Dislocations of Hnmcrns 257 



deltoid, and depressed by the subscapularis, infra-spinatus, and teres 

 minor, and by the pectoralis major, teres major, and latissimus dorsi. 

 It is drawn forwards by the pectoralis major, coraco-brachialis and 

 biceps, and by the anterior part of the deltoid; backwards by the 

 posterior part of the deltoid, the teres major, latissimus dorsi, and 

 triceps. 



The external rotators are the infra-spinatus and teres minor, and 

 the internal rotators are the pectoralis major, teres major, latissimus 

 dorsi, and subscapularis. 



Elevation of the arm above the head is accomplished by the action 

 chiefly of the trapezius and serratus magnus, and other muscles which 

 rotate or fix the scapula ; without rotation of the scapula the arm cannot 

 be raised above the head. 



In synovitis of the shoulder-joint there is impairment of move- 

 ment, and when the patient is stripped and the arms are raised the 

 scapula of the affected side moves with the humerus, its inferior angle 

 travelling forwards as the arm is abducted. If effusion occur there is 

 a deep-seated and elastic fulness beneath the deltoid. 



If abscess form in the joint, the pus may escape by the offshoot 

 of the synovial membrane which descends along the bicipital groove, 

 and so the abscess becomes diffused in the subdeltoid tissue, whence 

 it will work its way to the surface beneath the anterior or posterior 

 border of the muscle, for it is not likely that the pus would approach 

 the skin through the substance of the deltoid. Sometimes the articular 

 suppuration escapes by the gaps which exist beneath the insertions of 

 the subscapularis and infra-spinatus. The sinuses leading to the dis- 

 eased joint may open through the infra-clavicular fossa at any spot 

 along the anterior border of the deltoid, or even into the axilla, or 

 along the posterior border of the muscle. 



Dislocations of the humerus. The great freedom of movement 

 which the humerus enjoys renders it specially liable to dislocation, 

 and when once the bone has slipped out of the shoulder-joint the 

 luxation is apt to recur. 



The commonest cause of dislocation is a fall upon the elbow or 

 hand. When a man is falling he instinctively puts out his arm to 

 ' break the fall.' This is done by the energetic contraction of the 

 supra-spinatus and deltoid, the axillary muscles at the same instant 

 contracting, so as to fix the arm. Then, partly by the shock trans- 

 mitted to the socket, and partly by the downward muscular pull, 

 the head of the bone tears through the lower part of the capsule. 

 Sometimes the head of the humerus rests in the axilla as a sub- 

 glenoid dislocation. But more often it is dragged by the muscles, or 

 thrust by the shock, into the sub-coracoid or even into the sub- 

 clavicular region. 



If the elbow or hand happen to be advanced as well as raised 

 when the humerus tears through the capsule, the head of the bone 



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