We have here to do with cases in which the bone is pushed out of its place by some external force acting on it.

The bones are kept in their places mainly by the ligaments of the joints, but no inconsiderable aid in this regard is given by the muscles which act on the bones. As a general rule when external force is exercised on a bone at a joint, the muscles are so braced as to enable the bone to retain its place in spite of the external force. But if a force is exercised on a bone unexpectedly or when the muscles are generally relaxed as by alcoholic stupor, then it may be displaced, although the force under ordinary circumstances would be insufficient to produce this effect.

When the bone is dislocated, the same contraction of the muscles which normally aids in preventing dislocation, generally offers a serious obstacle to the return of the bone to its normal place.

A bone may even be displaced by the action of the muscles themselves, where a certain group acts very vigorously while their antagonists are relaxed. There are indeed persons who can produce Dislocation voluntarily of almost all the more movable joints, and that by mere muscular effort. In order to this we must suppose a certain laxness of the ligaments, but there is also a power acquired by education of strongly contracting certain muscles, while others which usually contract along with them are relaxed. We know that for the most part the muscles in their contractions are co-ordinated, and most people are unable to contract individual muscles apart from their co-ordinates, but there are exceptional persons who possess this power, some in a limited degree and others very remarkably. Many persons, for instance, cannot shut one eye without shutting the other, and most persons when they shut one require to make an active effort at opening the other, in order to prevent it shutting too. But there are persons who can close the eyelids of one eye as easily as they can close the fingers of one hand.

In traumatic dislocation there is usually tearing of the ligaments to some extent, and in the case of some joints much laceration is necessary before dislocation can occur.

If restoration does not occur soon, the bone acquires adhesions in its abnormal situation, the adhesions being the result of chronic inflammation set up by the irritation of the bone. The displaced bone generally comes to press with its head against a neighbouring part of the bone with which it formerly articulated, and the adhesions attach it to the periosteum in its new position. Friction by degrees wears down to some extent the opposed piece of bone, and as new bone is produced around by the irritation there may be a kind of hollow joint formed. By the wearing of the bone the cancellated tissue would be exposed, were it not that dense bone is produced on the surface so as to cover in the spongy tissue. A smooth hollow surface may thus be produced, and a tolerably perfect joint, although cartilage is not formed to cover it, but only a layer of smooth polished bone. The head of the displaced bone also loses its cartilage, and may even get worn away considerably. If the bone remains permanently displaced the old hollow of the joint becomes filled up, bone growing when the friction of the opposing bone is no longer exercised.