THORACIC CAVITY 139 



anterior rami 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. 39). 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 positions occupied by the 

 majority of the thoracic nerves and their general distribu- 

 tion have already been described (see p. 10), but the first, 

 second, and last nerves of the thoracic region require 

 special consideration. 



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 that branch, although 

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

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

 cutaneous 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 insignificant, but sometimes it is a large nerve ; 

 when that 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. 



The Last Thoracic Nerve. The twelfth thoracic nerve is 

 distributed to the wall of the abdomen and to the buttock. 

 It emerges from the vertebral canal between the last thoracic 

 and the first lumbar vertebrae, and, almost at once, passes 

 from the thorax to the abdomen behind the lateral lumbo- 

 costal arch (see p. 407) accompanying the subcostal artery. 



Venae Intercostales. The posterior intercostal veins differ 

 in their arrangement upon the two sides of the body. On 

 the right side they terminate in three different ways : 



1. The highest intercostal vein, from the first or highest space, joins the 



right innominate vein (sometimes the vertebral vein], in the neck. 



2. The intercostal veins of the second and third spaces (and sometimes 



that of the fourth space) unite into a common trunk, termed the 

 right superior intercostal vein, which joins the upper part of the 

 vena azygos. 



3. The intercostal veins of the lower eight spaces oin the vena azygos. 



On the left side of the body^///- modes of termination may 

 be recognised : 



