THE THORAX 1027 



axter\', which is a branch of the superior intercostal, and may even 

 replace it. 



The subcostal arteries, right and left, are the last branches of the 

 descending thoracic aorta. They are serially continuous with the 

 aortic intercostal arteries above, and with the lumbar arteries below. 

 Each vessel winds roimd the side of the body of the twelfth thoracic 

 vertebra, and, passing beneath the external arcuate ligament of the 

 diaphragm, it enters the wall of the abdomen, where it lies along 

 the lower border of the t^^'elfth rib. These vessels will be found 

 described in connection with the abdomen (see p. 773). 



The first and second intercostal spaces receive their arteries from 

 the superior intercostal artery, which is a branch of the second part 

 of the subclavian on the right side, and of the first part on the left 

 side. Having descended in front of the neck of the first rib to 

 the posterior extremity of the first intercostal space, the vessel 

 furnishes the first intercostal artery to that space, and it also gives 

 off the second intercostal artery, which descends in front of the neck 

 of the second rib to the second intercostal space. As stated, the 

 second intercostal artery receives a branch from the third intercostal 

 artery, which ascends over the neck of the third rib, the third inter- 

 costal being the first aortic intercostal. 



Development of the Descending Aorta. — The upper portion of the descend- 

 ing aorta is developed from that part of the left primitive dorsal aorta which 

 lies between the fourth left arterial arch and the place of junction of the two 

 primitive dorsal aortae. The greater portion of it, however, results from the 

 union of the two primitive dorsal aortae. The intercostal arteries are developed 

 from thoracic segmental arteries. 



Intercostal Veins. — ^The intercostal veins are eleven in number 

 on either side, and each lies in the subcostal groove above the 

 corresponding aortic intercostal artery. In the region of the 

 angle of the rib each vein receives the collateral intercostal vein, 

 which accompanies the artery of that name. At the posterior 

 extremity of an intercostal space each vein receives a large dorsal 

 branch, which returns blood from the muscles and integument of 

 the back, the dorsal spinal venous plexus, and the spinal canal. 

 With the exception of the upper three or four veins, all the other 

 intercostal veins pass inwards, behind the . corresponding sympa- 

 thetic cord, on to the bodies of the thoracic vertebrae, from which 

 they receive small twigs. Their mode of termination differs on the 

 two sides. On the right side the veins, hav'ing passed behind the 

 oesophagus, terminate in the right azygos vein. On the kft side 

 the lower four veins, namely, the eighth, ninth, tenth, and eleventh, 

 open into the lower left azygos vein ; and the succeeding three 

 (or four), namely, the fifth, sLxth, and seventh (and, it may be, 

 the fourth also), open into the upper left azygos vein. 



The first intercostal vein of each side accompanies the corre- 

 sponding superior intercostal artery, and terminates in the inno- 

 minate vein, or, it may be, in the vertebral vein, of its own side. 



The second and third intercostal veins (and, it may be, the fourth) 



