THE THORACIC NERVES 995 



to supply the second levator costae and the serratus posterior superior, and then 

 it divides into a lateral and an anterior branch. The two branches run forward 

 together to the mid-axillary line, where the lateral branch pierces the external 

 intercostal muscle and passes between two digitations of the serratus anterior 

 (magnus) into the axillary fossa; the anterior branch enters the substance of the 

 internal intercostal muscle. 



The lateral branch, the intercosto -brachial (intercosto-humeral) , may divide into a small 

 anterior and a large posterior division, or the anterior division may be absent. In either case 

 the lateral branch anastomoses with the medial brachial (lesser internal) cutaneous nerve, and 

 usuaUy with the lateral branch of the third intercostal nerve; it also anastomoses with the lateral 

 branch of the first nerve, if the latter is present. After forming these junctions it passes out 

 of the axillary fossa, pierces the deep fascia, and suppUes the integument in the upper and pos- 

 terior half of the arm. It also gives off a few filaments which terminate in the skin over the 

 axillary border of the scapula. The size of the intercosto-brachial nerve and the extent of its 

 distribution are usually in inverse proportion to the size of the other cutaneous nerves of the 

 upper arm, especially the middle brachial (lesser internal) cutaneous. When the latter nerve 

 is absent, the intercosto-brachial usually takes its place. 



The course and distribution of the anterior branch, when it is present, being similar to the 

 course and distribution of the anterior branches of the next four nerves, do not require a separate 

 description. 



The thoracic intercostal nerves (upper group). — The third, fourth, fifth, and 

 sixth thoracic nerves, in the posterior parts of the intercostal spaces, give muscu- 

 lar branches to the levatores costarum, the first to the fourth also giving branches 

 to the serratus posterior superior. They pass forward a short distance between 

 the external and internal intercostals, giving twigs to these muscles, and divide 

 into two branches, lateral and anterior. 



The lateral cutaneous branches continue forward between the intercostal 

 muscles, and, near the mid-axillary line, pierce the external intercostals and 

 serratus anterior (magnus) and divide into two branches, posterior and anterior. 

 The posterior branches pass backward over the latissimus dorsi to supply the skin 

 in the lower part of the scapular region. The anterior branches, in the four nerves, 

 increase in size from above downward. They pass around the lateral border of 

 the great pectoral muscle and are distributed to the integument over the front 

 of the thorax and mamma, sending filaments, the lateral mammary branches, into 

 the latter organ. The lowest two nerves also supply twigs to the upper digita- 

 tions of the external oblique muscle. 



The anterior branches run obliquely forward and medialward through the 

 substance of the internal intercostal muscles, reaching the deep surface of these 

 muscles at the extremity of the costal cartilages (fig. 762). They continue 

 forward between these muscles and the pleura, pass in front of the internal 

 mammary artery, turn abruptly ventralward a short distance from the sternum, 

 pierce the internal intercostals, the anterior intercostal membrane, and the pec- 

 toralis major, and give off three sets of terminal branches. One set supplies the 

 transverse thoracic muscle and the back of the sternum. A second set, cutaneous, 

 runs mesially. The third set passes laterally over the pectoralis major, supplying 

 the skin in that region, and, in the female, the mammary gland through the 

 medial mammary branches. The anterior branches in their course supply the 

 intercostal and subcostal muscles and give filaments that supply the ribs, the 

 periosteum, and the pleura. 



The thoraco -abdominal nerves (lower group). — The relations of the posterior 

 portions of the seventh, eighth, ninth, tenth, and eleventh thoracic nerves to the 

 thoracic wall are similar to those of the upper thoracic intercostal nerves. Each 

 divides in a similar manner into a lateral and an anterior branch, but these branches 

 are distributed partly to the abdominal and partly to the thoracic wall, and the 

 smaller muscular branches have also different distributions. 



The lateral branches, lateral cutaneous nerves of the abdomen, pierce the 

 external intercostal muscles and pass through or between the digitations of the 

 external oblique into the subcutaneous tissue, where they divide in the typical 

 way into anterior and posterior branches. The posterior branches pass backward 

 over the latissimus dorsi. The anterior branches give filaments to the digitations 

 of the external oblique and extend forward, medialward and downward to the 

 outer border of the sheath of the rectus. 



The anterior branches pass forward between the external and internal 

 intercostal muscles, to the ends of the intercostal spaces; there they insinuate 



