SUBCLAVIAN, AND ITS BRANCHES. 221 



The Posterior Arteries of the Mediastinum, (Arteria Me- 

 diastinahs Posteriores^) are numerous and small ; they come 

 from the anterior face of the aorta, as well as from the branches 

 last mentioned; and are spent upon the posterior mediastinum, 

 and upon its contents. 



The Intercostal Arteries (Jlrterice Intercostales Inferiores Aor- 

 ticce) of the aorta supply the ten lower intercostal spaces on 

 each side, as the two upper ones are supplied by the subclavian 

 artery. There is commonly an intercostal artery arising dis- 

 tinctly from the aorta for each space, but sometimes two of 

 them arise from a common trunk. Those for the right side 

 having to cross the spine behind the oesophagus and the vena 

 azygos, are, of course, longer than such as belong to the left. 

 The upper ones on either side have to ascend, in order to reach 

 their destination. 



Each artery joins the rib near its tubercle, and goes along 

 the groove in its lower margin, between the external and the 

 internal intercostal muscle, for two-thirds of the length of the 

 rib. It then abandons the groove, and divides into several 

 branches, which go to the intercostal muscles and contiguous 

 parts, and anastomose in front with the internal mammary ar- 

 tery. 



As each intercostal artery passes the head of the rib, it sends 

 a branch backwards, (ramus dorsalis,) between the transverse 

 processes of the adjoining vertebra?, which penetrates to the 

 posterior face of the trunk, and is distributed to the muscles and 

 skin on the side of the spine. A ramification from this branch 

 enters the intervertebral foramen, and is spent upon the me- 

 dulla spinalis and its membranes. 



Each intercostal artery also gives off, about the middle of 

 the rib, a branch, (ramus costalis inferior,) much smaller than 

 the trunk. This branch advances along the upper margin of 

 the rib below, and gives ramifications to its periosteum and to 

 the adjacent intercostal'muscles. 



The last intercostal artery is remarkable for its size. Its 

 origin is concealed by the small muscle of the diaphragm, to 

 which it gives some ramifications: it then passes, at the under 

 margin of the rib, behind the upper end of the quadratus lum- 

 borum muscle, where it divides into three branches; one of 



20* 



