922 THE NERVOUS SYSTEM 



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

 (fig. 680). One muscular branch is distributed to the teres minor; it swells out into 

 an ovoid or fusiform, reddish, ganglifonn enlargement before entering the muscle. 

 ( )ther branches supply the lower and posterior part of the deltoid. 



The Radial (Musculo-spiral) Nerve is the largest branch of the braclml plexus. 

 It contains fibres from the sixth, seventh, and eight h cervical and sometimes from the 

 fifth cervical and first thoracic nerves (figs. 675 and 678). It commences at the lower 

 border of the pectoralis minor, as the direct continuation of the posterior cord of the 

 plexus, and passes downwards and outwards in the axillary fossa behind the third 

 part of the axillary artery (fig. 679) and in front of the subscapularis, 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 inner side of the upper third of the humerus, be- 

 hind the brachial artery and in front of the long head of the triceps; then it runs ob- 

 liquely downwards and outwards behind the middle third of the humerus, in the 

 groove for the radial nerve (musculo-spiral groove) , and between the outer and inner 

 heads of the triceps. It is accompanied, in this part of its course, by the profunda 

 artery. At the junction of the middle and lower thirds of the humerus it reaches the 

 outer side of the arm, pierces the external intermuscular septum, and runs downwards 

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

 and the brachialis internally (fig. 682), 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 profunda 

 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 inner and long heads of the 

 triceps (fig.. 682), and an internal cutaneous branch. The branch to the long head of 

 the triceps at once enters the substance of the muscle, that to the inner 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 behind 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 (musculo-spiral) 

 nerve gives branches to the outer and inner heads of the triceps and to the anconeus. 

 The latter branch descends in the substance of the inner 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 outer 

 and inner heads of the triceps, divides near the elbow into its upper and lower branches 

 (fig. 680), each of which perforates either the outer 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 outer 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 condyle, and supplies the skin of the middle of the 

 back of the forearm as far as the wrist, anastomosing with the medial antibrachial 

 (internal) cutaneous and musculo-cutaneous nerves (fig. 683). 



After the radial (musculo-spiral) nerve has pierced the external intermuscular 

 septum it gives branches to the brachio-radialis, extensor carpi radialis longus, 

 and to the outer portion of the brachialis (fig. 683). From one of these branches an 

 articular filament is distributed to the elbow-joint. 



The terminal branches are: a motor branch, the deep radial (posterior inter- 

 osseous), and a sensory branch, the superficial radial (radial). 



The deep radial (posterior interosseous) nerve runs downwards in the interval 

 between the brachialis and extensor carpi radialis longus. It passes in front of the 

 outer part of the elbow-joint, and after giving off branches to supply the extensor carpi 

 radialis brevis and supinator, it is crossed in front by the radial recurrent artery 

 (fig. 683). It then runs downwards and backwards through the substance of the 

 supinator, and enters the interval between the superficial and deep layers of 



