MUSCULO-SPIRAL. 575 



the humerus in the spiral groove with the superior profunda artery and vein, 

 passing from the inner to the outer side of the bone, beneath the Triceps muscle. 

 At the outer side of the arm, it descends between the Brachialis anticus and 

 Supinator longus to the front of the external condyle, where it divides into the 

 radial and posterior interosseous nerves. 



The branches of the musculo-spiral nerve are : 



Muscular. Eadial. 



Cutaneous. Posterior interosseous. 



The muscular branches supply the Triceps, Anconeus, Supinator longus, 

 Extensor carpi radialis longior, and Brachialis 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. 



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 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 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 

 muscular branch. It is of small size, and passes across the axilla to the inner 

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

 the olecranon. 



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

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

 of the cephalic vein to the front of the elbow, supplying the integument of the 

 lower half of the upper 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 along the back part of the radial side of the 

 forearm to the wrist, supplying the integument in its course, and joining, near its 

 termination, with a branch of the external 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 posterior branch of the 

 external cutaneous nerve. 



The internal branch communicates, above the wrist, with a branch from the 

 external cutaneous, and, on the back of the hand, forms an arch with the dorsal 

 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. The latter nerve communicates with a filament from the dorsal branch 

 of the ulnar nerve. 



The posterior interosseous nerve pierces the Supinator brevis, winds to the back 

 of the forearm, in the substance of this muscle, and, emerging from its lower border, 



