1298 HUMAN ANATOMY. 



7. The Musculo-Cutaneous Nerve. 



The musculo-cutaneous nerve (n. musculocutaneus) (Fig. 1098) derives its fibres 

 from the fifth and sixth, and sometimes the seventh, cervical nerves and is a branch 

 of the outer cord. The nerve to the coraco-brachialis viiiscle, derived from the seventh 

 or sixth and seventh nerves, is usually found as an integral part of it. Leaving the 

 outer cord under cover of the pectoralis minor it pierces the coraco-brachialis and 

 passes obliquely downward and outward between the biceps and brachialis anticus 

 muscles. Reaching the outer margin of the biceps a short distance above the elbow, 

 the nerve pierces the deep fascia and passes under the median-cephalic vein. It then 

 becomes superficial (n. cutaneus antebrachii lateralis) and divides into its terminal 

 cutaneous branches. 



Branches. — These are: {a) the muscular, ib) the humeral, (r) the articular and {d) the 

 termijial. 



a. The muscular branches supply the coraco-brachialis, the biceps and the brachialis anticus. 

 T\\^nerve to the coraco-brachialis, which commonly has an independent origin, is usually double, 

 one filament going to each portion of the muscle. The nerves to the biceps and brachialis anticus 

 are given off while the musculo-cutaneous is in transit between those muscles. 



b. The humeral branch accompanies the nutrient branch of the brachial artery into the 

 humerus. 



c. The articular branch aids in the supply of the elbow joint. 



d. The terminal part (n. cutaneus antebrachii lateralis) (Fig. 1103) of the musculo-cutaneous 

 divides into two branches, {ad) an anterior and {bb) ?i posterior. 



aa. The anterior branch descends in the antero-lateral portion of the superficial fascia of the 

 forearm (Fig. 1104). It inosculates above the wrist with the radial nerve and supplies the in- 

 tegument of the antero-lateral part of the forearm. It also distributes fibres to the skin over the 

 thenar eminence, to the wrist joint and to the radial artery. 



bb. The posterior branch passes downward and backward and supplies the skin of the 

 postero-lateral portion of the forearm down to or slightly beyond the wrist joint (Fig. 1102 ). It 

 inosculates with the radial nerve and with the inferior external cutaneous branch of the musculo- 

 spiral. 



Variations. — Instead of piercing the coraco-brachialis the nerve may adhere to the median 

 or its outer head for some distance down the arm, and then either as a single trunk or as several 

 branches pass between the biceps and brachialis anticus muscles. Sometimes only a part of the 

 nerve follows this course, joining the main trunk after the latter' s transit through the muscle. 

 The muscular part only or the cutaneous part only may pierce the muscle. The nerve may be 

 accompanied through the muscle by fibres of the median which jejoin the latter below the 

 coraco-brachialis. The nerve may remain independent and fail to pierce the coraco-brachialis, 

 either passing behind it or between it and the associated head of the biceps. It may perforate 

 not only the coraco-brachialis but also the brachialis anticus or the short head of the biceps. 

 Rarely the entire outer cord, after giving off the external anterior thoracic, may traverse the 

 coraco-brachialis. Anomalies in distribution include a branch to the pronator radii teres, the 

 supply of the skin of the dorsum of the hand over and adjacent to the first metacarpal bone, 

 a branch to the dorsum of the thumb in the absence of the radial nerve and the giving off of 

 dorsal digital nerves to both sides of the ring finger and the adjacent side of the little finger. 



8. The Median Nerve. 



The median nerve (n. medianus) (Fig. 1098) consists of fibres which can be 

 traced to the sixth, seventh and eighth cervical and first thoracic nerves. It arises 

 by two heads, an outer and an inner, which are derived respectively from the outer 

 and inner cords of the plexus, the former containing fibres from the sixth and 

 seventh cervical and the latter fibres from the eighth cervical and first thoracic 

 nerves. The two heads, the inner of which usually crosses the main artery of 

 the upper extremity at about the point where the axillary becomes brachial, 

 unite either in front of or to the outer side of the artery. From the point of fusion 

 of the two heads the nerve passes down the arm in close relation with the brachial 

 artery, usually lying lateral or antero-lateral to the artery in the upper part of the 

 arm, and as the elbow is neared, gradually attaining the inner side by crossing 

 obliquely the anterior surface of the artery (Fig. 1098). It passes through the 

 cubital fossa beneath the median-basilic vein and the bicipital fascia, and enters the 

 forearm between the heads of the pronator radii teres muscle, the deep head of 



