its whole extent, and, in fact, the term necrosis was originally adopted on this supposition. [Illustration] [Illustration] The progress of necrosis is, as has been said, similar to that of sphacelation. The affected bone gradually changes its colour, and loses its sensibility; a line of demarcation is formed, and ultimately the dead portion is completely detached from the living. Previous to its separation, the surrounding parts, the portions of bone which are not doomed to perish, have commenced forming new osseous matter, which is secreted in nodules, and from continued deposition soon becomes consolidated. The commencement of the process is well seen in the following sketches from specimens in my collection. The disease, as represented in the two first cuts, was of the most acute kind, and a great part of the shaft of the tibia had perished. This is seen at various points through the sort of cortical deposit of new bone. The new bone, in its turn, secretes a texture similar to itself, whereby the deposit becomes more and more extended, and not unfrequently affords an almost complete encasement to the dead portion, or sequestrum, as represented in the cut on the right-hand side of the page. In general bone dies irregularly, so that the sequestrum presents an uneven surface, and its margins are rough and serrated by numerous sharp projections, as seen in the one taken from the tibia, and represented here. From the appearance of the dead bone, it was imagined that after its separation, portions of it were removed by absorption; and this opinion was strengthened by the thin exfoliations of the external lamina being found perforated at several points by minute apertures,—worm-eaten, as it was called. These cases of death of inner or medullary shell are irregularly separated, like any other slough; the remaining living outer shell is enlarged by inflammatory action and deposit. But a dead portion of bone, detached from the surrounding parts, is in every respect an extraneous body, and is not, and cannot be, acted on by the absorbents, any more than a piece of metal, wood, or stone. Some have gone so far as to affirm that portions of foreign bodies, ligatures, &c., are absorbed; but this opinion is altogether too absurd to require any contradiction; the knots of ligatures, like portions of glass, or other foreign substance, become surrounded with a dense cyst, and often remain in the body for a long time; so do portions of dead bone separated by the process here described. A series of experiments were made by Mr. Gulliver, in order to put this question at rest, many of which I witnessed and assisted at, and several I also repeated. Setons of bone were inserted and worn for a long time; thin plates of bone were confined on suppurating surfaces; pieces of bone were inserted in the medullary canal of various animals, and kept there for months, and in one instance for more than a year. These foreign