1. Nature, who, according to the wants of different species of animals, has varied the number of their articulations, knows also how to vary their structure, according to the uses of the different parts of their bodies. With great mobility, she has sometimes connected great solidity and strength, as is the case in the vertebral column; in other instances, parts very solid and compact, are capable of performing but feeble motions, as the carpus, the tarsus, &c. And, lastly, other parts, again, capable of great motion, possess so little solidity and firmness as to be easily deranged by the action of external bodies. Such, in man, is the articulation of the humerus with the scapula, of the sternum with the clavicle, &c. 2. Hence there exist three classes of articulations, very different from each other. To the last, as enumerated above, belongs, in a particular manner, the history of luxations, and, in this, as the solidity varies, the frequency of dislocations is equally various; no luxation occurs more frequently than that of the humerus; indeed, in a comparative catalogue of accidents of this kind, it alone has, during certain years, occurred oftener, in the Hotel-Dieu, than that of all the other bones, taken collectively. 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?