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.