THE SHOULDER-GIRDLE. 



233 



As a matter of fact the head usually comes to rest beneath the coracoid process 

 and permanent fixation of the bone either in the subclavicular or subglenoid positions 

 is very rare. As the symptoms and methods of treatment are identical they will all 

 be included under the one head of subcoracoid luxations. What are commonly 

 regarded as subglenoid luxations are really subcoracoid. 



Method of Production of Anterior Luxations. Anterior luxations are produced 

 by the arm being hyperabducted, rotated outward, and the head of the bone pushed 

 or pulled in toward the body. Rotation may not be essential, but it is largely 

 responsible for the wide detachment of the capsule which is present in these injuries. 

 When the arm is raised from the body much beyond a right angle the greater 

 tuberosity strikes the acromion process. If the hyperabduction is continued the acro- 

 mion process acts as a fulcrum and the head of the bone is lifted from its socket, 

 tearing away the capsule of the joint in front of and below the glenoid cavity. 



FIG. 245. Dislocation of the shoulder; action of the bones; by extreme abduction of the humerus over the 

 acromion process as a fulcrum the head is levered out of the socket. 



If now the arm rotates, the capsule is still farther detached and if the force con- 

 tinues to act, as in those cases in which a person is thrown forward and alights on 

 the outstretched arm, or if the axillary muscles contract, the head is thrust from its 

 socket. After once leaving the socket, subsequent movements may cause the head 

 to assume various positions around the glenoid cavity; as a matter of fact it is almost 

 always below the coracoid process. 



Parts Injured. When the luxation occurs the arm is hyperabducted and, owing 

 to the acromion process being somewhat posterior to the glenoid cavity, pointing 

 backward, this places it up almost or quite alongside of the head. The force which 

 thrusts the bone out acts downward toward the axilla and inward toward the body. 

 The posterior border of the scapula is prevented from descending by the levator 

 scapulae and rhomboid muscles, hence it is the joint which descends and tears loose 

 the capsule already stretched tightly over the head of the humerus. 



This is the reason why the lower portion of. the capsule is torn ; it is the longi- 



