6 THORAX 



intercostal muscles and are carried medially, first anterior 

 to the pleura, and then anterior to the transversus thoracis 

 muscle. Lastly, they cross anterior to the internal mammary 

 artery and turn anteriorly, at the side of the sternum, as the 

 anterior cutaneous nerves of the pectoral region. Each nerve, 

 before it reaches the surface, pierces (a) the internal inter- 

 costal muscle ; (fr) the anterior intercostal membrane ; (c} 

 the origin of the pectoralis major; and (d) the deep fascia 

 (Fig. 2). 



This description holds good for the upper five inter- 

 costal nerves only. The lower six nerves leave the anterior 

 ends of the intercostal spaces and run into the abdominal 

 wall. As they leave the thoracic wall the upper four of the 

 six pass posterior to the upturned costal cartilages, and all six 

 pass between the interdigitating slips of the diaphragm and 

 the transversus abdominis muscles. 



The intercostal nerves are the anterior branches of the 

 upper eleven thoracic nerves. As they traverse the thoracic 

 wall they give off (a) the lateral cutaneous branches, (b) 

 twigs to the intercostal, subcostal, and transversus thoracis 

 muscles. The terminal extremities of the upper five become 

 the anterior cutaneous nerves of the thorax. (For the 

 abdominal distribution of the lower six see Vol. I., p. 394.) 



The lateral cutaneous branches arise midway between the 

 vertebral column and the sternum. They pierce the external 

 intercostal muscles, and pass between the digitations of the 

 serratus anterior. 



The first intercostal nerve does not give a lateral branch, 

 and it does not become cutaneous anteriorly. The lateral 

 cutaneous branch of the second intercostal nerve is the so- 

 called intercosto-brachial nerve. 



It is not necessary to make a dissection of the intercostal nerves in more 

 than two or three of the spaces. 



Intercostal Vessels. The intercostal arteries should be 

 dissected in spaces in which the nerves have not been 

 traced, and in which, therefore, the internal intercostal 

 muscles are still entire. It is only in a well-injected subject 

 that a satisfactory view of these vessels can be obtained. In 

 each intercostal space one artery is found passing dorso-ven- 

 trally ; and in each of the upper nine intercostal spaces, two 

 anterior intercostal arteries run ventro-dor sally. 



