THE BACK. 



18T 



tebral foramen, the artery gives off a branch which imsses 

 inwards to siipijly the sjmial cord and its membranes. It 

 next sends a large branch to the muscles of the baclc, and on 

 £rst gaining the intercostal space is at the anterior margin 

 of the rib behind, but soon gains the posterior margin of 

 the rib in front, along which it passes protected by the bony 

 groove, and by the intercostal muscles, between which it is 

 placed. It sends ramuscules to the ])leura, muscles, ribs, 

 and skin in the neighbourhood, and the anterior intercostals 

 help to supxDly serratus magnus ; they are therefore the 

 largest. At the inferior part of the intercostal spaces 

 these vessels anastomose with the internal thoracic artery 

 anteriorly, posteriorly with that branch which it sends 

 along the inferior extremities of the false ribs. Thus the 

 posterior intercostals indirectly supply the abdominal mus- 

 cles and anastomose with the lumbars, circumflex ^ of the 

 ilium, epigastric, and abdominal branch of the internal 

 thoracic. 



The intercostal veins, after accompanying the arteries 

 as far as the thorax, for the most part empty themselves 

 into the 



Vena azygos, a vessel which arises under the loins and 

 passes with the thoracic duct and posterior aorta through 

 the hiatus aorticus (a foramen of the diaphragm) into the 

 thorax, where they pass in the superior part of the medias- 

 tinum underneath the bodies of the dorsal vertebrae, vena 

 azygos being on the right, posterior aorta on the left, and 

 the thoracic duct centrally placed. This vein passes to the 

 anterior vena cava, with one of the branches of which it 

 communicates by means of a subcostal branch, into which 

 the third, fourth, and fifth intercostal veins pass, thus 

 completing the analogy of the arrangement of the arteries 

 and the veins of this region, for the second intercostal and 

 subcostal veins pass to the dorsal vein, the first intercostal 

 to the posterior cervical. The blood from the intercostal 

 veins therefore passes to the anterior vena cava. The in- 

 tercostal nerves are derived from the dorsal intervertebral 

 nerves, each of which on emerging from the intervebral 

 foramen, after sending fibres to and receiving fibres from the 

 gangliated cord of the sympathetic, sends superior branches 

 to longissimus dorsi and the other structures on the back, 

 while its inferior branch runs between the ribs. Those 

 most posteriorly situated are continued from the inferior 



