304 THE SPINAL NERVES. 



(&) The internal cutaneous branch of the musculo-spiral nerve (n. cutaneus 

 brachii posterior) (8 c), often united in origin with the preceding, winds backwards 

 beneath the intercosto-humeral nerve, and extends, supplying filaments to the skin 

 over the long head of the triceps, nearly as far as the olecranon. This nerve is 

 accompanied by a small cutaneous artery. 



B. Posterior branches. 



These consist of a fasciculus of muscular branches (7, 8 c) which supply the outer 

 and inner heads of the triceps muscle and the anconeus. The branch of the anconeus 

 is slender, and remarkable for its length ; it descends in the substance of the inner 

 head of the triceps to reach its destination. 



C. External branches. 



(a) The external cutaneous branches, two in number, arise before the nerve 

 pierces the external intermuscular septum, and become superficial just below the 

 middle of the arm, between the outer head of the triceps and the brachialis anticus 

 muscles. 



The upper branch (6 c), the smaller of the two, is directed downwards to the fore 

 part of the elbow, along the cephalic vein, and distributes filaments to the lower 

 half of the arm on its outer and anterior aspect. The lower branch (n. cutaneus 

 antibrachii dorsalis) (6 c, or 6, 7 c, or 7 c, or 7, 8 c) descends in the interval between 

 the external condyle and the olecranon, and reaches as far as the wrist, distributing 

 its offsets to the lower half of the arm, and to the forearm, on their posterior 

 aspect. It is connected near the wrist with a branch of the external cutaneous 

 nerve, and often with the dorsal branch of the ulnar nerve. In some cases this 

 branch is prolonged to the back of the hand (fig. 201, A). 



(b) Muscular branches are furnished to the supinator longus (6 c), extensor carpi 

 radialis longior (6, 7 c) (the extensor carpi radialis brevior usually receiving its nerve 

 from the posterior interosseous), and frequently a small twig to the outer part of the 

 brachialis anticus (6 c), as the nerve lies between those muscles. Together with these 

 branches articular filaments are given to the outer part of the elbow-joint. 



According to Rauber a branch of the musculo-spiral nerve is given regularly to the periosteum 

 on the back of the humerus ; and in some cases it also furnishes the nerve to the shaft of that 

 bone, when the principal medullary artery is derived from the superior profunda. Filaments 

 are described as passing to the elbow-joint also from the ulnar collateral nerve, and from the 

 branch to the anconeus. 



Varieties. The musculo-spiral nerve has been seen passing backwards through the 

 quadrilateral space between the teres muscles with the circumflex nerve (Guy's Hosp. Rep., 

 xliv). A communication with the ulnar nerve in the arm is recorded by Villar (p. 300). The 

 lower external cutaneous branch was traced by Brooks in one case to the first phalanx of the 

 ring and the second phalanx of the little fingers ; and it has been observed furnishing the 

 dorsal digital nerves of the little and the ulnar half of the ring fingers in the place of the 

 ulnar nerve (Gruber, Gegenbaur). 



Radial nerve. The radial nerve (ramus superficialis n. radialis) receives its 

 fibres from the sixth cervical nerve, sometimes also the fifth or seventh. Continuing 

 straight down from the musculo-spiral nerve, it lies a little to the outer side of the 

 radial artery along the anterior border of the extensor radialis brevior, under cover 

 of the supinator longus, and resting on the supinator brevis, the insertion of the 

 pronator teres, and the radius. About three inches above the wrist the nerve turns 

 backwards beneath the tendon of the supinator longus and becomes subcutaneous. 

 It then divides into two branches, which ramify on the back of the hand in the 

 following manner : 



(a) The external portion passes to the radial side of the thumb, and is joined 

 by an offset of the external cutaneous nerve. It distributes filaments over the thenar 

 eminence. 



