856 ANATOMY of changes, and produce successive layers of new bone at the periphery. The importance of the periosteum as a centre of origin of new bone has, indeed, long been recog nised by both surgeons and pathologists. The parts of this membrane in which the special bone-producing power resides is the deep layer of osteo-blastic cells, whilst the blood-vessels furnish the pabulum for their nutrition. If strips of periosteum be removed, along with the cells of the deeper osteo-blastic layer, from a bone, and transplanted to other parts of the living body, bone will continue to be produced by their agency. The intra-membranous ossification of bone was first recognised by Nesbitt, and has been worked out in most of its details by Sharpey, Kolliker, and Gegenbaur. The tabular bones of the skull offer the best illustration of this mode of ossification. Sharpey has pointed out that a network of minute spicula of bone forms in the membrane, and extends in radiating lines from the centre of ossifica tion towards the circumference of the bone. The ossify ing tissue consists of fibres, of multitudes of granular cor puscles or osteo-blasts, and of blood-vessels. The osteo-blasts invest the fibres, but as the investing osteo-blastic cells calcify, from the periphery towards the nucleus, they assume a stellate configuration, and pass through a series of changes similar to those described in the intra-cartilaginous mode of ossification. The fibres, which are in the first instance soft, also calcify, and contribute to the formation of the bone. Here, however, as in the intra-cartilaginous ossifica tion, the active agents in the ossific process are the osteo- blastic cells. The lamellated structure is due to ossifica tion of successive layers of these cells, and the formation of the lacunae and canaliculi is owing to the persistence of their nuclei with a small proportion of unossified investing protoplasm. The increase in thickness of a membrane bone, like that of a cartilage bone, takes place through ossification in a deep periosteal layer of osteo-blasts. Hence it follows that, though the tissue which precedes the appearance of bone in the skeleton is not uniformly the same, in some cases being membrane, in others cartilage, there is an identity in the ossific process in the two forms of pre-existing tissue, in both of which the osteo-blastic cells are the active agents in ossification. The chemical differentiation which takes place in the protoplasm of the osteo-blasts during bone-formation is not merely a calcifica tion, but a coincident production of a gelatine-yielding substance, within which the minute calcareous particles are deposited. Stress has been laid by some anatomists, in discussing the homologies of the several bones of the skeleton, on the differences met with in the place of their formation. Thus, it has been supposed that a bone originally developed in cartilage cannot be homologous with one originally de veloped in membrane, and that a fundamental morpho logical d ! -itinction should be drawn between cartilage bones and membrane bones. But when it is considered that, though the place of formation may vary, the method of formation is the same in all localities, it does not appear that so much importance should be attached to the distinc tion between cartilage and membrane bones as it has some times received. Moreover, the differences between these two varieties of bones are, during the growth of the bone, etill further diminished, for in both cases increase in thick ness takes place in the same kind of pre-existing tissue, and in the same way, viz., by ossification of the deep periosteal layer of osteo-blasts. In the description of the development of bone in the fcetus and young person, the formation of medullary spaces was referred to. But the production of spaces in bone is by no means limited to its early stages of growth. The medullary canal in a long bone can scarcely be said to exist in the bones of an infant s limbs. The hollowing out of the shaft of a long bone into a large canal, and the enlargement of the spaces of the cancellated tissue, goes on not only up to the period of adult life, but even to ad vanced years ; so that in an old person the relative size of this canal is greater than in the prime of life. The Haver- sian spaces also, as Tomes and De Morgan pointed out, are produced by the absorption of the lamellae of the osseous tissue surrounding the Haversian canals, and the production of these spaces is constantly going on during the life of the bone. The air-sinuses in the cranial bones are also formed by the absorption of the diploe, and con sequent separation of the two tables of the skull. Bones, therefore, are organs which are continually undergoing change. During growth additions are being made to their length and thickness, and additional lamellae are being formed in the walls of the Haversian canals. At the same time a hollowing out of spaces in their interior is going on, so that an increase in weight commensurate with their growth does not take place. The interstitial absorptive changes, whether occurring during growth or after growth is completed, are due, as Kolliker has shown, to the action of many-nucleated colossal cells which line the walls of tho spaces where absorption is going on, which cells he has named ostco-klasts. The development and configuration of a bone is therefore, as has been well expressed by Kolliker, the product of the formation of osseous tissue by the agency of the osteo-blasts, and of its absorption or destruction by the action of the osteo-klasts. From the fact that osseous tissue may be produced either in the cartilaginous or in the fibrous tissues, and that all three contribute to the formation of the skeleton, it is evident that these tissues are closely allied. To express this alliance they have all been grouped together under the common term connective substances. MUSCULAR TISSUE. The muscular tissue is that which Muscle. is actively concerned either in the movement of parts of the body on each other, or in the movement of the entire body from place to place ; it is the active agent, therefore, both in motion and locomotion. It forms a large pro portion of the general mass of the body, is the essential constituent of the muscles or flesh, and enters into the formation of the walls of the hollow viscera. It consists structurally of threads or fibres, some of which are distin guished by being marked with transverse stripes or striae ; others have no such markings. Hence it is customary to divide the fibres of the muscular tissue into transversely striped fibres and non-striped fibres. As a rule, the striped fibres are collected together to form those muscles which are under the influence of the will, so that both the muscles and the fibres of which they are composed are called volun tary. One important exception to this rule is, however, met with, for the muscular fibres of the heart, though transversely striped, are involuntary ; the will exercises no control over the action of the heart. The non-striped fibres, and the muscles into the construction of which they enter, are in no instance, however, subject to the influence of the will ; so that, without exception, they may be named in voluntary. The Non-striped or Involuntary fibre, sometimes called pale or smooth muscular fibre, enters into the forma tion of the walls of the hollow viscera e.g., stomach, intestines, bladder, uterus of the walls of the air-tubes, gland-ducts, blood and lymph vessels, of the skin, and various mucous membranes. The fibres are usually col lected into bundles or fasciculi, which are not aggregated together into such compact red masses as in the voluntary muscles, but are of a paler red colour, and are set farther apart, and often cross and interlace with each other in the
walls of the tubes and hollow viscera, in which this form
