796 THE VASCULAR SYSTEM. 
between the adjacent transverse processes, beyond which they are continued in the 
abdominal wall. 
They le on the bodies of the corresponding lumbar vertebrae. In their back- 
ward course, and while still in relation with the vertebral bodies, each artery 1s 
crossed by the sympathetic cord, and then, passing under and being protected by 
the fibrous arches from which the muscle arises, it runs behind the psoas muscle 
and the lumbar plexus. The upper two arteries on each side also pass behind the 
erura of the diaphragm. Beyond the interval between the transverse processes of 
the vertebra each artery turns outwards and crosses the quadratus lumborum—the 
last usually passing in front, and the others behind the muscle; it then pierces the 
aponeurosis of origin of the transversalis, and proceeds forwards in the lateral 
abdominal wall in the interval between the transversalis and internal oblique 
muscles. The lumbar arteries anastomose with one another, with the lower inter- 
costal and subcostal arteries, and with branches of the superior and deep epigastric 
and of the deep circumflex ihac and ilio-lumbar arteries. 
Fine twigs also pass from the lumbar arteries to the extra-peritoneal fat, and 
anastomose with corresponding branches from the inferior phrenic and ilio- lumbar 
arteries, and with small branches from the hepatic, renal, and colic arteries, to form 
the subperitoneal plexus of Turner. 
The ‘abdominal aorta les somewhat to the left of the middle line, and con- 
sequently the right lumbar arteries are a little longer than the left, but the differ- 
ences between the arteries of opposite sides are limited to the first parts of the main 
trunks. On the right side the arteries, which near their origins lie more in front 
of the vertebral bodies, pass behind the inferior vena cava, the upper two arteries 
being separated from this vessel by the right crus of the diaphragm. The upper 
two right arteries also pass behind the receptaculum chyl and the lower end of the 
large azygos vein. 
Branches.—Dorsal (ramus dorsalis)—Each lumbar artery gives oft, opposite the 
interval between the vertebral transverse processes, a dorsal branch of considerable size. 
It is analogous with and distributed like the corresponding branch of an aortic intercostal 
artery (p. 793). Muscular branches are given off, both from the main trunk and its dorsal 
branch, to the adjacent muscles. 
3. The middle sacral artery (a. sacralis media, Fig. 566) is a single median 
vessel. It is commonly regarded as a caudal aorta and as the direct continuation 
of the abdominal aorta. It is, however, of small size, and almost invariably arises 
from the back of the aorta about half-an-inch above its bifurcation. It descends 
in front of the two lower lumbar vertebra and of the sacrum and coccyx, and*ends 
opposite the tip of the last-named bone by anastomosing with the lateral sacral arteries 
to form a loop from which branches pass to the coccygeal body. In its course it 
passes at first behind the lower part of the abdominal aorta. Opposite the fifth 
lumbar vertebra it is crossed by the left common ilae vein, below which it is 
covered by peritoneum and coils of small intestine as far as the third sacral seg- 
ment, and in the rest of its extent by the rectum. It is accompanied below by 
vene comites, which, however, unite above to form a single middle sacral vein. 
Small parietal branches pass transversely outwards on each side to the last 
lumbar vertebra and to the sacrum. They anastomose with the lateral sacral 
arteries, and they usually give off small spinal offsets which enter the anterior 
sacral foramina. Small and irregular visceral branches pass to the rectum and 
anastomose with the superior and middle hemorrhoidal arteries. 
COMMON ILIAC ARTERIES. 
4. The common iliac arteries (aa. iliacee communes, Figs.°566 and 572) are 
the terminal branches of the abdominal aorta. They are formed by the bifurcation 
of the main trunk, and commence opposite the middle of the body of the fourth 
lumbar vertebra a little to the left of the middle line. Each artery passes down- 
wards and outwards across the bodies of the fourth and fifth lumbar vertebrze 
and the intervening intervertebral disc, and terminates in front of the correspond- 
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