702 THE VASCULAR SYSTEM. 
THE SYSTEMIC ARTERIES. 
THE AORTA. 
The aorta is the main trunk of the arterial system. It commences at the base 
of the left ventricle and ascends, with an inclination to the right, to the level of 
the second right costal cartilage; then curving backwards and to the left, it 
reaches the left side of the lower border of the fourth dorsal vertebra, and finally 
descends through the thorax into the abdomen, where it terminates on the left 
side of the fourth lumbar vertebra by bifurcating into the two common iliac 
arteries. The portion of the aorta which is situated in the thorax is for con- 
venience termed the thoracic aorta, and the rest of the vessel is known as the 
abdominal aorta. 
THE THORACIC AORTA. 
The thoracic aorta is subdivided into the ascending portion, the arch, and the 
descencing portion. 
The ascending aorta (aorta ascendens) lies in the middle mediastinum. It 
springs from the base of the left ventricle, behind the left margin of the 
sternum, opposite the lower border of the third left costal cartilage and the 
body of the fifth dorsal vertebra. From its origin it passes upwards, forwards, 
and to the right, and it terminates in the arch of the aorta, behind the right 
margin of the sternum, at the level of the second costal cartilage. Its length 
is from to 2 to 2} inches (50 to 57 mm.), and its breadth is 14 inches (28 mm.). 
In the adult it is a little narrower at its commencement than the pulmonary 
artery is, but in old age it enlarges and exceeds the latter vessel in size. The 
diameter, however, is not uniform throughout the whole length of the ascending 
aorta; four distinct dilatations are present. Three of these, small and pouch-lke, 
are known as the sinuses of Valsalva (sinus aortze). They are situated at the origin 
of the aorta, immediately above the semilunar cusps of the valve which guards the 
aperture of communication with the left ventricle; therefore one is anterior im 
position, and two are situated posteriorly. The fourth dilatation is formed by 
a diffuse bulging of the right wall, and is known as the great sinus of the 
aorta. 
Relations.—The ascending aorta is completely enclosed within the fibrous peri- 
eardium, which blends above with the sheath of the vessel. It is enveloped, together 
with the stem of the pulmonary artery, in a tubular prolongation of the serous pericar- 
dium, and has the pulmonary artery in front, the anterior wall of the right auricle 
behind, and the right auricular appendix on its right side. In the upper part of its 
course the ascending aorta is overlapped by the anterior margins of the right lung and 
right pleural sac, whilst behind it are the right branch of the pulmonary artery, the upper 
right pulmonary vein, the right bronchus, and the left margin of the superior vena cava. 
The superior vena cava lies on the right side, and partly behind the upper part of the 
ascending aorta, whilst the pulmonary artery is at first in front of it and then, at a higher 
level, on its left side. 
Branches.— Two branches arise from the ascending aorta, viz. the right and the left 
coronary arteries. The former springs from the anterior, and the latter from the left 
posterior sinus of Valsalva (p. 755). 
The arch of the aorta (arcus aortze) lies in the superior mediastinum behind 
the lower part of the manubrium sterni, and connects the ascending with the 
descending aorta. It commences behind the right margin of the sternum, on a 
level with the second costal cartilage, and extends to the lower border of the fourth 
dorsal vertebra. As its name implies, it forms an arch; in this there are two 
curvatures, one with the convexity upwards, and the other with the convexity — 
forwards and to the left. From its origin it runs for a short distance upwards, — 
backwards, and to the left, in front of the trachea; then it passes backwards — 
. 
round the left side of the trachea to the left side of the body of the fourth dorsal — 
