Formation of Bone.—The commencement of the growth of bone is generally preceded by the formation of a cartilaginous matter which occupies the place afterwards taken by bone. From this circumstance it has been supposed that bone is formed from the ossification of cartilage. This however is not the case, as it is found that although ossification takes place in the first instance in cartilage, the bony matter thus formed has not a permanent character. The formation of bone always takes place in the first instance in the immediate neighbourhood of blood-vessels in canals excavated in the cartilaginous substance, and the spots where these canals are formed are called centres of ossi fication. There is usually one of these in the centre of a long hone and one at each end, and frequently another for any considerable process or projection, such as the trochanter in the femur of the human body. Up to the time that the bone is fully formed these centres are only connected by cartilage, and this arrangement serves the obvious purpose of allowing the increase of the whole bone by means of cartilage until the time comes when no further increase of size is needed. In the early conditions of the skeleton of the vertebrate animals there is a much closer correspondence in this respect than is subsequently seen ; for according to the habits of the animal the whole of the cartilage is converted into bone, or by its absorption and disappearance separate bones are formed. It is upon this fact that the science of transcendental anatomy rests, in which the whole of the modifications of the vertebrate skeleton are referred to departures from a type which is found generally to exist in the embryo condition of the whole class. Thus it is found, from an extensive comparison, that the regular number of distinct bones in the wrist (carpus) and instep (tarsus) is ten, but in the human wrist the number is reduced to eight and in the instep to seven, whilst the reduction is still greater in the hoofed mammalia. [SKELETON.] The structure of the temporary cartilage of bone is precisely the same as permanent cartilage. The first calcareous deposit is made in the space between the proper cells of the cartilage. These cells subsequently disappear leaving large open areelm having no very definite form. In these areohe there is subsequently deposited a fluid blastema containing cells, and through the agency of this blastema the Haversian canals and cancelli appear to be formed, whilst the interspersed cells are changed into the bone-cells and their projecting canaliculi. Although in most instances the formation of bone is preceded by that of cartilage, yet this is net universally the case, as we see bone formed in the substance of the tissues, as for instance the fibrous membrane. This is seen in the development of
the bones of the roof of the skull, and also in the growth of bones subsequently to their first development by the progressive calcifica tion of the inner layers of the periosteum, or fibrous covering of the bones.
It has been stated that the central cavities of some of the larger bones are filled with the substance called marrow, an oily matter contained in a series of membranous cells, which, like those in which the fat is deposited do not communicate with each other. Even the pores and cancelli of bone also contain a kind of oily matter, which is supposed to differ from marrow only in possessing a greater degree of fluidity. This oily matter is deposited in longitudinal canals, which pass through the solid substance of the bone, together with its nutrient vessels. The use of the marrow, and of the modification of it which constitutes the oily matter, is not well understood. Without doubt it serves the same general use in the economy as the other oily secretions. [AmeosE 'Plasm] All bones are covered by a membrane, named, on account of its affording them an external envelope, Periosteum. The outer surface of this enveloping membrane is connected to the surrounding parts by cellular tissue, but its inner surface is firmly adherent to the substance of the bone. This adhesion is effected by innumerable fibres or threads, which on examination are found to consist of blood-vessels. The periosteum is in fact the membrane on which the nutrient arteries of the bone rest, divide, and ramify in order to enter the osseous substance. These threads are much more numerous in the child than in the adult ; and accordingly the adhesion of the perios teum to the bone is much firmer in the former than in the latter, as the quantity of blood distributed to the bone is greater. Moreover, in general the inner surface of bones is also lined by a fine and delicate membrane, commonly termed the internal periosteum, the continuation of which forms the membranous bags in which the marrow is contained.
(ltoget, Animal and Vegetable Physiology ; Southwood Smith, Philosophy of Health ; Simon, Animal Chemistry ; Sharpy, Quoin s Anatomy; Cyclopa•dia of Anatomy and Physiology, articles 'Bone,' 'Osseous Tissue'; Todd and Bowman, Physiological Anatomy; Carpenter, Principles of Physiology, General and Comparative ; Tomes and De Morgan, Structure of Bone, Phil. Trans. ; Quekett, Lectures on Histology.)