816 



NERVOUS SYSTEM OF THE HORSE 



Axillary Nerve 

 The axillary nerve (Figs. 566, 652)^ derives its fibers chiefly from the eighth 

 cervical root of the brachial plexus. It runs downward and backward across 

 the distal part of the subscapularis, and, in company 

 with the posterior circumflex artery, dips in between that 

 muscle and the subscapular artery at the level of the 

 shoulder joint. Continuing outward in the interval between 

 the teres minor and the long and lateral heads of the triceps, 

 it reaches the deep face of the deltoid and divides into several 

 divergent branches . The muscular branches supply the teres 

 major, capsularis, teres minor, deltoid, and brachiocephali- 

 cus. The cutaneous branch (N. cutaneus brachii lateralis) 

 runs downward and a little forward across the lateral head 

 of the triceps and ramifies on the fascia on the front of the 

 forearm and on the superficial pectoral muscle. 



Radial Nerve 

 The radial nerve^ (Figs. 566, 568, 652) is sometimes the 

 largest branch of the brachial plexus. Its fibers are derived 

 chiefly from the first thoracic root of the plexus. It passes 

 do^vnward and backward over the medial surface of the origin 

 of the subscapular artery and the distal part of the teres 

 major. In this part of its course it is related in front to the 

 ulnar nerve, which separates it from the brachial vein.^ It 

 detaches a branch to the tensor fascise antibrachii, passes out- 

 ward in the interval between the teres major and the long 

 and medial heads of the triceps, and gains the musculo-spiral 

 groove of the humerus. Accompanied by a branch of the 

 deep brachial artery, it runs obliquely downward and outward 

 in the groove, covered laterally by the lateral head of the tri- 

 ceps and the extensor carpi radialis, and reaches the flexion 

 surface of the elbow joint. In this part of its course it gives 

 off muscular branches to the three heads of the triceps and 

 to the anconeus, and a cutaneous nerve (N. cutaneus anti- 

 brachii dorsalis); branches of the latter emerge below or 

 through the lateral head of the triceps and ramify on the 

 dorso-lateral surface of the forearm. At the elbow the nerve 

 descends with the anterior radial vessels on the joint capsule, 

 between the brachialis and extensor carpi radialis, and sup- 

 plies branches to the extensor carpi and the common extensor 

 of the digit and (inconstantly) to the brachialis. Below the 

 elbow joint the nerve detaches a large branch which passes 

 back between the lateral extensor and the radius to the ulnaris 

 lateralis, and terminates by small branches which descend on 

 the radius to enter the radial and ulnar heads of the common 

 and lateral extensors of the digit, and the oblique extensor of 

 the carpus. Thus the radial nerve innervates the extensors 

 of the elbow, carpal, and digital joints, and supplies also 

 the lateral flexor of the carpus.* 



^ Also termed the cii'cumfl'ex nerve. 



- This is also known as the musculo-spiral nerve. 



^ The relative positions of the vessels and nerves here are variable; not rarely the ulnar nerve 

 passes between the subscapular artery and vein. 



^ Morphologically the last-named muscle belongs to the extensor group. In man and in 

 many animals it is an extensor in function. 



Fig. 652, — Cutaneous 

 Nerves of Right 



FORELIMB OF HoRSE; 



Lateral Face. 



a, Cutaneous branch 

 of axillarj' nerve; h, cu- 

 taneous branches of radial 

 nerve; c, posterior cutane- 

 ous branch of ulnar nerve ; 

 d, cutaneous branch of me- 

 dian nerve; e, superficial 

 branch of ulnar nerve; /, 

 lateral volar nerve; g, anas- 

 tomotic branch connecting 

 medial and lateral volar 

 nerves; -?, lateral head of 

 triceps: 2, extensor carpi 

 radialis; 3, common ex- 

 tensor; 4-^ ulnarig lateralis. 

 (After Ellenberger, in Leis- 

 ering's Atlas.) 



