neck, and
upper extremities, and the primitive iliac arte¬
ries which arise from the lower part of the
abdominal aorta supplying the pelvis and the
lower extremities. 2nd order.—Branches some¬
what smaller going to the thoracic and abdomi¬
nal viscera and the parietes of the chest and
abdomen ; the coronary arteries which supply
the heart arise from the aorta immediately after
its origin ; the bronchial arteries which supply
the substance of the lungs, and the intercostal
arteries supplying the parietes of the chest
arise from the thoracic aorta; the cceliac, su¬
perior and inferior mesenteric, which supply
the digestive organs ; the renal arteries which
supply the kidnies ; the spermatic going to the
organs of generation, the inferior phrenic sup¬
plying the diaphragm, and the lumbar arteries
going to the parietes of the abdomen and lum¬
bar region of the spine, are the vessels of this
order which arise from the abdominal portion
of the aorta. 3rd order.—Branches of much
smaller size are sent from the aorta to se-
* Experimental Inquiry on some parts of the
Animal Structure. London, 1740.