THE THORACIC NERVES. 1317 



The second thoracic nerve sometimes contributes fibres to the brachial plexus. 

 The posterior ramus of its lateral cutaneous branch is called the inter costo- humeral nerve. 



The intercosto-humeral nerve (n. intercostobrachialis) (Fig. 1105) is quite 

 large and pierces the inner axillary wall between the second and third ribs. Enter- 

 ing the axilla, it crosses that space toward the arm and communicates with the lesser 

 internal cutaneous nerve from the brachial plexus. After piercing the deep fascia, 

 the intercosto-humeral nerve supplies the internal and posterior portion of the integ- 

 ument of the upper half of the arm, a few of its fibres extending slightly beyond the 

 margin of the scapula. 



The third thoracic nerve may form an inosculation with the lesser internal 

 cutaneous nerve. 



The twelfth thoracic or the subcostal nerve lies below the last rib and 

 therefore does not occupy an intercostal space, but passes outward below the 

 external arcuate ligament and anterior to the quadratus lumborum muscle. It 

 contributes a twig to the lumbar plexus which passes down to join the first lumbar 

 nerve. Its lateral cutaneous branch is not confined in its distribution to the 

 abdominal wall, since, after piercing the internal oblique and sending a filament 

 to the lower digitation of the external oblique, it penetrates the substance of the 

 latter muscle at a point from 210 cm. above the crest of the ilium and supplies the 

 integument of the gluteal region as far down as the upper margin of the great 

 trochanter (Fig. 1083). 



Branches of the thoracic nerves are : (i) the muscidar and (2) the cutaneous. 



i. The muscular branches (rr. musculares) may be divided into two groups: (a) the thoracic 

 and (b) the abdominal. 



a. The thoracic muscular branches arise from the first to the seventh inclusive and supply 

 the external and internal intercostals, the subcostals, the levatores costarum, the serratus 

 posticus superior, the triangularis sterni and the rectus abdominis. 



The branches to the intercostal and subcostal muscles are distributed throughout the course 

 ">f each nerve. The first to be given off is the largest and courses forward for some distance 

 along the lower part of the intercostal space. The others vary greatly in number and size. 



The branches to the levatores costarum consist of fine threads, one arising from each nerve 

 beyond the anterior costo-transverse ligament. They pierce the external intercostal muscles 

 and enter the deep surface of the muscles which they supply. 



The branches to the serratus posticus superior arise from the upper four nerves. After 

 piercing the external intercostal muscles they pass along the outer margin of the ilio-costalis and 

 supply the four digitations of their muscle. 



The branches to the triangularis sterni are terminal continuations of the third to the seventh 

 intercostal nerves. After piercing the internal intercostal muscles they pass forward between 

 the triangularis sterni and the internal intercostals or, in the case of the seventh, anterior to the 

 transversalis muscle. In addition to supplying the triangularis sterni the seventh sends fibres to 

 the first digitation of the transversalis. 



The branches to the rectus arise from the fifth, sixth and seventh and enter the deep 

 surface of the muscle. 



b. The abdominal muscular branches arise from the eighth to the twelfth inclusive and are 

 distributed to the intercostals, the subcostals, the levatores costarum, the serratus posticus inferior, 

 the external obique, the internal oblique, the transversalis, the rectus, the pyramidalis and the 

 diaphragm. 



The branches to the intercostal, subcostal and levatores costarum muscles, with the excep- 

 tion of arising from the lower thoracic nerves, resemble in origin, course and distribution those 

 arising from the upper nerves. 



The branches to the serratus posticus inferior are larger than those to the serratus posticus 

 superior. They arise from the ninth, tenth and eleventh nerves and pass around the lateral 

 margin of the ilio-costalis to reach their destination. 



' The branches to the external oblique, the internal oblique and the transversalis comprise 

 numerous fine twigs which supply those muscles and arise from the lower five thoracic nerves as 

 they course forward between the transversalis and the internal oblique. 



The branches to the rectus arise from the eighth to the twelfth nerves inclusive after they 

 have entered the sheath and as they pierce the rectus on their way to the surface. 



The branches to the pyramidalis are derived from the twelfth thoracic and first lumbar 



nerves. 



The branches to the diaphragm are supplied to its costal portion and consist of fine 

 filaments which are given off by the lower six thoracic nerves (Luschka). 



