(Chapter 21) Footnote 72:

See the beginning of De articulis and Vectiarius 5.1: “The shoulder is dislocated downwards. I have no knowledge of any other direction.” (Loeb tr. by E. T. Withington). In an extreme forceful abduction of the arm, the humeral head may slip out of the glenoid, being displaced to the subglenoid region and eventually to a subcoracoid position.