THE THORACIC NERVES 



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The First Thoracic Nerve. The anterior division of the first thoracic nerve divides 

 into two branches : one, the larger, leaves the thorax in front of the neck of the first 

 rib, and enters the brachial plexus; the other and smaller branch, the first intercostal 

 nerve, runs along the first intercostal space, and ends on the front of the chest as 

 the first anterior cutaneous branch of the thorax. Occasionally this anterior cuta- 

 neous branch is wanting. The first intercostal nerve as a rule gives off no lateral 

 cutaneous branch; but sometimes it sends a small branch to communicate with 

 the intercostobrachial. From the second thoracic nerve it frequently receives a 

 connecting twig, which ascends over the neck of the second rib. 



Recurrent branch Sympathetic 

 ganglion 



Transversus Internal 

 thoracis mam. art. 



Lateral cutaneous 



Anterior cutaneous 

 FIG. 819. Diagram of the course and branches of a typica intercostal nerve. 



The Upper Thoracic Nerves (nn. Intercostales). The anterior divisions of the 

 second, third, fourth, fifth, and sixth thoracic nerves, and the small branch from the 

 first thoracic, are confined to the parietes of the thorax, and are named thoracic 

 intercostal nerves. They pass forward (Fig. 819) in the intercostal spaces below the 

 intercostal vessels. At the back of the chest they lie between the pleura and the 

 posterior intercostal membranes, but soon pierce the latter and run between the 

 two planes of Intercostal muscles as far as the middle of the rib. They then enter 

 the substance of the Intercostales interni, and, running amidst their fibers as far as 

 the costal cartilages, they gain the inner surfaces of the muscles and lie between 

 them and the pleura. Near the sternum, they cross in front of the internal mammary 

 artery and Transversus thoracis muscle, pierce the Intercostales interni, the anterior 

 intercostal membranes, and Pectoralis major, and supply the integument of the 

 front of the thorax and over the mamma, forming the anterior cutaneous branches 

 of the thorax; the branch from the second nerve unites with the anterior supra- 

 clavicular nerves of the cervical plexus. 



Branches. Numerous slender muscular filaments supply the Intercostales, the 

 Subcostales, the Levatores costarum, the Serratus posterior superior, and the Trans- 

 versus thoracis. At the front of the thorax some of these branches cross the costal 

 cartilages from one intercostal space to another. 



Lateral cutaneous branches (rami cutanei laterales) are derived from the intercostal 

 nerves, about midway between the vertebrae and sternum; they pierce the Inter- 

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