no THORAX 



arches, and they run, in company with the last thoracic 

 nerves, along the lower borders of the last pair of ribs. 



Arteriae Intercos tales Supremse. The superior intercostal 

 arteries, which supply the upper two intercostal spaces on 

 each side, are derived from the costo-cervical branches of 

 the subclavian arteries (Fig. 5). Each superior intercostal 

 artery commences at the level of the upper border of the 

 neck of the first rib. It descends anterior to the neck of 

 the rib, posterior to the parietal pleura and between the first 

 thoracic ganglion of the sympathetic trunk medially and 

 the first thoracic nerve ; which is passing upwards to the 

 brachial plexus, laterally (Fig. 5). At the lower border of 

 the neck of the first rib it gives off the posterior inter- 

 costal artery to the first intercostal space ; then it crosses 

 anterior to the neck of the second rib, and, turning later- 

 ally, it becomes the posterior intercostal artery of the second 

 space. 



Nervi Intercostales. The intercostal nerves are the 

 anterior branches of the thoracic nerves. They pass laterally 

 in company with the arteries. The twigs which connect 

 them with the sympathetic ganglia have been noted already 

 (p. 26). Each nerve lies at a lower level than the corre- 

 sponding artery, and is at first placed between the posterior 

 intercostal membrane and the pleura, and then between 

 the two muscular strata. The further course of the nerves 

 is described on p. 5. 



The first thoracic nerve runs upwards, anterior to the neck 

 of the first rib, to join the brachial plexus. It gives a small 

 branch to the first intercostal space, but this nerve, although 

 it is disposed after the manner of an intercostal nerve, does 

 not furnish, as a rule, a lateral cutaneous or an anterior 

 branch. The second intercostal nerve, as a rule, sends a branch 

 upwards, anterior to the neck of the second rib, to join that 

 portion of the first thoracic nerve which enters the brachial 

 plexus. This communicating twig is usually minute and in- 

 significant, but sometimes it is a large nerve; when this is 

 the case, the intercosto- brachial nerve (O.T. intercosto- 

 humeral), or lateral cutaneous branch of the second intercostal 

 nerve, is very small or altogether absent. 



Venae Intercostales. The intercostal veins differ in their 

 arrangement upon the two sides of the body. On the right 

 side they terminate in three different ways : 



