926 THE VASCULAE SYSTEM. 



costal groove the artery lies between the corresponding vein above and the inter- 

 costal nerve below, and it terminates anteriorly 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 into 

 the abdominal wall, and anastomose with branches of the superior epigastric, sub- 

 costal, and lumbar arteries. The first right aortic intercostal artery frequently 

 gives off the right bronchial artery, 



Branches. (a) The posterior branch passes backwards, accompanied by the posterior 

 branch of a spinal nerve, medial to the anterior 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 a medial and a lateral cutaneous terminal 

 branch. The medial cutaneous branch passes backwards and medially, either over or 

 through the multifidus, giving branches to the muscles between which it passes and to the 

 vertebral column. The lateral branch runs laterally under cover of the longissimus dorsi to 

 the interval between it and the iliocostalis dorsi. It terminates in the skin of the back, 

 after giving branches to the adjacent muscles. Before it divides, each posterior branch 

 gives off muscular branches, and a spinal branch which passes through the corresponding 

 intervertebral foramen, and enters the vertebral canal, to the contents and walls of which 

 it is distributed. The spinal branch divides into three branches neural, post-central, 

 and pre-laminar. The neural branch divides into two branches which run medially, on 

 the roots of the spinal nerve ; they pierce the dura mater and arachnoid, and divide into 

 branches some of which pass to the membranes of the spinal medulla, whilst others are 

 continued on to reinforce the posterior and anterior 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 backs 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 vertebral canal. 



(b) A collateral branch arises near the angle of the rib. It descends and runs] 

 forward along the lower border of the intercostal space, to anastomose in front with an 

 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 to the adjacent muscles are given off both by the main trunk 

 and its collateral branch. 



(d) A lateral cutaneous offset accompanies the lateral cutaneous branch of the inter- 

 costal nerve. 



In addition to the secondary branches above-named, the trunk of the first aortic inter 

 costal, on each side, anastomoses with the superior intercostal, and may supply the wholf 

 or the greater part of the second intercostal space. The upper three or four aortic inter 

 costal arteries, on each side, give branches to the mammary gland which anastomose witl 

 branches of the lateral thoracic and internal mammary arteries. Longitudinal anastomose: 

 between adjacent trunks and also between adjacent posterior branches of intercosta 

 arteries sometimes exist near the necks of the ribs, or near the transverse processes. Thesi ' 

 longitudinal anastomoses are of considerable morphological interest. 



2. The subcostal arteries are the last pair of parietal branches given off from th 

 thoracic aorta. They are in series with the aortic intercostal arteries, but ar 

 situated below the last ribs. Each gives off a posterior branch which behaves ii 

 the same manner as the posterior branch of an ordinary aortic intercostal artery 

 The trunk runs along the lower border of the twelfth rib in company with th 

 last thoracic nerve. It passes below the arcus lumbocostalis lateralis to th 

 abdomen, and there crosses anterior to the quadratus lumborum, and posterior t 

 the kidney and the adjacent part of the colofi. It next pierces the aponeurosis c 

 origin of the transversus abdominis, and runs between the transversus and th 

 internal oblique muscles, anastomosing with the lower intercostal arteries, wit; 

 the lumbar arteries, and with branches of the superior epigastric artery. 



3. Superior phrenic branches are given off from the lower part of the thoraci 

 aorta. They are small vessels which ramify on the upper and posterior surfaces c 

 the diaphragm, and anastomose with branches of the pericardiaco- phrenic an 

 musculo-phrenic arteries. 



