INTERCOSTAL. 577 



they lie between the pleura and the External intercostal muscle, but are soon 

 placed between the two planes of Intercostal muscles as far as the costal car- 

 tilages, where they lie between the pleura and the Internal intercostal muscles. 

 Near the sternum, they cross the internal mammary artery, and Triangularis 

 sterni, pierce the Internal intercostal and Pectoralis major muscles, and supply the 

 integument of the mamma and front of the chest, forming the anterior cutaneous 

 nerves of the thorax ; that from the second nerve becoming joined with the cla- 

 vicular nerve. 



Branches, Numerous slender muscular filaments supply the Intercostal and 

 Triangularis sterni muscles. Some of these branches, at the front of the chest, 

 cross the costal cartilages from one to another intercostal space. 



Lateral cutaneous nerves. These are derived from the intercostal nerves, midway 

 between the vertebrae and sternum, pierce the External intercostal and Serratus 

 magnus muscles, and divide into two branches, anterior and posterior. 



The anterior brandies are reflected forwards to the side and forepart of the 

 chest, supplying the integument of the chest and mamma, and the upper digitations 

 of the External oblique. 



The posterior branches are reflected backwards, to supply the integument over 

 the scapula and Latissimus dorsi. 



The first intercostal nerve has no lateral cutaneous branch. The lateral cutaneous 

 branch of the second intercostal nerve is of large size, and named, from its origin 

 and distribution, the intercosto-humeral nerve. It pierces the External intercostal 

 muscle, crosses the axilla to the inner side of the arm, and joins with a filament 

 from the nerve of Wrisberg. It then pierces the fascia, and supplies the skin of 

 the upper half of the inner and back part of the arm, communicating with the 

 internal cutaneous branch of the musculo-spiral nerve. The size of this nerve is 

 in inverse proportion to the size of the other cutaneous nerves, especially the 

 nerve of Wrisberg. A second intercosto-humeral nerve is frequently given off 

 from the third intercostal. It supplies filaments to the arm-pit and inner side of 

 the arm. 



The Lower Intercostal Nerves (excepting the last) have the same arrangement 

 as the upper ones as far as the anterior extremities of the intercostal spaces, where 

 they pass behind the costal cartilages, and between the Internal oblique and Trans- 

 versalis muscles, to the sheath of the Rectus, which they perforate. They supply 

 the Rectus muscle, and terminate in branches which become subcutaneous near the 

 linea alba (anterior cutaneous nerves of the abdomen), supplying the integument 

 in front of the abdomen, being directed outwards to the lateral cutaneous nerves. 

 The lower intercostal nerves supply the Intercostal and abdominal muscles, and, 

 about the middle of their course, give off lateral cutaneous branches, which pierce 

 the External intercostal and External oblique muscles, and are distributed to the 

 integument of the abdomen, the anterior branches passing nearly as far forwards 

 as the margin of the Eectus ; the posterior branches passing to supply the skin 

 over the Latissimus dorsi, where they join the dorsal cutaneous nerves. 



PECULIAR DORSAL NERVES. 



First dorsal nerve. Its roots of origin are similar to those of a cervical nerve. 

 Its posterior or dorsal branch resembles, in its mode of distribution, the dorsal 

 branches of the cervical nerves.; Its anterior branch enters almost wholly into the 

 formation of the brachial plexus, giving off, before it leaves the thorax, a small 

 intercostal branch, which runs along the first intercostal space, and terminates on 

 the front of the chest, by forming the first anterior cutaneous nerve of the thorax. 

 The first intercostal nerve gives off no lateral cutaneous branch. 



The last dorsal is larger than the other dorsal nerves. Its anterior branch 



runs along the lower border of the last rib in front of the Quadratus lumborum, 



perforates the aponeurosis of the Transversalis, and passes forwards between it 



and the Internal oblique, to be distributed in the same manner as the preceding 



37 



