Proper function of the shoulder depends on the intactness of the humeroscapular joint, both clavicular joints, the subacromial bursa and other periarticular structures, and the muscular mechanism. The humeroscapular joint allows a great amount of motion, which, however, is checked to some extent by the acromion, in order to give sufficient stability and strength. Again, a great deal of motion which is checked by the acromion is compensated by the mobility of the scapula, which swings around the clavicular joint and is regulated by the complicated muscular apparatus.
The great frequency of lame and stiff shoulders is easily explained by their complicated apparatus, and by their free exposure to strain and injury. For the latter reason, it is evident that the periarticular structures, especially the subacromial bursa, are much more frequently affected than the humeroscapular joint itself, a fact which the investigations of Küster and Codman have clearly demonstrated. It