ANGEIOLOGY. 81 
plete rings. By the aid of the microscope, this coat may be divided into 
three lamine: an external, thin, elastic, and yellow; a middle, composed 
of circular fibres similar to those of unstriped involuntary muscle; and an 
internal, similar to the last, but with the fibres longitudinal. The cnternal 
coat of an artery is smooth and polished, and may be subdivided into two 
lamin, the internal composed of tesselated epithelium, and resting on a 
basement membrane with longitudinal and internal fibres. Although the 
external system is devoid of exact symmetry, yet, with a few exceptions, 
one description will apply to either side. Arteries are all supplied with 
nutrient vessels and nerves from the adjacent parts. 
A. The Arch of the Aorta. 
The aorta, arising from the superior posterior end of the left ventricle, 
passes beneath the pulmonary artery, and is entirely concealed in front by 
it. Keeping to the right, it emerges at the base of the heart, between the 
right auricle and the trunk of the pulmonary artery, being bounded on the 
right side by the descending vena cava. Continuing its ascent, it forms a 
curvature with the convexity upwards, the summit of which arises to within 
about an inch of the superior edge of the sternum. This curvature is in 
front of the third and fourth dorsal vertebra, and in its course the aorta 
passes over the right pulmonary artery, across the left bronchus, and applies 
itself to the left side of the spine about the third or fourth dorsal vertebra. 
This bend is known as the aortic arch, arcus aortce. In its descent down 
the thorax the aorta is in contact with the left surface of the bodies of the 
dorsal vertebre. At the lower part of the thorax, it inclines towards the 
middle line of the vertebree in order to reach the hiatus aorticus of the 
diaphragm, through which it penetrates to the abdomen. Here it descends 
in front of the lumbar vertebrze, somewhat on their left side, ceasing at the 
intervertebral space between the fourth and fifth vertebrz by division into 
two large trunks, the primitive iliacs, one for each lower extremity and the 
corresponding side of the pelvis. In the course of the aorta from the heart 
to the loins, it first gives off the branches which supply the heart, then those 
for the head and superior extremities, then those for the sides of the thorax, 
and afterwards in the abdomen it detaches the trunks which supply the 
viscera and sides of the abdomen. 
From the arch of the aorta there arise five arteries: the right and left 
coronary, the innominata, the left carotid, and the left subclavian. Excep- 
tions to this arrangement not unfrequently oecur, as in addition to the two 
coronary there are sometimes but two arteries, sometimes six. : 
CoRoNARY ARTERIES. The right and left coronary arteries are the 
nutrient vessels of the heart. They arise above two of the sigmoid valves, 
and communicate freely with each other by their ramifications, 
The arteria innominata, about an inch and a half in length, arises from die 
upper part of the arch, at the junction between the ascending and horizontal 
portion; it ascends anal to the right side in front of the trachea and 
of the right pleura, and opposite to hie sternal end of the clavicle divides 
into the right subclavian and right carotid arteries. 
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