Bone differs materially in structure and function from most of the other tissues of the body, and these peculiarities exercise an important influence on the pathological changes to which it is liable.
This structure seems to subserve a double function in the economy. In the period of development and growth it is actively engaged in the building up of the skeleton, and even in the adult it contains structures capable of renewing the process of bone-formation. On the other hand, it is evidently an organ intimately related to the formation of blood, and this function, though most active during development and growth, is retained in the adult state.
Much attention has been paid of late years to the bone-marrow as intimately related to the formation and replenishment of the red blood-corpuscles. The spaces left by the solid and rigid structures of the bone are not entirely taken up by the blood-vessels designed to nourish the bone, but nature has taken advantage of them to hold tissue having the function mentioned. This matter has already been- referred to in the General Part of this work.
The normal bone-marrow is divisible into the Red marrow and the Yellow or Fatty marrow. The former is found in the foetus in all the medullary spaces, whether the hollows of the shafts of the long bones or the cancellated tissue. It yields to the yellow or fatty marrow in the shafts of the long bones and even in the cancellated tissue of these, but in the adult it remains in most of the cancellated tissue of the short and flat bones. The red marrow is the active tissue in relation to the blood. The yellow marrow is really adipose tissue, and this replaces the red marrow as the requirements of blood-formation become limited. In the foetus and growing child there is a great new-formation of blood, both to replenish the loss incident to the processes of life and to increase the actual amount in proportion to the increase in the tissues. As the adult period is reached the latter requirement diminishes and disappears, and yellow marrow takes, to some extent, the place of the red marrow. Even the yellow marrow, is not a pure adipose tissue, as the constituents of the red marrow are still distinguishable in it, although sparsely present, and there may be, under pathological conditions, a partial or complete return to the condition of the red marrow.
The red marrow is a highly cellular and highly vascular substance. The cells are of various sorts. There are larger and smaller colourless cells, some of which contain eosinophil granules, and there are cells of a yellowish colour like the red blood-corpuscles, but which, unlike these, are nucleated. The nucleated red cells (or erythroblasts) are regarded as concerned in the formation of the red blood-corpuscles, and they present the appearances of nuclear division by karyokinesis. The capillaries and veins of the red marrow are wide and thin-walled; some authors have stated that they do not possess proper walls, but are bounded directly by the marrow cells.
During the period bf growth bone is in a condition of great activity, the growth of bone being effected by processes which are, in some respects, special and peculiar. The proper bony tissue (tela ossea) is composed of a rigid calcified matrix, enclosing the living cells or bone-corpuscles in lacunae. This tissue once formed is, like the mason-work of a house, fixed in form and insusceptible of expansion or plastic alteration in shape. It can only be altered by being taken down and rebuilt. But in the growing bones there is necessarily a very active process of reconstruction going on. The bone as a whole is expanding, and this is effected, not merely by apposition of new bony tissue to the old, but it involves a process of destruction and reconstruction.
The Internal architecture of the bones is of great interest both from a physiological and pathological point of view. From the observations of Meyer, which have been amplified and expanded by Wolff, it appeals that the bones are carefully constructed with a view to the function of resisting the mechanical forces of pressure and traction. Pressure is mainly exercised at the articular ends of the bones, and the cancellated tissue existing there consists of series of trabecule which arch in various directions so as to carry the weight. These trabecular mostly end in the dense bone of the shaft or otherwise, which thus becomes the ultimate repository of the pressure. This elaborate architecture is preserved during the process of growth, and this can only be effected by gradual destruction and reconstruction.
It is a further point of much interest that when the normal architecture of the bones is interfered with by imperfectly adjusted fractures, by bendings, or by dislocations of the bones, there is a reconstruction so as to meet the new lines of pressure which are thus brought about. Thus even in the adult there is a moulding of the affected bones according to a definite plan. Where, in the new form of the bone, the pressure is increased in any particular direction, new-formation of bone occurs along the lines of increased pressure, whilst absorption occurs in places where the pressure is diminished. Thus in the annexed illustration from a case of genu valgum or knock-knee (Fig. 265) it is seen how the head of the right tibia shows a marked increase in the trabecular under the outer condyle, along with a corresponding thickening of the shaft, due to the fact that, in the faulty position of the leg the pressure,was much greater on the outer than the inner condyle. The extent to which reconstruction is necessary is emphasized when one remembers that the shaft of a young child's femur is no greater in diameter than the medullary cavity of that of an adult. In fact, in the course of growth the whole bone is, in many cases, taken down and reconstructed several times over.
Fig. 2<>5. - Head of right tibia from genu valgum. There is great thickening of the bony trabeculae on the outer side. (Wolff).
The growth of bone is effected by the process of Ossification. In a longitudinal section of a growing long bone, such as that illustrated in Fig. 266, the process of ossification from cartilage may be seen. At the upper part of the figure (a) there is the normal appearance of hyaline cartilage. As the ossifying margin is approached the cells multiply, and at first they form irregular groups, as at b. Nearer the margin the multiplying cells become arranged in rows parallel to the long axis of the bone, the elongated groups being separated by narrower elongated pieces of hyaline matrix. The beginning and the end of this zone of multiplying cartilage cells are quite abrupt, and form nearly straight transverse lines. This zone appears to the naked eye as a narrow bluish band. Next occurs an infiltration of the hyaline matrix between the groups with lime salts, the matrix becoming opaque and granular (c, c in figure). This forms a yellow zone as-seen with the naked eye. This zone is also suddenly interrupted, and at a definite level the groups of cartilage cells give place to cavities filled with Foetal marrow, consisting of round cells and blood-vessels (d). It is from these cells that the formation of true bone occurs. The cells contained in the spaces have the power of producing bone, and are hence called Osteoblasts. These fix themselves to the walls of the cavities, which, as we have seen, are composed of calcified cartilaginous matrix, and each cell forms around itself a certain portion of bone-tissue, apparently by a process of secretion.
Fig. 260. - Longitudinal section of bone ossifying from cartilage: a, commencement of proliferation of cartilage cells; b, cells arranged in longitudinal groups: c, matrix of cartilage impregnated with lime, this calcified matrix is continued beyond the ossifying border (c on right side); d, medullary spaces containing round cells replacing cartilage cells; e, c, formation of bone at borders of calcified matrix, the osteoblasts each forming a piece of bone and remaining as the bone corpuscle. x 90. (After Thier- FELDER).
The tela ossea thus formed encloses the cell which has formed it, and the latter remains as a bone corpuscle. In such a section as that under review one constantly sees at the borders of the cavities layers of osteoblasts with oval areas of new-formed osseous tissue around them, e in figure. Thus the calcified cartilaginous matrix is encroached upon, and the bone is formed.
Besides this ossification from cartilage, we have what is commonly but not quite correctly called Ossification from membrane. In all such cases we have cells similar to those in the cavities mentioned above, which have the characters of osteoblasts, and form bone in a similar fashion. These osteoblasts form a layer under the periosteum, they exist also in the spongy parts of bones, and at the ossifying margins of flat bones.
The Absorption of bone is effected by active cells which are distinguished by the name of Osteoclasts. The giant-cells or myelo-plaques which exist in foetal bone-marrow have this function. Where absorption is going on these cells are found attached to the bone, and lying in gaps evidently formed by them and called Howship's lacuna?.
When the period of growth is finished, the osteoblasts and the osteo-clasts have completed their work, but the bone remains penetrated in every part by soft tissue. This soft tissue, as we have seen, subserves blood-formation, but it also contains the remains of the structures which were concerned in the building up of the bone. We may accept the view of Ranvier, according to which the bone in the adult state is enveloped in, and penetrated by the remaining potential osteoblasts. This tissue constitutes the bone-marrow and the internal layer of the periosteum (called Periosteal marrow by Ranvier), while between these two there is a connecting network traversing the Haversian canals.
From these observations it appears that in childhood the bones are in an extraordinary state of activity, a process of enlargement and reconstruction continually going on. At the same time, the function of the bones as substantial props is in full activity. It is not remarkable, therefore, that in childhood the bones are peculiarly liable to disease, which may take origin from without or be due to inherent weakness.
In the diseases both of children and adults again, there may be a partial return to the conditions of the period of growth, and osteoblasts and osteoclasts resume their operations, the processes being now pathological.
Another characteristic of pathological processes in bone is the occurrence, formerly alluded to, of Metaplasia. Bone, cartilage, and con nective tissue are fundamentally the same tissue, and apparently the same cells may be, according to circumstances, osteoblasts, fibroblasts, or chondroblasts. This will be illustrated under Fractures.
Quain's Anat., 10th ed., vol. i., p. 268, 1891; Meyer, Die Archi-tectur der Spongiosa, Reichert and Du Bois-Reymond's Arch., 1867, and Jubilaumsschrift fur von Bischoff, 1882; Wolff (who scarcely does justice to Meyer), Centralbl. f. d. med. Wissensch., 1869, Virch. Arch.; 1., 1870, and Das Gesetz der Transformation der Knochen, 1892; Ranvier, Le developpement du tissu osseux, 1865.