THE BRACHIAL PLEXUS 1038 



The Anterior Thoracic Nerves (nn. thoracales anter lores) (Figs. 758 and 759), 

 two in number, supply the Pectoral muscles. 



The external anterior thoracic nerve (Figs. 757 and 704), the larger of the two, 

 arises from the outer cord of the brachial plexus, through which its fibres may be 

 traced to the fifth, sixth, and seventh cervical nerves. It passes inward, across 

 the axillary artery and vein, pierces the costocoracoid membrane, and is dis- 

 tributed to the under surface of the Pectoralis major muscle. It sends down 

 a .communicating filament to join the internal anterior thoracic nerve, and this 

 communicating filament forms a loop around the inner side of the axillary artery. 



The internal anterior thoracic nerve arises from the inner cord and through it 

 from the eighth cervical and first thoracic nerves. It passes behind the first part 

 of the axillary artery, then curves forward between the axillary artery and vein, 

 and joins with the filament from the external nerve. It then passes to the under 

 surface of the Pectoralis minor muscle, where it divides into a number of branches, 

 which supply the muscle on its under surface. Some two or three branches 

 pass through the muscle and reach the Pectoralis major. 



The Subscapular Nerves (nn. wbseapulares) (Figs. 758 and 759), three in number, 

 arise from the posterior cord of the plexus and supply the Subscapularis, Teres 

 maior. and Latissimus dorsi muscles, and give filaments to the shoulder-joint. 

 The fasciculi of which they are composed may be traced to the fifth, sixth, seventh, 

 and eighth cervical nerves. 



The upper or short subscapulax nerve, the smallest, arises from the posterior divi- 

 sion of the upper trunk of origin of the brachial plexus, and enters the upper part 

 of the Subscapularis muscle; this nerve is frequently represented by two branches. 



The lower subscapular nerve arises from the posterior cord of the brachial plexus, 

 enters the axillary border of the Subscapularis, and terminates in the Teres major. 

 The latter muscle is sometimes supplied by a separate branch. *j &* 



The middle or long subscapular nerve (n. thoracodorsalis) (Fig. faS), the largest 

 of the three, arises from the posterior cord of the brachial plexus and follows 

 the course of the subscapular artery, along the posterior wall of the axilla to the 

 Latissimus dorsi muscle, through which it may be traced as far as its lower border. 



The Circumflex Nerve (n. axillaris) (Figs. 759 and 765) supplies some of the 

 jnuscles, the shoulder-joint, and the integument of the shoulder (Figs. 7G1 and 

 7ii2)~ It arises from the posterior cord of the brachial plexus, in common with 

 the musculospiral nerve, and its fibres may be traced through the posterior cord 

 to the fifth and sixth cervical nerves. It is at first placed behind the axillary 

 artery, between it and the Subscapularis muscle, and passes downward and out- 

 ward to the lower border of that muscle. It then winds posteriorly in company 

 with the posterior circumflex artery, through a quadrilateral space bounded above 

 by the Teres minor muscle, below by the Teres major muscle, internally by the 

 long head of the Triceps muscle, and externally by the neck of the humerus. 

 The nerve then divides into two branches. 



The upper branch (Fig. 765) winds posteriorly around the surgical neck of the 

 humerus, beneath the Deltoid, with the posterior circumflex vessels, as far as the an- 

 terior border of that muscle, supplying it, and giving off cutaneous branches, which 

 pierce the muscle and ramify in the integument covering its lower part (Fig. 762). 



The lower branch (Fig. 765), at its origin, distributes filaments to the Teres 

 minor and back part of the Deltoid muscles. Upon the filaments to the former 

 muscle an ova] enlargement usually exists. The nerve then pierces the deep 

 fascia, and supplies the integument over the lower two-thirds of the posterior 

 surface of the Deltoid (n. cutaneus brachii lateralis), as well as that covering 

 the loirj 1 head of the Triceps (caput longum n. tricipitis brachii) (Fig. 766). 



The circumflex nerve, before its division, gives off an articular filament, which 

 enters the shoulder-joint below the Subscapularis muscle. 



