TEXTURES.] ANATOMY 855 together with the periosteum, the medulla, and their blood vessels, are active agents in the development, growth, and nutrition of the osseous tissue. velop- In the description of the development of the skeleton, nt of it was stated that the bones are formed by ossification in le> cartilage and fibrous membrane, so that bones are pro duced by secondary changes in a pre-existing material. The mode of production of the osseous tissue in the car tilaginous and fibrous tissues will now be considered, and it should be clearly understood at the outset that, in normal ossification, bone is not formed by a mere calcification of the matrix of the pre-existing tissue, and a conversion of the cartilage or connective tissue corpuscles into bone corpuscles; but, as the researches of Sharpey, Bruch, H. Miiller, Love"n, and Gegenbaur have made known, is due to a development of new corpuscles, which Gegenbaur has named osteo-blasts, accompanied by an abundant forma tion of blood-vessels. When the process of ossification in temporary cartilage begins, a change takes place in the arrangement of its cells at the centre, or point, or nucleus of ossification. The cells, instead of preserving their irregularly scattered arrange ment in the matrix, are now collected into longitudinal parallel rows, not unlike what was described in a previous section, in the deeper cells of encrusting cartilage. In each row the cells lie with their long axes transverse, and apparently multiply by a process of fission. The cells at the end of the rows which lie nearest the centre of ossific change swell out and become more rounded. Calcification of the matrix substance, which separates not only the parallel rows of cells, but also the cells in the same row, from each other, then takes place, which calcification in cludes also the capsules of the cartilage cells. A general opacity of the cartilage is the result of this calcification, and the further progress of ossification is rendered obscure. It is necessary, therefore, to dissolve out by an acid the calcareous matter, in order to follow the steps of the process, i Spaces or canals now form in the ossifying cartilage, into which blood-vessels, continuous with the vessels of the perichonclrium, are prolonged. These spaces are lined by concentric layers of small rounded cells, not unlike lym- phoid cells in size and appearance, and form the medullary spaces of foetal cartilage, whilst the cells and blood-vessels form the medulla. Respecting the source of origin of the cells of this medulla, there have been difficulties in arriving at a correct conclusion. Some have believed them to be descended from the cartilage cells, though no demonstration of their derivation from this source has ever been obtained. Henke conceived that they might be blood corpuscles migrated from, the blood-vessels within the spaces. But the recent observations of Stieda seem satisfactorily to show that the layers of medulla cells are continuous with similar layers beneath the perichondrium, which layers are prolonged along with the blood-vessels into the medullary spaces as they form in the ossifying cartil age. But, whatever be their derivation, there can be no doubt that these cells undergo certain modifications which are of the utmost importance in the further stages of the ossific process. A few become elongated into fusiform or stellate corpuscles, like those of connective tissue ; others have oil drops forming in their interior, and become the cells of yellow marrow ; others become the corpuscles of red marrow; others, again, which form the osteo-blasts, properly so-called, are the direct agents in the production of the osseous tissue itself. The formation of the medullary spaces in cartilage is owing to an absorption of the calcified cartilaginous tissue. Kolliker points out that the absorption is effected through the agency of colossal, many-nucleated cells (myeloplaxes), winch he believes to be derived from the osteo-blastic cells of the medulla already described, so that a destruction of the calcified cartilage precedes the formation of the proper osseous tissue. As the absorption of the cartilage goes on, an irregular series of medullary spaces communicating more or less freely with each other is produced. But along with the destructive changes in the cartilage the pro duction of the new osseous tissue takes place. Certain of the cells of the medulla are arranged in layers around the walls of the medullary spaces, and undergo an important change both in composition and shape. They become granular, their protoplasm hardens from the periphery towards the nucleated centre of the cell, so as to give origin to the dense matrix substance of a bone lamella ; but the nucleus, and the protoplasm immediately investing it, do not harden, they form the soft contents of the lacunas and canaliculi. A second layer of osteo-blastic medulla cells then passes through a similar metamorphosis, and a second lamella is formed. By a repeti tion of this process around the walls of the several medullary spaces, the lamellae of the bone are produced. Hence it would appear that the dense solid matrix of the osseous tissue is produced by a special hardening of the protoplasm of the osteo-blastic cells in the medullary spaces, and as layer after layer of these cells is ossified successive lamellae are produced. The per sistence, however, of the nucleus of each osteo-blast, and of a small portion of its investing protoplasm, preserves within the hard matrix a certain amount of soft material, which being destroyed when a bone is macerated, leaves the lacunary and canalicular system already described. The formation of successive lamcllce diminishes the size of the medullary spaces, which then form the Haversian canals. The vascular and cellular contents of these canals are therefore the remains of the contents of the medullary spaces of the foetal cartilage, and are continuous with the deeper layer of the periosteum. So long as any cartilage remains in a foetal or young bone the process of replacement of the cartilaginous tissue by the proper osseous tissue goes on, until none of the cartilage is left, except the thin layer of encrusting cartil age at each articular extremity. Bones grow in length by an ossification in cartilage ; and a provision for their longitudinal increase is furnished up to, and even beyond the age of puberty, by the plate of cartilage which separ- rates the epiphysis from the shaft of a bone. The ossifica tion of this plate of cartilage marks the period when growth ceases in the long axis of the bone. But bones also grow in thickness, and this addition to their girth takes place by an ossification of material situated at their circumference. It has already been pointed out that a bone is invested by a fibrous membrane, the periosteum, which fulfils for it the same purpose as does the peri chondrium for the cartilage. On the deeper surface of the periosteum, i.e., next the bone itself, are osteo-blastic cells, similar to those which lie in the medullary spaces of the f octal cartilage. These cells pass through a similar series FIG. d3. Section through a foetal bone to illustrate its development. B, 13, the dense osseous tissue, in which the lacunfe, with their soft nucleated contents, may be seen. M, 51, the medullary tissue in the medullary spaces. OB, OB, layer of osteo-blastic cells of the medulla, next the osseous tissue, some of which in places are obviously becoming included in it. V,V, trans versely divided blood-vessels, surrounded by medulla cells, situated in medullary spaces,vhich>
are assuming the form of Haversian canals.
