THE BRACHIAL PLEXUS. 775 



they supply the Triceps, Anconeus, Supinator longus, Extensor carpi radialis 

 longior, and Brachialia anticus. These branches are derived from the nerve at 

 the inner side, back part, and outer side of the arm. 



The internal muscular branches supply the inner and middle heads of the 

 Triceps muscle. That to the inner head of the Triceps is a long, slender filament 

 which lies close to the ulnar nerve, as far as the lower third of the arm, and is 

 therefore frequently spoken of as the ulnar collateral. 



The posterior muscular branch, of large size, arises from the nerve in the groove 

 between the Triceps and the humerus. It divides into branches which supply the 

 outer and inner head of the Triceps and Anconeus muscles. The branch for the 

 latter muscle is a long, slender filament which descends in the substance of the 

 Triceps to the Anconeus. 



The external muscular branches supply the Supinator longus, Extensor carpi 

 radialis longior, and (usually) the outer part of the Brachialis anticus. 



The cutaneous branches are three in number, one internal and two external. 



The internal cutaneous branch arises in the axillary space with the inner mus- 

 cular branch. It is of small size, and passes through the axilla to the inner side 

 of the arm, supplying the integument on its posterior aspect nearly as far as the 

 olecranon. In its course it crosses beneath the intercosto-humeral, with which 

 it communicates. 



The two external cutaneous branches perforate the outer head of the Triceps 

 at its attachment to the humerus. The upper and smaller one passes to the 

 front of the elbow, lying close to the cephalic vein, and supplies the integu- 

 ment of the lower half of the arm on its anterior aspect. The lower branch 

 pierces the deep fascia below the insertion of the Deltoid, and passes down along 

 the outer side of the arm and elbow, and then along the back part of the radial 

 side of the forearm to the wrist, supplying the integument in its course, and join- 

 ing, near its termination, with the posterior cutaneous branch of the musculo- 

 cutaneous nerve. 



The radial nerve passes along the front of the radial side of the forearm to 

 the commencement of its lower third. It lies at first a little to the outer side of 

 the radial artery, concealed beneath the Supinator longus. In the middle third 

 of the forearm it lies beneath the same muscle, in close relation with the outer 

 side of the artery. It quits the artery about three inches above the wrist, passes 

 beneath the tendon of the Supinator longus, and, piercing the deep fascia at the 

 outer border of the forearm, divides into two branches. 



The external branch, the smaller of the two, supplies the integument of the 

 radial side and ball of the thumb, joining with the anterior branch of the musculo- 

 cutaneous nerve. 



The internal branch communicates, above the wrist, with the posterior cuta- 

 neous branch from the musculo-cutaneous, and on the back of the hand forms an 

 arch with the dorsal cutaneous branch of the ulnar nerve. It then divides into 

 four digital nerves, which are distributed as follows : The first supplies the ulnar 

 side of the thumb ; the second, the radial side of the index finger ; the third, 

 the adjoining sides of the index and middle fingers; and the fourth, the adjacent 

 borders of the middle and ring fingers. 1 The latter nerve communicates with a 

 filament from the dorsal branch of the ulnar nerve. 



The Posterior Interosseous Nerve winds to the back of the forearm round the 

 outer side of the radius, passes between the two planes of fibres of the Supinator 

 brevis, and is prolonged dowmvard between the superficial and deep layer of 

 muscles, to the middle of the forearm. Considerably diminished in size, it de- 

 scends on the interosseous membrane, beneath the Extensor longus pollicis, to the 

 back of the carpus, where it presents a gangliform enlargement from which fila- 

 ments are distributed to the ligaments and articulations of the carpus. It supplies 



1 According to Hutchinson, the digital nerve to the thumb reaches only as high as the root of 

 th nail ; the one to the forefinger as higli as the middle of the second phalanx , and the one to the 

 middle and ring fingers not higher than the first phalangeal joint (London Hosn. Gaz., vol. in., p. 319). 



