932 



THE NERVOUS SYSTEM 



The lateral branch, the intercosto-brachial (intcrcosto-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 usually 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 anastomoses it passes out of the axillary fossa, pierces 

 the deep fascia, and supplies the integument in the upper and posterior 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. The third, fourth, fifth, and sixth thoracic 

 nerves, in the posterior parts of the intercostal spaces, give muscular branches to the 



FIG. 685. DIAGRAM OF THE DISTRIBUTION OF A TYPICAL THORACIC NERVE. 



INTERNAL BRANCH 



Longissimus dorsi 



Semispinalis dorsi 

 Multifidus spinse 



Superior costo-transverse 

 ligament 



DORSAL ROOT 

 VEXTRAL ROOT 



RECURRENT BRANCH 

 SYMPATHETIC 

 GANGLION 



MEDIAL BRANCH 

 BRANCH TO AORTA 



(Esophagus 



Infernal mammary artery 

 Transverse thoracic muscle 



Ilio-costalis dorsi 



EXTERNAL BRANCH 



pa* Ti:i:iin: i-n DIARY 



D1V1S10S 

 A STERIOR PRIM A R Y 



II I VISION 

 Internal intercostal muscle 



External intercostal muscle 



LATERAL CUTANEOUS 

 BRANCH- 



ANTERIOR BRANCH 



Anterior intercostal membrane 



levatores costarum, the second and sometimes the third also giving branches to the 

 serratus posterior superior. They pass forwards a' short distance between the ex- 

 ternal and internal intercostals, giving twigs to these muscles, and divide into two 

 branches, lateral and anterior. 



The lateral cutaneous branches continue forwards 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 pos- 

 terior branches pass backwards over the latissimus dorsi to supply the skin in the 

 lower part of the scapular region. The anterior branches increase in size from above 

 downwards. The}' pass around the outer 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 also 

 supply twigs to the upper cligitations of the external oblique muscle. 



The anterior branches run obliquely forwards and inwards, through the sub- 

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

 at the outer extremity of the costal cartilages (fig. 685). They continue forwards 

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



