3. Every thing seems to favour the escape of this bone from its natural cavity. 1st, On the part of the articulating surfaces, a cavity somewhat oval and very shallow, aided by a slight cartilaginous ring, receives a half-spherical head, twice its own diameter from above downwards, and three times as large from before backwards. 2dly, On the part of the ligaments, this articulation is strengthened by only a simple capsule. This capsule is thin and weak on its lower side, a direction in which there is nothing to prevent a luxation, while it is thicker on its upper side, where the acromion and coracoid apophyses, and a strong ligament, present an obstacle almost insurmountable. 3dly, As far as respects the muscles and the motions of the joint, strong and numerous bundles of fibres surrounding the articulating surfaces, communicate to them motions easily performed in every direction, and which, by pushing the head of the humerus against the different parts of the capsule, distend it, predispose it to laceration, and indeed even rupture it, when the quantum of their force is superior to its resistance. 4thly, As far as relates to external bodies, what bone is more exposed to their action than this, particularly among that class of persons, engaged, for a livelihood, in hard labour?
4. Subject to the influence of these different predisposing causes, the humerus would be constantly liable to luxations, did not the scapula, moveable like itself, furnish it, by accompanying its motions, with a point of support, differently disposed, according to the different position of its superior extremity; so that, to this two-fold mobility of the articulating surfaces, is to be attributed, in a great measure, the stability of their connexion.
5. The upper articulation of the humerus, though predisposed, in general, to luxations, is not equally so in every direction. There is a point at which luxation cannot take place. There are others, where, though possible, this accident has never been observed. It is necessary, therefore, before examining the mechanism of this luxation, to mention with precision, the directions in which it may occur. On this point, writers have differed in a very singular manner. Sometimes, to express the same thing, they have used a different language; and, at other times, have, by the same words, expressed things widely different. Always agreeing as to certain modes of dislocation, they have been divided as to others; while, in the midst of those contrarieties, the surgeon being embarrassed, is at a loss on what ground to found his practice.