The connection between bones is sometimes unnaturally close and intimate, and sometimes unnaturally loose: when the latter condition is very decided, it is usually combined with some deviation from their relative position, that is, with dislocation.

The former state, or that in which bones are bound too closely together, is found, both when their articulation with each other is movable, and when it is immovable. It is known as synostosis or anchylosis, though the latter term is chiefly employed to designate the fixed state of a joint. Synostosis is sometimes congenital, but much more frequently it is acquired.

Congenital synostosis may be the result of an unnatural fusion of points of ossification belonging to separate bones; it is then almost always manifestly prejudicial to the full development of one or both of the united bones, and it accompanies other and more important malformations, such as acephalus, cyclopia, etc.: or it may consist of premature union of bones, which do not naturally unite till various periods after birth: thus the cranial bones are sometimes found united even in the foetus.

Allied to this, is the case in which certain bones coalesce at some period subsequent to birth, but earlier than that at which their union normally takes place. Thus the cranial bones sometimes unite prematurely with each other, and so do the two halves of the lower jaw, epiphyses with their diaphyses, etc.

Synostosis, when acquired, is either incomplete, that is to say, adjoining bones become bound together by bridges of new bone (osteophytes), which pass over the intervening synchondroses and articular cavities, and enclose them in a more or less perfect bony capsule: or it is complete; the synchondrosal cartilages, or the soft tissues of a joint, having been removed by atrophy, suppuration, etc, the bones are brought into immediate contact with one another, and become conjoined. Vertebrae, the pubic bones, or the bones composing a joint, unite thus with one another; so too does the sacrum with the ossa innominata.

Other bones, also, when brought into permanent and close mutual contact, may become fixed together in the same manner; the ribs, for instance, in cases of lateral curvature.

Synostoses are to be met with under any of the above-mentioned circumstances, sometimes between single bones only, sometimes at several parts of a skeleton; and sometimes they are almost universal. Phoebus has recently seen and described an example of congenital synostosis, in which there had been a fusion of original points of ossification. There is a very similar case in the museum at Vienna, of congenital fusion of the second and third cervical vertebrae; only it obtains further importance from the fact, that the atlas is also congenitally united with the occiput. In another specimen the bones of the right forearm are continuous with the humerus, without the intervention of a joint. A similar synostosis, and one presenting considerable interest, is that of Nägele's obliquely narrowed pelvis, in which the sacrum is united with one of the innominata.

Anchylosis, in the restricted sense of extinction of a joint, especially that which is acquired, will be considered in the chapter on Joints.

Loosening of the natural connection of bones which are immovably articulated to each other, is denominated Osteodiastasis: when the same thing occurs between bones which move upon upon one another, the result is dislocation. In diastasis the change produced in the connecting medium depends upon circumstances: it is either stretched, attenuated and loosened in texture, or torn through. Gradually increasing extension leada to the first change, as is instanced by the cranial bones in cases of hydrocephalus, by the bones of the face, when stretched over fibrous polypi, or by the pelvic bones in parturition; while the second is caused by quickly acting force, and is exemplified by disjunction of the sutures when the skull is shattered, by the laceration of the synchondroses of the pelvis in very difficult parturition, or by the separation of the epiphyses in consequence of injury.

Moreover, the occurrence of diastasis is not only favored by previous disease, especially by inflammatory softening and loosening of the connecting substance, but it may be the immediate result of destructive suppuration of that substance, as is shown by the consequences which the pelvis sustains from the worse forms of puerperal disease.

The subject of dislocations will also be considered in the chapter on Joints.