(From exfolio, to cast the leaf). Desquamatio. Exfoliation. The process by which the dead part of the bone separates from the sound. One principal cause of an exfoliation of a bone is an interruption of the continuity of the vessels which nourish it. The coldness of the air, by contracting and drying up the extremities of the small vessels of the bone, also checks the circulation of the nutritious matter through them. Mr. John Hunter observes, "that one part of a bone is never separated from another by the rotting of the dead part, for that which comes away is as sound as it ever was. Exfoliation takes place soonest in bones wherein are the fewest cells, and whose texture is the closest. Before any part of a bone can be thrown off by exfoliation it must be dead. But even then, till the process of exfoliation begins, the bone adheres as strongly as ever, and would remain for years before it could be separated by putrefaction alone. Bones arc composed of two substances, viz. a true animal matter, and an earthy one, which are only intermixed with each other. A dead bone acts on the system in the same manner as any other extraneous body. It stimulates the adjacent living parts; in consequence of which, such a process is begun that must terminate in its being thrown off. The effects of this stimulus are, first, that the living adjacent bone becomes more vascular; a circumstance which always takes place when a part hath more to do than is just sufficient for the support of life. Secondly, that the earth of the living part, when it is in contact with the dead bone, is absorbed; hence the bone becomes softer, and adheres by its animal matter only. Thirdly, that the living animal part is at last absorbed along the surfaces of contact: this part of the process commences long before the last is finished. Both of them begin first at the surface, though in their course they do not every where take place in an equal degree at the same time. Fourthly, in proportion to the waste made by the last part of the process, a fungus arises from the living surface, and fills up the intermediate space, so that there may be no vacuum. These different stages, taken together, constitute ulceration. When any part of a bone is once loose, it will be pushed to the surface in the same manner as most other inanimate bodies would be; and this stage is partly mechanical, partly a continuation of ulceration. A proof of the third stage above mentioned may be derived from those cases where people die while exfoliation is going on. A small groove or worm-eaten canal can then be discovered, which becomes gradually deeper, and follows the irregularities of the living and dead surfaces. After the application of the trepan, a circular piece of bone is frequently thrown off, which is always less than the space from whence it came. This, however, would never be the case, were there not a loss of substance."

When a bone is laid bare by any accident, and an exfoliation is feared, if several perforations are made in the bone, the exfoliation will be prevented: in such cases the wound should be kept clean, and defended from unctuous and watery applications: pledgets of lint are very proper; or they may be dipped in the mixture of the oleum terebinthinae and tincture myrrhae. Neither caustics nor the actual cautery, in Mr. Hunter's opinion, hasten exfoliation: they produce death only in part of the bone, which is the first step towards exfoliation. "If caustics ever hasten exfoliation where the bone is already dead, it must be by producing inflammation in the adjacent living bone; this brings about a change in it, and makes it exert a power which it was incapable of before." See Caries.