218 SURGICAL APPLIED ANATOMY. [Chap, xi. 



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 

 portion of the capsule being tightly stretched in this 

 position, it requires no extraordinary force to tear the 

 ligament and drive the bone into the axilla. 



Thus it happens that in luxations at this joint the 

 rent in the capsule is at its inferior and inner aspect. 

 The head of the bone being thus driven downwards 

 into the axilla, may, for certain reasons, remain there 

 (subglenoid form), or more usually it will be drawn 

 forwards and inwards by the powerful pectoralis 

 major, aided by other muscles whose action is now 

 less resisted (subcoracoid form) ; and lastly, the direc- 

 tion of the violence being applied markedly from in 

 front, the head of the bone may be thrust backwards 

 under the acromioii or spinous processes (subspinous 

 form). The overwhelming frequency of the subcora- 

 coid variety is explained by the greater advantage at 

 which those muscles act that draw the bone forwards, 

 in comparison with those that would draw it back- 

 wards, and by the very trifling opposition offered to the 

 passage of the head forwards when compared with the 

 substantial obstacles in the way of its passage back- 

 wards under the scapular spine. 



Features common to all dislocations at 

 the shoulder. As the roundness of the deltoid 

 depends to a great extent upon the presence beneath 

 it of the humeral head, and as in all these luxations 

 (save perhaps in the slighter grades of the subspinous 

 form) the head is removed practically from its 

 connection with the deltoid, that muscle is always 

 more or less flattened. This flattening is augmented 

 by the stretching of the muscle, whicli in some degree 

 is constantly present. Stretching of the deltoid in- 

 volves abduction of the arm, and this symptom is 

 fairly constant in all the luxations. The biceps being 



