THE RADIAL NERVE 985 



The inferior division divides into cutaneous and muscular branches. The 

 cutaneous branch (the lateral brachial cutaneous nerve) turns around the pos- 

 terior border of the deltoid, pierces the deep fascia, and supplies the skin covering 

 the lower third of the deltoid and a small area of integument below the insertion of 

 the muscle (fig. 756). One muscular branch is distributed to the teres minor; it 

 swells out into an ovoid or fusiform, reddish, gangliform enlargement before 

 entering the muscle. Other branches supply the lower and posterior part of the 

 deltoid. 



The radial (musculo-spiral) nerve is the largest branch of the brachial plexus. 

 It contains fibres from the sixth, seventh, and eighth cervical and sometimes 

 from the fifth cervical and first thoracic nerves (figs. 751, 754). It commences 

 at the lower border of the pectoralis minor, as the direct continuation of the 

 posterior cord of the brachial plexus, and passes downward and lateralward in the 

 axillary fossa behind the third part of the axillary artery (fig. 755) and in front of 

 the subscapulars, latissimus dorsi, and teres major muscles. From the lower 

 border of the axillary fossa it descends into the arm, where it lies, at first, on the 

 medial side of the upper third of the humerus, behind the brachial artery and in 

 front of the long head of the triceps; then it runs obliquely downward and lateral- 

 ward behind the middle third of the humerus, in the groove for the radial nerve 

 (musculo-spiral groove), and between the lateral and medial heads of the triceps. 

 It is accompanied, in this part of its course, by the profunda artery. At the junc- 

 tion of the middle and lower thirds of the humerus it reaches the lateral side of the 

 arm, pierces the external intermuscular septum, and runs downward and forward 

 between the brachio-radialis and extensor carpi radialis longus externally, and 

 the brachialis internall}^ (fig. 758), and it terminates, a short distance above the 

 capitulum, by dividing into deep and superficial terminal branches. In the last 

 part of its course it is accompanied by the anterior terminal branch of the pro- 

 funda artery. 



Branches.— The branches of the radial or musculo-spiral nerve are cutaneous, 

 muscular, articular, and terminal, but for practical purposes it is best to consider 

 them in association with the situations of their origins. While it is in the axillary 

 fossa the radial (musculo-spiral) nerve gives branches to the medial and long 

 heads of the triceps (fig. 758), and a medial cutaneous branch. The branch to the 

 long head of the triceps at once enters the substance of the muscle, that to the 

 medial head breaks into branches which terminate in the muscle at different levels, 

 and one of them, the ulnar collateral nerve, accompanies the ulnar nerve to the 

 lower part of the arm. The posterior brachial (internal) cutaneous branch 

 crosses the tendon of the latissimus dorsi, passes dorsal to the intercosto-brachial 

 (intercosto-humeral) nerve, pierces the deep fascia, and is distributed to the skin 

 of the middle of the back of the arm below the deltoid. 



While it lies behind the middle third of the humerus, the radial nerve gives 

 branches to the lateral and medial heads of the triceps and to the anconeus. The 

 latter branch descends in the substance of the median head of the triceps, close to 

 the bone, and it is accompanied by a small branch of the profunda artery. The 

 dorsal antibrachial (external) cutaneous branch, passing down between the 

 lateral and median heads of the triceps, divides near the elbow into its upper and 

 lower branches (fig. 756), each of which perforates either the lateral head of the 

 triceps muscle near its attachment to the humerus or the external intermuscular 

 septum. 



The upper branch, much the smaller, pierces the deep fascia in the line of the external inter- 

 muscular septum; it accompanies the lower part of the cephalic vein, and supplies the skin over 

 the lower half of the lateral and anterior aspect of the arm. The lower branch is of considerable 

 size. It pierces the deep fascia a little below the upper branch, runs behind the external con- 

 dyle, and supplies the skin of the middle of the back of the forearm as far as the wTist, an- 

 astomosing with the medial antibrachial (internal) cutaneous and musculo-cutaneous nerves 

 (fig. 759). 



After the radial nerve has pierced the external intermuscular septum it gives 

 branches to the brachio-radialis, extensor carpi radialis longus, and to the lateral 

 portion of the brachialis (fig. 759). From one of these branches an articular 

 filament is distributed to the elbow-joint. 



The terminal branches of the radial nerve are: — a motor branch, the deep 

 radial, to the supinator and extensor muscles of the forearm, and a sensory 



