BRACHIAL PLEXUS. 25 



the thoracic side of its artery, and receives corresponding thoracic extent and 

 and shoulder branches. Opposite the subscapular muscle it is relations ' 

 joined externally 1 >y a large vein, which is formed by the union of branches - 

 the venae comites of the brachial artery ; and near the clavicle the 

 cephalic vein opens into it. 



Dissection. The continuity of the axillary with the subclavian Dissection 

 artery will now be displayed by removing the middle third of the 

 clavicle and the subjacent portion of the subclavius muscle and 

 cleaning the vessel l>eneath the bone. After this the dissector will 

 follow out the branches of the brachial plexus, cut through the 

 pectoralis minor near its insertion into the coracoid process, and 

 turn it towards the chest, but without injuring the thoracic nerves. 

 The axillary vessels are next to be ligatured, divided below the 

 second rib above the ligature, and to be drawn down with hooks, 

 care being taken to preserve the loop of communication between 

 the external and the internal anterior thoracic nerves ; and their 

 thoracic branches may be removed at the same time. A dense 

 fascia is to be cleared away from the large nerves of the plexus. 



The BRACHIAL PLEXUS (figs. 7 and 8) results from the interlace- Xerve.s 

 ment of the anterior branches of the lower four cervical nerves and bracMaf 

 the larger part of the first dorsal ; and a slip is added to it above plexus, 

 from the fourth cervical nerve. It is placed successively in the Its situation 

 neck and the axilla, and ends opposite the coracoid process in the 

 nerves of the limb. The part of the plexus alx>ve the clavicle is 

 described in the dissection of the head and neck. The part and reia- 

 below the clavicle has the same relations to surrounding muscles tlons - 

 as the axillary artery ; and in it the nerve-trunks are disposed as 

 follows : 



As the plexu> enters the axilla it consists of three cords, inner, The nerves 

 outer, and posterior, which lie together in a bundle on the outer Jonis^ 

 side of the artery. Beneath the pectoralis minor the three cords **J" n . d the 

 embrace the vessel, being placed as their names indicate the first "" 

 inside, the second outside, and the third behind the artery. Near 

 the lower edge of the small pectoral muscle, the cords divide to 

 form the large nerves of the limb. 



The branches of the plexus below the clavicle arise from the branches: 

 .several cords in the following way (fig. 8) : 



The outer cord furnishes one anterior thoracic branch (eat), the outer cord ; 

 musculo-cutaneous (me), and the outer head of the median nerve (m). 



The inner cord gives origin to a second anterior thoracic nerve inner cord ; 

 (iat), the internal cutaneous (ic), the nerve of Wrisberg (w), the 

 inner head of the median (m), and the ulnar nerve (u). 



The posterior cord furnishes the subscapular branches (si, s2, and posterior 

 s3), and ends in the circumflex (c) and musculo-spiral (ms) trunks. 



Only the thoracic and subscapular nerves are exposed to their ^ e a ^ n s e g' n 

 termination at present ; the remaining nerves will be seen in the now, vi*. 

 subsequent dissec t ions. 



The anterior thoracic branches (fig. 6, 1 and 2 , p. 21, and fig. 7, two anterior 

 p. 23), two in number, are named outer and inner, like the cords 

 from which thev come. 



