984 THE NERVOUS SYSTEM 



from the first thoracic nerve alone. It runs downward on the medial side of the 

 axillary vein, being separated by that vessel from the ulnar nerve, and it continues 

 downward with a slight inclination dorsalward under cover of the deep fascia on 

 the inner side of the arm. At the middle of the arm it pierces the deep fascia, 

 and near the bend of the elbow it turns somewhat sharply dorsalward to supply 

 the integument which covers the olecranon process (fig. 756). 



As it traverses the axilla the nerve of Wrisberg communicates with the intercosto-brachial 

 nerve, forming one, or sometimes two loops (fig. 754). In its course down the arm it gives a 

 few fine twigs to the integument. This nerve may be absent, its place being taken by the inter- 

 costo-brachial or by part of the posterior brachial (internal) cutaneous branch of the radial 

 (musculo-spiral) or, rarely, by a branch from the first intercostal nerve. 



The medial antibrachial (internal) cutaneous nerve (figs. 751 and 754) arises 

 from the medial cord in close relation with the ulnar nerve. It contains fibres 

 from the eighth cervical and first thoracic nerves. At its origin it lies directly on 

 the medial side of the axillary artery (fig. 755), but it soon becomes more super- 

 ficial and then lies in the groove between the artery and the vein. In the upper 

 two-thirds of the arm it lies in front and to the medial side of the brachial artery. 

 It divides into two branches (volar and ulnar) which supply the medial aspect 

 of the forearm. 



At the junction of the middle and lower thirds of the arm this nerve pierces the deep fascia, 

 in company with the basilic vein, and divides into an anterior and a posterior branch. Previous 

 to its division it gives off twigs which pierce the deep fascia and supply the integument of the 

 upper and medial part of the arm. The volar (anterior) branch is larger than the ulnar (pos- 

 terior) ; it passes in front of or dorsal to the median basilic vein, and divides into several twigs 

 which run down the forearm, supplying the integument covering its anterior and medial aspect 

 as far as the WTist, and anastomosing with the branches of the ulnar nerve. The ulnar (posterior) 

 branch passes downward and dorsalward in front of the medial condyle of the humerus, and di- 

 vides into branches which supply the skin on the postero-medial aspect of the forearm. It 

 anastomoses with the dorsal antibrachial (inferior external) cutaneous branch of the radial 

 (musculo-spiral) nerve and the dorsal branch of the ulnar nerve). 



The subscapular nerves are branches of the posterior cord (fig. 754). They 

 are three in number, are distinguished as upper, thoraco-dorsal or middle, and 

 lower, and are distributed to the subscapularis, latissimus dorsi, and teres major 

 muscles. 



The upper or short subscapular nerve is derived from the fifth and sixth cervical nerves. 

 It hes in the upper and posterior part of the axillary fossa, and it is distributed exclusively to 

 the subscapularis muscle. It is occasionally double. 



The thoraco-dorsal, middle, or long subscapular nerve consists mainly of fibres from the 

 seventh and eighth cervical nerves, but it may contain fibres from the fifth or the sixth nerve. 

 It passes behind the axillary artery, accompanies the subscapular artery along the axillary margin 

 of the subscapularis muscle, and ends in the latissimus dorsi (fig. 755). 



The lower subscapular nerve, carrying fibres from the fifth and sixth cervical nerves, 

 passes behind the subscapular artery, below the circumflex branch (dorsaUs scapula;), and is 

 distributed to the teres major, and furnishes to the subscapularis one or two twigs which enter 

 that muscle near its axillary margin. 



The terminal branches of the plexus are two from each cord. The posterior 

 cord divides into the axillary (circumflex) and the radial (musculo-spiral) nerves. 

 The lateral cord divides into the musculo-cutaneous nerve, and the lateral root of 

 the median nerve; the medial cord divides into the ulnar nerve, and the medial 

 root of the median nerve, the median nerve as a whole being one of the five ter- 

 minal branches of the plexus. 



The axillary (circumflex) nerve is the smaller of the two terminal branches of 

 the posterior cord, and contains fibres from the fifth and sixth cervical nerves 

 (figs. 751 and 754). At the lower border of the subscapularis it passes dorsalward 

 and accomi)anies the posterior circumflex artery through the quadrilateral space, 

 which is bounded l)y the teres major, long head of triceps, and subscapularis mus- 

 cles, and the surgical neck of the humerus, and it divides into a smaller superior 

 and a larger inferior division. Previous to its division it furnishes an articular 

 twig to the shoulder-joint. This twig pierces the inferior i)art of the articular 

 capsule. 



The superior division accompanies the posterior circumflex artery around the 

 neck of the humerus, and gives off a number of stout twigs which enter the del- 

 toid muscle (fig. 755). A few fine filaments pierce the deltoid and end in the integ- 

 ument which covers the middle third of that muscle. 



