SPINAL NERVES. 



759 



External 



spiral in the arm are numerous, and may be 

 arranged into 



{Internal cutaneous. Branch 

 for the internal head of the 

 triceps. 



f Branches for the long head of 

 Posterior i the triceps. Outer head and 



ancongeus. 

 Cutaneous filaments to the 



arm. External cutaneous. 

 The internal cutaneous is the first branch of 

 the musculo-spiral, and continues for some 

 distance deeply seated to the fascia, which it 

 perforates above the middle of the arm, and 

 descends as one or two filaments along the 

 inner and back part of the arm to the elbow, 

 where they communicate with the posterior 

 filaments of the accessory internal cutaneous. 

 The branch for the internal head of the tri- 

 ceps is the next that is given off. It is a' 

 delicate, long nerve, which is directed along 

 the surface of the inner portion of the triceps, 

 running behind the ulnar nerve to within three 

 or four inches of the elbow-joint, when it enters 

 the substance of the muscle. 



The branches for the long head of the triceps 

 are numerous, and enter its anterior surface. 

 The superior branch is reflected upwards, and 

 can be traced as far as the axillary origin of 

 the muscle. The inferior or descending branch 

 is the longest, and courses downwards to near 

 the olecranon before entering it. 



The branch for the outer head of the triceps and 

 anconceus, given off externally to the branches 

 for the long head, is a long slender nerve. It 

 passes down between the outer and middle 

 head to the outside of the olecranon, sup- 

 plying the outer head in its course, and ter- 

 minating in the anconceus by entering at its 

 anterior surface. 



The external cutaneous branch is given off 

 below the middle of the arm, as the musculo- 

 spiral is commencing its anterior and outer 

 course. It passes along the outer and back of 

 the arm, and divides into two or three delicate 

 descending filaments which supply the skin, 

 and terminate on the back of the carpus be- 

 tween the posterior branches of the external 

 cutaneous, radial, and dorsalis carpi ulnaris 

 with which they communicate. 



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 supi- 

 nator longus 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 muscular) is larger than the 

 anterior, passes downwards and backwards 

 along the inner aspect of the exterior carpi 

 radialis 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 communis 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 gangliform enlargement, from which 

 numerous filaments radiate for the supply of 

 the ligaments and carpal articulations. 



The circumflex ncrrc (axillary) is the most 

 posterior of the terminal branches of the bra- 

 cliial 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 wlreh it is situated. 

 Alter a short course in the axilla, it soon 

 kavcs 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 tercs major, below the 



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