657 



The intercostal arteries (aa. intercostales) (Fig. 466) arise from the back of the 

 aorta. The aortic intercostals are usually n'ne in number on each side, the two 

 superior"mtercostal spaces being supplied by the superior intercostal, a branch 

 of the subclavian. The right aortic intercostals are longer than the left, on account 

 of the position of the aorta on the left 

 side of the vertebral column; they pass 

 across the bodies of the vertebrae, behind 

 the oesophagus, thoracic duct, and the 

 vena azygos major, and are covered by the 

 right lung and pleura. The left aortic 

 intercostals run backward on the sides 

 of the vertebrae and are covered by the 

 left lung and pleura; the two upper are 

 crossed by the left superior intercostal 

 vein, the lower vessels by the azygos minor 

 veins. Opposite the heads of the ribs 

 the sympathetic cord passes downward 

 in front of them, and the splanchnic 

 nerves also descend in front of the lower 

 arteries. Each artery crosses the cor- 

 responding intercostal space oblique ly 

 toward the angle of the upper rib and 

 thence is continued forward in the sub- 

 costal groove. It is placed at first be- 

 tween the pleura and the posterior inter- 

 costal membrane, then pierces this 

 membrane and lies between it and the 

 External intercostal muscle as far as the 

 rib angle; from this onward it runs be- 

 tween the External and Internal inter- 

 costal muscles and anastomoses with the 

 anterior intercostal branches of the inter- 

 nal mammary or musculophrenic. Each 

 intercostal artery is accompanied by a 

 vein and nerve, the former being above, 

 and the latter below, except in the upper 

 intercostal spaces, where the nerve is at 

 first above the artery. The first aortic 

 intercostal anastomoses with the superior intercostal branch of the subclavian, 

 and the last two intercostals continue anteriorly from the intercostal spaces into 

 the abdominal wall, anastomosing with the superior epigastric, subcostal and 

 lumbar arteries. 



Branches. Each intercostal artery gives off numerous muscular and cutaneous 

 branches. 



FIG. 466. Thoracic aorta. (Testut.) 



Posterior or dorsal. 

 Muscular. 



Collateral intercostal. 

 Lateral cutaneous. 



The posterior or dorsal branch (ramus posterior) runs with the posterior division 

 of a spinal nerve and passes backward through a small opening which is bounded 

 above and below by the necks of the ribs and adjacent transverse processes, 

 internally by the vertebral body, and externally by the anterior costotransverse 

 ligament. It gives off a spinal branch, which enters the vertebral canal through 

 the intervertebral foramen, is distributed to the spinal cord and its membranes, 



42 



