Such is an outline of the chief opinions en tertained on this interesting subject, and it is probable that, to a certain extent, they are all correct. When the periosteum has not been removed or spoiled, there can be no doubt that it is deeply and even principally engaged in the process of reproduction. In the museum at Park-street, the specimens exhibiting the earliest period of the disease show the periosteum as slightly thickened, smooth on its internal, but more rough and flocculent on its external sur face, detached from the bone, the surface of which is smooth, and scarcely appears changed from its natural and healthy condition. At a more advanced period, the periosteum is still thicker, but is not softened ; on the contrary, it has nearly the firmness of ligament, and there are small osseous depositions within it ; the bone then being rough and uneven on its surface and evidently having lost its vitality.
But although we concede to the periosteum the principal office in the process of reproduction, we can also conceive that the adjacent tissues are also more or less engaged, for the thicken ing of parts is found to extend on the outside of this membrane, and Dr. Macartney himself speaks of the cellular tissue external to the periosteum beconiing altered and condensed. Now, supposing the periosteum to be destroyed, these structures may be capable of supplying its place and producing the secretion of gelati nous substance, which is afterwards to become bone, just as we see that if the periosteum is torn off a bone, the adjacent tissues laid down upon it may prevent exfoliation, and answer every purpose of nutrition and preservation that the original membrane did. Frorn what ever source derived, this deposition ber,ins while yet the original hone is in a state of in flamination, and the part that is to die still un detached. If tendons or muscles are inserted into this part of the bone, they, being living and organized substances, separate from that which is dead : but the previous deposition has ex tended about them, and fastened them in their situations, and hence not only is the limb capa ble of support during the progress of necrosis, but unless in exceedingly rapid, acute, and un favourable cases, its motions may not be very materially impaired.
Soon after the investing shell has been form ed, the dead portion of the bone separates from its attachments, and lies within its osseous case. It is now termed the sequestrum, and presents some remarkable and peculiar charac ters that distinguish it from diseased bone otherwise circumstanced. Its extremities are always jagged, pointed, and uneven : its mar row and internal periosteum have disappeared : its length and its diameter are always much less than ought to be anticipated from consi dering the size of the bone that has died ; and its surface is uneven and marked with slight depressions, as if part of its substance had been taken up by the absorbents. This ap
pearance is more distinctly observable, and the sequestrum is always smaller where the surface of the new shell is covered IA ith granulation, than when it is only smeared over with lymph. And here, as in other cases, it may be observed that the existence of granulation or of lymph on the new bone seems greatly to depend on the free admission of air to the cavity. Where the bone is deep-seated, as in the thigh, and there are but a few sinuous apertures that can scarcely render the cavity analogous to an open sore, the surface is covered by a layer of lymph; but where it is more superficial, as when the shaft of the tibia has come. away and left the new osseous deposit totally uncovered, its entire surface is seen studded over with healthy gra nulations, which, on passing the handle of a scalpel over them, are found to be gritty, and give sensible indications of containing bony matter.
From the first formation of the new deposit, small holes or perforations exist in it, the edges of which are bevelled down and thin, and not withstanding that the new bone may and usually does become extremely thick and spongy, these apertures still remain thin : it is through thein the matter makes its way to the surface and forms the fistulous ulcers that attend on this disease, and are to be described hereafter. These apertures remain as long as there is a single spicula of sequestrum within to keep up irritation and protract the suppuration. After the sequestrurn has completely disap peared, the growth of osseous material still continues internally until the new shaft appears one solid mass devoid of any cancellated or medullary cavity whatever. At this period the ulcers are healed up, and the patient enjoys a wonderful use of his swollen and deformed limb, but the pathological condition of the bone is still deserving of attention. At first it is a. mass of soft and spongy texture. After the lapse of a few years, though still clumsy in shape, and undiminished in diameter, the bone has become much rnore firm and solid, and in these respects, at least, equals the original structure. At a more remote period the osseous part is wonderfully solidified, being, in some instances, as firm as ivory, and a new medul lary cavity, with an internal periosteum, is formed. When a transverse section of a tibia. so circumstanced is made, the osseous walls are found to be hard, thick, and very firm, the medullary cavity much narrower than in the healthy bone, being scarcely capable of admit ting more than a goose-quill, and it does not seem to be cancellated or reticulated, but merely to consist of one continuous cell. In this state the bone possesses nearly three times the weight of one in the natural condition, and when dried is of a dirty brown colour, never assuming the white tint or polished appearance of the remainder of the skeleton.