#The Axillary (Circumflex) Nerve.#—­In the majority of cases in which paralysis of the deltoid follows upon an injury of the shoulder, it is due to a lesion of the fifth cervical nerve, as has already been described in injuries of the brachial plexus.  The axillary nerve itself as it passes round the neck of the humerus is most liable to be injured from the pressure of a crutch, or of the head of the humerus in sub-glenoid dislocation, or in fracture of the neck of the scapula or of the humerus.  In miners, who work for long periods lying on the side, the muscle may be paralysed by direct pressure on the terminal filaments of the nerve, and the nerve may also be involved as a result of disease in the sub-deltoid bursa.

The deltoid is wasted, and the acromion unduly prominent.  In recent cases paralysis of the muscle is easily detected.  In cases of long standing it is not so simple, because other muscles, the spinati, the clavicular fibres of the pectoral and the serratus, take its place and elevate the arm; there is always loss of sensation on the lateral aspect of the shoulder.  There is rarely any call for operative treatment, as the paralysis is usually compensated for by other muscles.

When the supra-scapular nerve is contused or stretched in injuries of the shoulder, the spinati muscles are paralysed and wasted, the spine of the scapula is unduly prominent, and there is impairment in the power of abducting the arm and rotating it laterally.

The musculo-cutaneous nerve is very rarely injured; when cut across, there is paralysis of the coraco-brachialis, biceps, and part of the brachialis, but no movements are abolished, the forearm being flexed, in the pronated position, by the brachio-radialis and long radial extensor of the wrist; in the supinated position, by that portion of the brachialis supplied by the radial nerve.  Supination is feebly performed by the supinator muscle.  Protopathic and epicritic sensibility are lost over the radial side of the forearm.

#Radial (Musculo-Spiral) Nerve.#—­From its anatomical relationships this trunk is more exposed to injury than any other nerve in the body.  It is frequently compressed against the humerus in sleeping with the arm resting on the back of a chair, especially in the deep sleep of alcoholic intoxication (drunkard’s palsy).  It may be pressed upon by a crutch in the axilla, by the dislocated head of the humerus, or by violent compression of the arm, as when an elastic tourniquet is applied too tightly.  The most serious and permanent injuries of this nerve are associated with fractures of the humerus, especially those from direct violence attended with comminution of the bone.  The nerve may be crushed or torn by one of the fragments at the time of the injury, or at a later period may be compressed by callus.