chap. XL] THE SHOULDER-JOINT. 193 



followed the course of the musculo-spiral nerve, and 

 opened on the outer side of the elbow. 



The various forms of anchylosis are common at the 

 shoulder-joint, and to afford a freer range of move- 

 ment to the limb in the more intractable of these 

 cases, Tillaux proposes to divide the clavicle. 



Dislocations. Dislocations at this joint are 

 more common than at any other joint in the body. 

 This is explained by the shallowness of the glenoid 

 fossa, the large size and globular shape of the head of 

 the humerus, the extensive movements of the arm, 

 the long leverage it affords, and the dependence of the 

 articulation for its strength mainly upon muscles. 

 The upper limb and shoulder are also peculiarly ex- 

 posed to injury. 



The principal forms of luxation of the humerus at 

 the shoulder are : 1. Subcoracoid, forwards and a 

 little downwards ; the usual form. 2. Subglenoid, 

 downwards and a little forwards ; rare. 3. Subspinous, 

 backwards ; rare. 



In all complete dislocations, the head of the bone 

 leaves the joint cavity through a rent in the capsule. 

 In so-called "false luxations" the capsule is not torn. 

 For example, in the cadaver, if the deltoid be divided 

 the humeral head can be displaced under the coracoid 

 process without rupture of the capsule, and the same 

 thing may occur during life, in cases where the muscle 

 has long been paralysed. 



In all cases of dislocation at this joint the primary 

 displacement is always downwards into the axilla. It 

 is well known that dislocations at the shoulder are 

 usually due to violence applied to the limb while 

 the arm is abducted, or to severe direct violence 

 forcing the bone downwards. Now when the limb is 

 abducted the head of the humerus projects below the 

 glenoid fossa, and rests upon the inferior and least 

 protected part of the capsule. The fibres of this 



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