710 THE NEKVOUS SYSTEM. 



arm on its medial side. At the elbow it divides into two terminal branches 

 volar and ulnar, which, crossing superficial or deep to the median basilic vein, are 

 distributed to the medial side of the forearm. 



The volar branch can be followed to the wrist and supplies the whole of the 

 volar surface of the forearm in the medial half; the ulnar branch is not so large, 

 and, passing obliquely backwards and distally over the origins of the pronator 

 and flexor muscles, it is distributed to the proximal two-thirds or three-fourths of 

 the dorsal aspect of the forearm on the medial side. 



Communication. The medial cutaneous nerve of the forearm communicates 

 with the volar branch of the ulnar nerve in the distal part of the forearm. 



NERVUS CUTANEUS BEACHII MEDIALIS. 



The medial cutaneous nerve of the arm (O.T. lesser internal cutaneous nerve) 

 arises from the medial cord of the brachial plexus, and ultimately from the first 

 thoracic nerve (Fig. 614, p. 701). It lies at first between the axillary artery and 

 vein ; and after descending over, under, or even, in some cases, through the axillary 

 vein, it perforates .the deep fascia and is distributed to the skin of the arm for the 

 proximal half or 'more on its medial side. 



The nerve varies considerably in size. It may be absent, its place being taken by branches 

 of the intercosto-brachial or by branches from the posterior cutaneous branch of the radial nerve. 

 It generally bears a distinct relation in size to the intercosto-brachial, due to the fact that the size 

 of the latter depends upon the size of the part of the second thoracic nerve connected with 'the 

 first in the thorax. If an intra- thoracic connexion occurs between the first and second thoracic 

 nerves, the intercosto-brachial may be deprived of a certain number of its fibres, which in 

 that case reach the upper limb through the medial cutaneous nerve of the arm. When traced 

 up to the plexus the medial cutaneous nerve of the arm is found to have an origin from the 

 posterior part of the cord formed by the eighth cervical and first thoracic nerves, and usually 

 receives fibres from the first thoracic nerve only. In cases where " axillary arches " are present 

 they may be supplied by this nerve. 



NERVUS AXILLARIS. 



The axillary nerve (O.T. circumflex) at its origin is just below the supra- 

 scapular and comes from the same spinal nerves the fifth and sixth cervical 

 nerves (Fig. 614, p. 701). Extending distally and laterally behind the axillary 

 artery, it leaves the axilla by passing round the lateral border of the subscapularis 

 muscle, in company with the posterior circumflex artery of the humerus, in a quadri- 

 lateral space bounded by the humerus, subscapularis, triceps (long head), and teres 

 major. Winding round the surgical neck of the humerus from medial to lateral 

 side, it terminates by supplying the deltoid muscle (Fig. 619, p. 708). 



Branches. Muscular branches are supplied to the teres minor and deltoid 

 muscles. The nerve to the teres minor enters the lateral aspect of the muscle. It 

 possesses a pseudo-ganglion, a thickening of fibrous tissue, on its trunk. 



Articular branches enter the posterior part of the capsule of the shoulder-joint. 



A cutaneous branch of considerable size the lateral cutaneous nerve of the arm 

 passes obliquely distally and forwards from beneath the deltoid muscle, becoming 

 superficial at its posterior border. Sometimes branches pierce the muscle. It 

 supplies the skin over the insertion of the deltoid and the proximal half of the arm 

 on its lateral aspect (Figs. 617, p. 706, and 618, p. 707). 



NERVUS EADIALIS. 



The radial nerve (O.T. musculo-spiral) appears to be the continuation into the 

 upper limb of the posterior cord of the brachial plexus. It usually takes origin 

 from all the nerves which form the posterior cord the fifth, sixth, seventh, and 

 eighth cervical and first thoracic nerves (Fig. 614, p. 701). In some cases the first 

 thoracic contributes no fibres, and often the fifth cervical nerve is excluded 

 from it. It extends from the axilla, round the back of the humerus, to the bend of 

 the elbow, where it ends by dividing into its superficial and deep terminal branches. 



