MUSCULO-SPIBAL NERVE 795 



sujiply the palniaris l)rrvi,s muscle, several twigs, whieh an* distrilmted to the skin 

 covering the hypotlienar eminence, and then it divides into two branches, an inner 

 and an outer. The inner branch is distributed to the inner side of the little linger 

 <tn its volar aspect. The outer branch comnmnicates with the median nerve, and 

 then divides to supply the contiguous margins of the ring and little lingers. The 

 distriI)ution of these branches to the lingers ri'sembles that of the digital branches 

 of the median nerve, which has been already described. 



(f ) The deep branch of the ulnar nerve accompanies the deep branch of the 

 ulnar artery int(j the interval between the abductor and flexor lirevis minimi digiti 

 muscles, and then passes through the libres of the o})ponens minimi digiti to reach 

 the deep surface of th(? flexor tendons. It supplies the abductor, flexor l)revis, 

 and op})onens minimi digiti, the two inner lumbrieales, all the interossei, the 

 adductors, and the inner head of the flexor brevis pollicis. Occasionally it also 

 gives a twig to the outer head of the flexor brevis ])ollicis. 



(7) The musculo-spiral nerve is the largest branch of the brachial plexus. 

 It arises from the posterior cord, and contains fibres derived from the fifth, sixth. 

 seventh, and eighth cervical nerv'es. It is i)laced at first behind the third stage of 

 the axillary artery; low'er down it has a similar relation to the upper part of tlie 

 brachial artery. It soon leaves the latter vessel and accompanies the superior pro- 

 funda artery into the inter\'al betw^een the inner and outer heads of the tricejis. 

 Having followed the si)iral groove around the humerus, it pierces the external 

 inter-muscular septum and runs downwards in the interval between the supinator 

 longus (brachio-radialis) and the brachialis anticus muscles. A little above the 

 bend of the elbow it terminates by dividing into the radial and the posterior inter- 

 oss(M)us nerves. 



Branches. — The musculo-spiral nerve gives oflf three cutaneous branches, one 

 internal and two external, and supplies the following muscles: triceps, anconeus, 

 brachialis anticus in part, supinator longus, and extensor carpi radialis longior. 



(a) The internal cutaneous branch *irises within the axilla, usually in 

 common with tlie branch to the long head of the triceps. It crosses the tendon of 

 the latissimus dorsi muscle and passes l)ehind the intercosto-humeral nerve to 

 the back of the arm, supplying a strip of integument in the middle of the dorsal 

 surface extending nearly dow^n to the elboAV, and placed betw^een the areas of 

 distribution of the circumflex and intercosto-humeral nerves. 



(h) The external cutaneous branches are distinguished as U])per and lower. 

 The upper branch, much the smaller, i)ierces the deep fascia in the line of the 

 external intermuscular septum; it accompanies the lower part of the cei)halic vein 

 and su]>plies the skin over the lower half of the outer and anterior asjjcct of the 

 arm. The lower branch is of consideral>le size. It pierces the deep fascia a little 

 below the upi)er branch, runs behind the external condyle, and supj)lies the skin of 

 the middle of the back of the forearm as far as the wrist, anastomosing with tlie 

 internal cutaneous and musculo-cutaneous nerves. 



(<■) Muscular Branches. — ^Muscular branches are given off in the axilla to 

 the long and internal heads of the tricejos; one of the latter is a long filament which 

 accompanies the ulnar nerve and is known as the ulnar collateral. At tlie l)ack of 

 the humerus muscular branches are given to the outer and imier heads of the tri- 

 ceps and to the anconeus; the latter descends in the substance of the inner head of 

 the triceps accompanied by a branch of the superior ])rofimda artery. At the outi-r 

 side of the humerus branches are supplied to the supinator longus, the extensor 

 carpi radialis longior. and to a section of the brachialis anticus; from one of these 

 an articular twig is given off to the elliow-joint. 



(^d) The posterior interosseous nerve passes downwards in thi' interval 

 between the brachialis anticus and the extensor carpi radialis longior, and. having 

 given oft' branches to sup))ly the extensor carpi radialis brevior and sui)inator 

 brevis, it is crossed by the radial recurrent artery; then it runs downwards and 

 backwards through the substance of the sui)inator brevis, and enters the cellular 

 interval between the suiierficial and deep layers of muscles at the back of the fore- 

 arm; here it comes into relation with the ])osterior interosseous artery, which it 

 accompanies across the extensor ossis metacar]>i jiollicis. At the lower border of 

 the latter muscle it gives off a branch to the extensor secundii interno<lii ]>ollicis, 



