THE BRACHIAL PLEXUS. 1295 



I. Supraclavicular Branches 



1. Suprascapular 4. Muscular 



2. Posterior scapular 5. Communicating to the phrenic 



3. Posterior thoracic nerve 



II. Infraclavicular Branches 



A. From Outer Cord : B, From Inner Cord: 



6. External anterior thoracic 9. Internal anterior thoracic 



7. Musculo-cutaneous 10. Lesser internal cutaneous 



8. Median (outer head) 11. Internal cutaneous 



12. Ulnar 



13. Median (inner head) 



C. From Posterior Cord : 



14. Subscapular 



15. Circumflex 



16. Musculo-spiral 



I. The Supraclavicular Branches. — These are given off at various levels 

 while the plexus is still in the neck. 



1. The suprascapular nerve (n. suprascapularis) (Fig. 1092) arises from 

 the posterior surface of the outer trunk, most of its fibres coming from the fifth 

 cervical nerve and the remainder from the sixth. It traverses the posterior cervical 

 triangle above the upper border of the plexus and under cover of the omo-hyoid and 

 trapezius muscles. Reaching the superior margin of the scapula, it passes through 

 the suprascapular notch, under the suprascapular ligament, and enters the supra- 

 spinous fossa. After giving off a branch for the supply of the supraspinatus muscle 

 and a tiny filament to the posterior portion of the capsular ligament of the 

 shoulder, it passes through the great scapular notch in company with the supra- 

 scapular artery and vein. Having become an occupant of the infraspinous fossa, the 

 nerve supplies the infraspinatus muscle and often gives off a branch to the 

 shoulder joint. 



Variations. — It may receive additional fibres from the fourth cervical nerve or may arise 

 entirely from the fifth. A rare anomaly is the giving off of a branch to the teres minor or to the 

 upper part of the subscapularis. Twigs to the scapula and its periosteum and to the acromio- 

 clavicular articulation have been described. Division into two parts may occur, the upper part 

 passing through the notch and the lower through a bony foramen below the notch. 



2. The posterior scapular nerve or the branch to the rhomboid 

 muscles (n. dorsalis scapulae) (Fig. 1082) arises, in common with a root to the 

 posterior thoracic nerve, from the dorsal aspect of the fifth cervical nerve. After 

 traversing the substance of the scalenus medius, it passes downward and backward 

 toward the vertebral border of the scapula, lying upon the deep surface of the 

 levator anguli scapulae and the rhomboidei. It supplies a filament to the levator 

 anguli scapulae and occasionally one to the upper digitation of the serratus posticus 

 superior, and terminates by entering the substance of the rhomboideus major and 

 minor muscles. 



Variation. — It may pierce the levator anguli scapulae, 



■^ The posterior thoracic (n. thoracalis longus), also called the long 

 thoracic or external respiratory nerve of Bell arises from the fifth, sixth and 

 seventh cervical nerves, the largest contribution coming from the sixth (Fig. 1092). 

 The roots from the fifth and sixth nerves pass through the scalenus medius and unite 

 either in the substance of that muscle or as they reach its surface. The root from the 

 seventh nerve passes anterior to the middle scalene muscle and unites with the main 

 trunk at about the level of the first rib. Entering the axilla the nerve descends on 



