THORACIC WALL n 



junction of the bone with the cartilage of the rib ; from 

 that point it continues towards the median plane, lying 

 in the cases of the first two nerves between the internal 

 intercostal muscle and the pleura, and in the cases of the 

 third, fourth, and fifth, between the internal intercostal muscle 

 and the transversus thoracis. About 13 mm. from the sternum 

 it crosses in front of the internal mammary vessels ; then, 

 it turns forwards, and after piercing the internal intercostal 

 muscle, the anterior intercostal membrane, the pectoralis 

 major muscle, and the deep fascia, it enters the superficial 

 fascia of the thorax as an anterior cutaneous nerve. 



The above description holds good for the upper five nerves 

 only ; after the lower six nerves have reached the deep sur- 

 faces of the corresponding internal intercostal muscles they 

 leave the anterior ends of the intercostal spaces and enter the 

 wall of the abdomen. As they leave the thoracic wall the 

 upper four of the six pass posterior to the upturned costal 

 cartilages, and all 'six pass between interdigitating slips of the 

 diaphragm and the transversus abdominis. Then they con- 

 tinue towards the median plane between the transversus 

 abdominis and the obliquus internus, enter the sheath of the 

 rectus abdominis, turn forwards, and, after piercing the rectus 

 abdominis, the anterior wall of its sheath, and the deep fascia, 

 they enter the superficial fascia as anterior cutaneous nerves 

 of the abdomen. 



The dissector of the thorax traces the lower intercostal nerves only as 

 far as the anterior ends of the intercostal spaces. The dissector of the 

 abdomen has displayed them in the wall of the abdomen. 



Whilst the intercostal nerves are between the intercostal 

 muscles each gives off branches to the muscles between which 

 it lies, and a lateral cutaneous branch. The lateral cutaneous 

 branch pierces the external intercostal muscle and then passes, 

 according to its position, either between digitations of the 

 serratus anterior or external oblique muscles and divides into 

 anterior and posterior branches which are distributed to the 

 skin. The first intercostal nerve does not give a lateral 

 cutaneous branch. Nor does it become cutaneous at its 

 termination. The lateral cutaneous branch of the second is 

 distributed to the arm as the intercosto-brachial nerve. The 

 lateral cutaneous branches of the lower intercostal nerves 

 supply muscular twigs to the digitations of the external oblique 



