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PARIETAL BRANCHES OF DESCENDING THORACIC AORTA. 793 
muscles. In the subcostal groove the artery lies between the corresponding vein 
above and the intercostal nerve below, and it terminates in front by anastomosing 
with an anterior intercostal branch of the internal mammary or of the musculo- 
phrenic artery. The lower two intercostal arteries on each side extend beyond 
their spaces to the abdominal wall, and anastomose with branches of the superior 
epigastric, subcostal, and lumbar arteries. 
Branches.—(a) Dorsal (ramus posterior).—As each artery enters its intercostal 
space it gives off a posterior or dorsal branch which passes backwards accompanied by 
the posterior primary division of a spinal nerve, internal to the superior costo-transverse 
ligament, between the necks of the ribs which bound the space, and between the 
adjacent transverse processes, to the vertebral groove, where it divides into internal and 
external terminal branches. The znternal branch (ramus cutaneus medialis) passes back- 
wards and inwards either over or through the multifidus spine, giving branches to the 
muscles between which it passes and to the vertebral column. The external branch (ramus 
cutaneus lateralis) runs outwards beneath the longissimus dorsi to the interval between it 
and the musculus accessorius. It terminates in the skin of the back, after giving 
branches to the adjacent muscles. A spinal branch (ramus spinalis) from each dorsal 
artery passes through the corresponding intervertebral foramen, and enters the spinal 
canal, to the contents and walls of which it is distributed. It divides into three branches 
—neural, post-central, and pre-laminar. The newral branch runs inwards on the roots of 
the spinal nerve, pierces the dura mater and arachnoid, and divides into branches some 
of which pass to the membranes of the cord, whilst others are continued on to reinforce 
the dorsal and ventral spinal arteries. The post-central branch divides into ascending 
and descending branches which, anastomosing with similar branches above and below, 
form a series of vertical arches on the back of the bodies of the vertebrae. The arches of 
opposite sides are connected by short transverse anastomoses. The pre-laminar branch is 
small, and its ascending and descending branches are distributed in a similar though less 
regular manner on the posterior wall of the spinal canal. 
(6) A collateral branch springs from the trunk of each intercostal artery near the 
angle of the rib. It descends to the lower border of the intercostal space, along which it 
runs forwards to anastomose in front, like the intercostal artery itself, with a separate 
anterior intercostal branch of the internal mammary or musculo-phrenic artery. The 
collateral branches of the lower two intercostal arteries on each side are inconstant ; 
when present they are small, and terminate in the abdominal wall. 
(c) Muscular branches (rami musculares) to the adjacent muscles are given off both 
by the main trunk and its collateral branch. 
(d) A lateral cutaneous (ramus cutaneus lateralis) offset from the intercostal artery 
accompanies the lateral cutaneous branch of the intercostal nerve. 
In addition to the above-named branches the first aortic intercostal on each side 
anastomoses with the superior intercostal, and may supply the whole or the greater part of 
the second intercostal space, and the first right aortic intercostal frequently gives origin to 
the right bronchial artery. The upper three or four aortic intercostals on each side give 
branches to the mammary gland which anastomose with branches of the long thoracic and 
internal mammary arteries. Longitudinal anastomoses between adjacent intercostal arteries 
and their dorsal branches sometimes exist near the necks of the ribs, or near the transverse 
processes. These longitudinal anastomoses are of considerable morphological interest. 
2. The subcostal arteries are the last pair of parietal branches given off from 
the thoracic aorta. They are in series with and very similar to the aortic intercostal 
arteries, but are situated below the last ribs. Each runs along the lower border of 
the twelfth rib in company with the last dorsal nerve. It passes beneath the liga- 
mentum arcuatum externum to the abdomen, and there crosses in front of the quad- 
ratus lumborum, and behind the kidney and the adjacent part of the colon. It 
next pierces the aponeurosis of origin of the transversalis abdominis, and runs 
between the transversalis and the internal oblique muscles, anastomosing with the 
lower intercostals, with the lumbar arteries, and with branches of the superior 
epigastric artery. 
3. Diaphragmatic branches (aa. phrenice superiores) are given off from the 
lower part of the thoracic aorta. They are small vessels which ramity on the 
upper surface of the diaphragm, and anastomose with branches of the superior 
phrenic and musculo-phrenic arteries. 
4. The vas aberrans is a variable and inconstant branch of the thoracic aorta ; 
