The exact knowledge we possess of the minute structure of osseous tissue is largely the result of the careful investigation of the late Professor Sharpey, whose account, published in the fifth edition of this work in 1845, has needed no erasure, and but little addition, even to the present day. His labours in this field have been to a certain extent recognized in the adoption of the name "fibres of Sharpey" for the perforating fibres discovered by him, but it is only of late that the facts which he demonstrated are becoming understood and their significance appreciated by histologists. THE PERIOSTEUM. The periosteum, as already stated, is a fibrous membrane which covers the bones externally. It adheres to them very firmly, and invests every part of their surface, except where they are covered with cartilage. It is composed of two layers; the outer, consisting chiefly of white fibres, and containing occasional fat-cells, is the means of supporting numerous blood-vessels destined for the bone, which ramify in the membrane, and at length send their minute branches into the Haversian canals of the compact substance, accompanied by processes of filamentous tissue derived from, or at least continuous with, the periosteum. The inner layer is largely made up of elastic fibres, frequently in several distinct strata. Between it, however, and the proper osseous tissue there is a fibrous stratum containing in the young bone a number of granular corpuscles (osteoblasts), while in the adult bone these have become flattened out into an epithelioid layer covering the osseous substance, and are in many places separated by a cleft-like space. (serving probably for the passage of lymph) from the rest of the periosteum (Schwalbe). By treating the membrane with nitrate of silver, lymphatics are discovered in it accompanying the blood-vessels in the outer layer; and, as in other aponeurotic structures, extensive epithelioid markings, covering a great part of the surface, are brought into view. Fine nerves spread out in the periosteum; they are chiefly associated with the arteries, and for the most part destined for the subjacent bone; but some are for the membrane itself, and some of these end in Pacinian corpuscles. The chief use of the periosteum is to support the vessels going to the bone, and afford them a bed in which they may subdivide into fine branches, and so enter the dense tissue at numerous points. Hence, when the periosteum is stripped off at any part, there is great risk that the denuded portion of the bone will die and exfoliate. The periosteum also contributes to give firmer hold to the tendons and ligaments where they are fixed to bones. Its relation to the growth and renewal of bone will be referred to later on. THE MARROW. The marrow (medulla ossium) is lodged in the interior of the bones; it fills up the hollow shaft of long bones and occupies the cavities of the cancellated structure; it extends also into the Haversian canals-at least into the larger ones-along with the vessels. A fine layer of a highly vascular areolar tissue lines the medullary canal, as well as the smaller cavities which contain marrow; this has been named the medullary membrane, internal periosteum, or endosteum; but it cannot be detached as a continuous membrane. Its vessels join on the one side those of the osseous substance, and on the other side are continuous with the capillaries of the marrow. The marrow differs considerably in different situations. Within the shaft of the long bones in man it is of the character of ordinary adipose tissue, but the fat-cells are supported by a kind of retiform tissue, and between them elements occur similar to those immediately to be mentioned in the red marrow. In short bones, and in the cancellated ends of long bones, but especially in the cranial diploë, the bodies of the vertebræ, the sternum, and the ribs, the marrow is red or reddish in colour, of more fluid consistence, and with very few fat-cells. While, however, the fat-cells are scanty in the red-coloured marrow, it contains numerous round leucocytes -the proper marrow-cells of Kölliker (fig. 307, e-i). These in general appearance resemble the pale corpuscles of the blood, but are larger, with a clearer protoplasm and a relatively larger nucleus. Like the pale corpuscles, they exhibit amoeboid movements. Amongst them are smaller cells which have a reddish colour, and resemble in appearance the primitive nucleated corpuscles of the embryo (fig. 307, j-t); these are the cells (erythroblasts) which are concerned in the formation of the red blood-disks, and according to some authors are themselves Fig. 307.-CELLS OF THE RED MARROW OF THE GUINEA-PIG. HIGHLY MAGNIFIED. (E. A. S.) a, a large cell the nucleus of which appears to be partly divided into three by constrictions; b, a cell the enlarged nucleus of which shows an appearance of budding into a number of smaller nuclei; c, a so-called giant-cell or myeloplaxe with many nuclei; d, a smaller myeloplaxe with three nuclei ; e-i, proper cells of the marrow; j-t, various forms of coloured nucleated cells, some undergoing division. derived from colourless marrow-cells or from pale blood-corpuscles (p. 219). It is, however, doubtful if this is so; it is more probable that they are the direct descendants of the nucleated red corpuscles of the early embryo. Like these they are amoeboid and divide by karyokinesis. They appear to be formed into blood-disks by the disappearance of the nucleus and the moulding of the cell protoplasm into the biconcave discoid shape. It is probably by virtue of their amoeboid properties that they pass into the venous capillaries of the marrow, but some may be contained within the vessels, and in birds all the erythroblasts are found within the lumen of the venous capillaries (Bizzozero and Torre). Cells have occasionally been noticed containing one or more red corpuscles in their interior (Osler): whether these have been developed in situ in a manner similar to that previously described in connective tissue corpuscles of the young animal, or have been taken into the interior of an amoeboid cell, there to be transformed into pigment-granules, is not certainly known. Cells containing reddish pigment granules are, however, not uncommon. There further occur in the marrow, especially in the neighbourhood of the osseous substance, large multi-nucleated protoplasmic masses (myeloplaxes of Robin, fig. 307, a-d), which, as pointed out by Kölliker, appear to be more especially concerned with the process of absorption of bone, under which head they will subsequently be further alluded to. The myeloplaxes vary much in size, but are always larger than the proper marrow-cells. Their nucleus is not always multiple, but when single it is usually enlarged, and presents indications of division (fig. 307, a); it may even be so constricted as to exhibit an irregularly moniliform appearance (fig. 307, b). Blood-vessels.-The bones are well supplied with blood-vessels. A network of periosteal vessels covers their outer surface; fine vessels run from this through all parts of the compact tissue in the Haversian canals; others penetrate to the cavities of the spongy part, in which they ramify; and a considerable artery goes to the marrow in the central part of the bone. In the long bones this medullary artery, often, but improperly, called "the nutritious artery," passes into the medullary canal, near the middle of the shaft, by a hole running obliquely through the compact substance. The vessel, which is accompanied by one or two veins, then sends branches upwards and downwards in the middle of the marrow; from these branches arterial capillaries pass radially towards the periphery. The comparatively narrow arterial capillaries pass suddenly at the periphery of the marrow into the wide venous ones, which form a close network of large thin-walled vessels throughout the medullary tissue, so that the current of blood must be considerably retarded both in these and in the large thin-walled veins. The ramifications of the medullary artery anastomose with the arteries of the compact and cancellated structure; indeed, there is a free communication between the finest branches of all the vessels which proceed to the bone, and there is no strictly defined limit between the parts supplied by each. In the thigh bone there are frequently two medullary arteries entering at different points. The veins of the cancellated texture are peculiar and deserve special notice. Their arrangement is best known in the bones of the skull, where, being lodged in the diplöe or spongy texture between the outer and inner compact tables, they have received the name of the diploic veins. They are large and numerous, and run separately from the arteries in canals formed in the cancellated structure, the sides of which are constructed of a thin lamella of bone, perforated here and there for the admission of branches from the adjoining cancelli. Being thus inclosed and supported by the hard structure, the veins have exceedingly thin coats. They issue from the bone by special apertures of large size. A similar arrangement is seen in the bodies of the vertebræ, whence the veins come out by large openings on the posterior surface. In the long bones numerous apertures may be seen at the ends, near the articular surfaces; some of these give passage to arteries, but the greater number, as well as the larger of them, are for the veins of the cancellated texture, which run separately from the arteries. According to Hoyer and Rindfleisch the venous eapillaries and veins of the red marrow have incomplete walls, or rather are channels bounded only by the medullary parenchyma, so that the blood-corpuscles which are being formed from marrow-cells can readily get into the circulation. Langer, on the other hand, found the vascular system of the marrow to be a closed one. In birds, this is certainly the case according to the testimony of Bizzozero and of Denys, but in mammals it is doubtful if the vascular walls are everywhere complete. The blood coming from the marrow possesses a large number of pale corpuscles, and sometimes nucleated red corpuscles can be detected in it. Lymphatics. In addition to the lymphatics in the periosteum (which have already been mentioned), there are others in the Haversian canals accompanying the VOL. I. T vessels (fig. 297, 7), and often partially or wholly enclosing them (perivascular). The lymph or plasma of the blood is enabled to penetrate the hard bony substance by means of the lacunæ and communicating canaliculi, which appear to bear the same relation to the lymphatics as do the cell-spaces of ordinary connective tissue to the lymphatics of that tissue. The fine nerves which may be seen entering the bones along with the arteries are probably chiefly destined for those vessels; it is not known whether any end in the bony tissue itself. As far as can be judged from observations on man and experiments on the lower animals, the bones, as well as their investing periosteum, are scarcely if at all sensible in the healthy condition, although they are painfully so when inflamed. FORMATION AND GROWTH OF BONE. The foundation of the skeleton is laid at a very early period; for, among the parts that appear soonest in the embryo, we distinguish the rudiments of the vertebræ and base of the skull, which afterwards form the great median column to which the other parts of the bony fabric are appended. But it is by their outward form and situation only, that the parts representing the future bones are then to be recognised; for at that early period they do not differ materially in substance from the other structures of the embryo, being made up of mesoblastic cells, with a small amount of intercellular substance. Very soon, however, they become cartilaginous, and ossification in due time beginning in the cartilage and continuing to spread from one or from several points, the bony tissue becomes gradually formed. But, while it is true with respect to the bones generally that their ossification commences in cartilage, it is not so in every instance. The tabular bones forming the roof of the skull may be adduced as a decided example to the contrary; in these the ossification goes on in connective tissue altogether unconnected with any cartilage; and even in the long bones, in which ossification undoubtedly commences and to a certain extent proceeds in cartilage, it will be afterwards shown that there is much less of the increment of the bone really owing to that mode of ossification than was at one time generally believed. It is necessary, therefore, to distinguish two species or modes of ossification, which for the sake of brevity may be called the intramembranous and the intracartilaginous. INTRAMEMBRANOUS OSSIFICATION: OSSIFICATION IN CONNECTIVE TISSUE. The tabular bones of the cranium, as already said, afford an example of this mode of ossification. The base of the skull in the embryo is cartilaginous; but in the roof, that is to say, the part comprehending the parietal, the frontal, and a certain portion of the occipital bones, we find (except where there happen to be commencing muscular fibres) only the integuments, the dura mater, and an intermediate layer, in which the ossification proceeds. The commencing ossification of the parietal bone, which may be selected as an example, appears to the naked eye in the form of a network in which the little bars or spicula of bone run in various directions, and meet each other at short distances. By-and-by the ossified part, becoming extended, gets thicker and closer in texture, especially towards the centre, and the larger bony spicula which now appear, run out in radiating lines to the circumference. The ossification continues thus to spread and consolidate until the parietal meets the neighbouring bones, with which it is at length united by sutures. Fig. 308 represents the parietal bone of an embryo sheep about two inches and a half long, and shows the character of the ossification as it appears when the object is magnified about twelve diameters. The bone is formed in membrane as in the human foetus, but a thin plate of cartilage rises up on its inside from the base of the skull. The ossification, however, is decidedly unconnected with the cartilage, and goes on in a membrane lying outside of it. When further examined with a higher magnifying power, the tissue or membrane in which the ossification is proceeding, appears to be made up of fibres and granular corpuscles, with a ground-substance between, and, in point of structure, may not unaptly be compared to connective tissue in a certain stage of development. The corpuscles are large and angular, and they are densely packed all over the area of ossification, covering the bony spicula, and filling up their interstices. On observing more closely the points of the growing osseous rays at the circumference of the bone, where they shoot out into the soft tissue, it will be seen that the portion of them already calcified is granular and rather dark in appearance (fig. 309), but that this character is gradually lost as they are traced further out Fig. 308.- PARIETAL BONE OF AN EMBRYO SHEEP. The small upper figure represents the bone of the natural size, the larger figure is magnified about 12 diameters The curved line, a, b, marks the height to which the subjacent cartilaginous lamella extended. A few insulated particles of bone are seen near the circumference, an appearance which is quite common at this stage. wards in the membrane, in which they are prolonged for a little way in form of soft and pliant bundles of transparent fibres (fig. 309, B, of). These are termed osteogenic fibres, the soft transparent matter of which they are composed being known as osteogenic substance, or simply as osteogen. They exhibit faint fibrillation, and have been compared to bundles of white connective tissue fibres, with which, in some situations, they appear to be continuous (Gegenbaur). But although similar in chemical composition, they are somewhat different from these in appearance, having a stiffer aspect and straighter course, besides being less distinctly fibrillated. The fibres become calcified by the deposition within them of earthy salts in the form of minute globules, which produce a darkish granular opacity, until the interstices between the globules also become calcified, and the minute globules becoming thus fused together, the new bone again looks comparatively clear (fig. 309, B, b). As already stated the fibrils which compose the osteogenic fibres themselves, are, according to v. Ebner, not calcified, but the calcification affects only the matrix which unites them. The bundles of osteogenic fibres which prolong the bony spicules, generally spread out from the end of each spicule so as to come in contact with those from adjacent spicules. When this happens, the innermost or proximal fibres frequently grow together (fig. 309, B, c), whilst the other fibres partially intercross as they grow further into the membrane. The ossific process extends into the osteogenic |