844 THE NERVOUS SYSTEM. 



The cutaneous brandies 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 

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

 side of the thumb and the radial side of the index finger; the second, the adjoin- 

 ing sides of the index and middle fingers ; and the third, 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 through 

 the fibres of the Supinator brevis, and passes down, between the superficial 

 and deep layer of muscles, to the middle of the forearm. Considerably dimin- 

 ished in size, it descends on the interosseous membrane, beneath the Extensor 

 longus pollicis, to the back of the carpus, where it presents a gangliform 

 enlargement from which filaments are distributed to the ligaments and artic- 

 ulations of the carpus. It supplies all the muscles of the radial and posterior 

 brachial regions, excepting the Anconeus, Supinator longus, and Extensor carpi 

 radialis longior. 



Surgical Anatomy. The brachial plexus may be ruptured by traction on the limb leading 

 to complete paralysis. In these cases the lesion would appear to be rather a tearing away of 

 the nerves from the spinal cord than a solution of continuity of the nerve-fibres themselves. In 

 the axilla any of the nerves forming the brachial plexus may be injured in a wound of this part, 

 the median being the one which is most frequently damaged from its exposed position, and the 

 musculo-spiral, on account of its sheltered and deep position, being the least often wounded. 

 The brachial plexus in the axilla is often damaged from the pressure of a crutch, producing the 

 condition known as "crutch paralysis." In these cases the musculo-spiral appears to be the 

 nerve which is most frequently implicated to the greatest extent, the ulnar nerve being the one 

 that appears to suffer next in frequency. 



The circumflex nerve is of particular surgical interest. On account of its course round the 

 joint it is liable to be torn in fractures of the surgical neck of the humerus and in dislocations 

 of the shoulder-joint, leading to paralysis of the deltoid, and, according to Erb, inflammation of 

 the shoulder-joint is liable to be followed by a neuritis of this nerve from extension of the 

 inflammation to it. 



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

 nail ; the one to the fort-linger as high as the middle of the second phalanx; and the one to the mid 

 die and ring lingers not higher than the first phalangeal joint (London Hasp. Gas. vol. iii. p. 319.) 



