I026 A MANUAL OF ANATOMY 



parietal pleura. Each artery, on entering an intercostal space, 

 lies at first between the parietal pleura and the posterior intercostal 

 aponeurosis. Its course is outwards and slightly upwards, and 

 partly on this latter account, but chiefly on account of the down- 

 ward slope of the rib, it soon gains the lower border of the upper 

 rib. At a point corresponding to the angle of the rib the artery 

 pierces the posterior intercostal aponeurosis, and, passing l>etween 

 the external and internal intercostal muscles, it enters the sub- 

 costal groove of the upper rib. In this position it courses forwards, 

 and ends by anastomosing with the upper anterior intercostal 

 branch of the internal mammary artery, or of its musculo-phrenic 

 branch, according to the level. 



The companion intercostal vein lies above the artery, and the 

 corresponding intercostal nerve lies below it. The order of struc- 

 tures, therefore, in the subcostal groove, from above downwards, 

 is as follows : intercostal vein, intercostal artery, and intercostal 

 nerve. The upper seven aortic intercostal arteries are confined to 

 the intercostal spaces which they occupy ; but the lower two, 

 namely, those in the tenth and eleventh intercostal spaces, ulti- 

 mately leave these spaces, and pass into the abdominal wall, where 

 they have been described in connection with the abdomen. 



Branches. — ^These are dorsal, giving off a spinal branch ; collateral ; 

 and lateral cutaneous. 



The dorsal branch arises from the aortic intercostal artery as that 

 vessel enters the posterior extremity of an intercostal space. It 

 passes backwards, in company with the posterior primary division 

 of the corresponding spinal nerve, between the adjacent transverse 

 processes, where it lies internal to the superior costo-transverse 

 ligament. Opposite the intervertebral foramen it gives off its 

 spinal branch, which enters the spinal canal through the foramen 

 to be distributed to its osseous and ligamentous walls, as well as 

 to the spinal cord and its membranes. The dorsal branch, con- 

 tinuing its course baclcwards, divides into an internal and external 

 branch, which supply the muscles and integument of the back. 



The collateral intercostal artery arises from the main aortic inter- 

 costal opposite the angle of the rib. It passes obliquely downwards 

 and outwards to the upper border of the lower rib, along which it 

 courses, lying between the external and internal intercostal muscles. 

 Anteriorly it ends by anastomosing with the lower anterior inter- 

 costal branch of the internal mammary artery, or of its musculo- 

 phrenic branch, according to the level. 



The lateral cutaneous branches accompany the lateral cutaneous 

 branches of the corresponding intercostal nerves to the integu- 

 ment. 



The first aortic intercostal artery, namely, that which lies in 

 the third intercostal space, furnishes a branch, of varitible size, 

 which ascends over the neck of the third rib to the second inter- 

 costal space. This branch anastomoses with the second intercostal 



