DISLOCATIONS OF THE SHOULDEE. 115 



most freely movable joint in the body. The head of the humerus 

 is much larger than the glenoid fossa with which it articulates, 

 the capsule is loose and, below and internally, unsupported by 

 muscles. The joint depends chiefly for its stability upon (1) the 

 long tendon of the biceps, which, arising from the supra-glenoid 

 tubercle, passes through the joint above the head of the humerus, 

 a little internal to its middle line, and leaves through an opening 

 in the capsule beneath the transverse humeral ligament, to pass 

 vertically downwards in the bicipital groove : (2) certain muscles, 

 notably the subscapularis, supraspinatus, infraspinatus and 

 teres minor ; these muscles are often termed capsular muscles : 

 (3) the coraco-acromial arch, consisting of the coracoid process, 

 the acromion process, and the coraco-acromial ligament thrown 

 between the two : (4) atmospheric pressure. Unless distended 

 with fluid, there is no true cavity in the joint, and its interior 

 practically constitutes a vacuum, so that if the capsule is perforated 

 and air permitted to enter, the laxity of the capsule will allow 

 the articular surfaces to become very distinctly separated. 



It will thus be seen that, although there are these four powerful 

 factors maintaining the stability of the joint, they are to a great 

 extent counteracted by the want of co-aptation between the head 

 of the humerus and the glenoid fossa, together with the laxity 

 of the capsule. 



The shoulder- joint is the one which is most commonly the seat 

 of dislocation, generally the outcome of falls upon the outstretched 

 hand, upon the elbow, or upon the point of the shoulder. In any 

 of these accidents the head of the humerus tends to be pushed 

 forcibly against the anterior part of the capsule, and if the 

 muscles surrounding the joint are taken off their guard, a rent 

 in the capsule will occur, through which the head of the humerus 

 will be protruded. 



The most frequent dislocation is that in which the head is 

 displaced forwards and inwards, so that it comes to lie beneath 

 the coracoid process, and the displacement is therefore termed 

 sub-coracoid. In the majority of such dislocations the head 

 usually reaches a point which is somewhat internal to the line of 



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