28 Anatomy Applied to Medicine and Suigery. 



unsupported portion that the head of the bone, in exten- 

 sion, i.e., elevation of the elbow from the side, rests, hence, 

 sudden over-extension of the arm may cause the head of 

 the humerus to distend and tear through this lower part 

 of the capsule, and dislocation result. There are three 

 varieties of dislocation, the subcoracoid, the subglenoid 

 and the subspinous (a fourth is sometimes added, the sub- 

 clavicular) , and this arrangement corresponds to the rela- 

 tive frequency of the three forms of displacement. The 

 head of the bone having left the joint will be found, in the 

 subcoracoid form, resting internally on the neck of (the 

 scapula, close to the glenoid rim, and, therefore, beneath 

 the coracoid process ; in the subglenoid form, the head 

 will be below and slightly in front of the glenoid fossa, 

 res-ting on the anterior surface of the triceps ; while, in /the 

 subspinous, it will lie on the posterior surface below the 

 spine of the scapula. The production of the different 

 forms may be understood by considering what happens 

 when a person falls so that the palm comes in contact with 

 the ground, the arm ait the same time being extended and 

 adducted or abducted. In the first case, the arm, acting 

 as a long lever, dislodges the head of the bone from the 

 glenoid fossa, and, at the same time, the momentum of the 

 body, continuing, drives it (the body) onward, so that the 

 head is displaced backwards subspinous form. This 

 displacement of the head backwards may be assisted by 

 the action of the latissimus dorsi. Should, however, the 

 patient have saved himself sufficiently, so that, after (dislo- 

 cation, there would be no onward -propulsion of the body, 

 and should the action of the latissimus dorsi neutralize that 

 of the pectoralis major, then the head remains where it was 

 first displaced, viz., in the subglenoid position. When, 

 however, the patient falls, with arm extended and a'bdudt- 

 ed, then the bone, on rupturing the capsule, is driven for- 



