The musculo-spiral nerve, before giving off its terminal branches, sends filaments to the muscles between which it passes, viz. the bra chialis anticus, supinator longus, and extensor carpi radialis longior.
The anterior terminal branch (radial nerve) is the apparent continuation of the musculo spiral nerve, though smaller than the posterior terminal branch. It passes between the stipi nator Jongus and brevis, lying on the latter, and over-lapped by the former, and gradually approaches, in its descent of the fore-arm, the radial artery ; so that at the middle it is in close contact with, but external to, the vessel. Having arrived at the lower third of the fore arm, or a little above, it twists round the deep surface of the tendon of the supinator longus, and appears beneath the fascia on the outer part of the fore-arm, and after a short sub aponeurotic course, perforates the fascia, and divides about a couple of inches above the sty loid process into an external large, and internal terminal-branch. The external branch passes along the outer aspect of the styloid process ; and at the proximal part of the wrist sends a communicating loop inwards, to be connected with the cutaneous palmar branch of the me dian. It then descends on the dorsum of the thumb, and supplies its external border. The internal branch crosses obliquely the extensor ossis metacarpi and primi internodii pollicis, and divides into a series of branches which supply the ulnar side of the thumb : both sides of the index finger, and the radial side of the middle. These different branches furnish, in their course along the carpus, several cutaneous filaments, and some small twigs which com municate with the perforating interosseous of the deep branch of the ulnar nerve. The most internal division communicates with the dorsalis carpi ulnaris. The two terminal branches of the radial are subject to much variation : the external being sometimes larger than the internal, and supplying either both sides of the thumb, or both sides of the thumb and the radial side of the index finger. The internal branch occasionally unites with the outer division of the dorsalis carpi ulnaris, and supplies the opposed sides of the middle and ring fingers.
The deep terminal branch (the posterior interosseous or nzusczilar) is larger than the anterior, passes downwards and backwards alont, the inner aspect of the exterior carpi ' radia°lis brevis, gives filaments to it, and reaches the surface of the supinator brevis, supplies it, as it passes obliquely downwards, backwards, and inwards through its substance, to emerge at its lower and posterior portion.
It here divides into a posterior and anterior series : the former supplying the extensor carpi ulnaris, the comtnunis digitorum, and minimi digiti, entering at their anterior aspect the latter the deep-seated muscles. One of the latter has a somewhat remarkable course ; is longer and larger than the rest ; and passes along the posterior surface of the extensor ossis metacarpi and primi internodii; and at the lower part of the fore-arm becomes placed between the interosseous ligament and the extensor secundi internodii, and indicator, sup plies these muscles with one or two twigs, and is conducted in front of the posterior annular ligament to the back of the carpus, where it assumes a onngliform enlargement, from which numerous °filaments radiate for the supply of the ligaments and carpal articulations.
The circumflex nerve (axillary) is the most posterior of the terminal branches of the bra chial plexus, and is occasionally given off from the musculo-spiral, usually, however, taking its origin from a trunk common to it, and to that nerve, external to wh;ch it is situated. After a short course in the axilla, it soon leaves that space by passing downwards and outwards over the upper part of the axillary border of the subscapularis to enter the quadri lateral space above the teres major, below the teres minor, and between the humerus and long head of the triceps to tertninate in the deep surface of the deltoid. It gives off in this course branches to the subscapularis and teres minor; that for the latter entering the lower border of the muscle, and prior to dividing into its deltoid branches. — The cutaneous nerve of the shoulder passes from behind the posterior border of the deltoid, perforates the fascia, and divides into a series of radiating branches, which supply the skin at the upper and back part of the shoulder. The deltoid branches ramify through the sub stance of the muscle as far as its insertion, and from one of them a filament is given off to the capsular ligament of the shoulder joint.