specimen all these cavities have been filled up with finely divided débris and hence appear opaque. In the living bone these spaces are filled with a tissue or a cell or with fine protoplasmic processes. Thus the Haversian canal contains an artery and vein, some capillaries, a flattened lymph space, fine medullated nerve fibres— the whole being supported in a fine fatty tissue. Each lacuna is filled with a cell—the bone corpuscle—and the canaliculi contain fine branching processes of these cells. On comparing such a section with one taken parallel to the long axis of the shaft of a bone it is seen that the Haversian canals run some distance along the length of the bone, and that they frequently unite with one another or communicate by obliquely coursing channels. The spaces between the Haversian systems are filled in with further bony tissues which may or may not be arranged in laminae. Finally, the systems are as it were bound together by other laminae running parallel to the surface of the bone. If a piece of fresh bone be decalcified so that a thin section can be cut from it, the bone corpuscles can be seen filling up the lacunae but the section does not give so typical a picture as that already examined because it is not possible to make the protoplasmic structures filling the lacunae and canaliculi stand out in marked contrast with the surrounding matrix.
Fig. 8.—Section of Bone. Showing four Haversian systems and
interlying bone material. This is a dry preparation, hence all the
cavities (such as the Haversian Canals, the lacunae and canaliculi),
being filled with débris from the grinding, appear dark.
Cancellous bone only differs from compact bone in the arrangement of the bone tissue. This encloses a number of irregular spaces which communicate with one another to form a kind of spongework. Commonly the framework is so constructed that a number of trabeculae running parallel to one another are produced. This is for the purpose of especially strengthening the bone in that direction. This direction is in all cases found to be that in which the bone has to support its maximum strain while in position within the body. Usually the bone trabeculae are so narrow that there is no need for Haversian systems within them, and they therefore usually consist of a few laminae arranged parallel to the surface. These laminae include bone corpuscles as in the rest of the bone tissue.
Bone Marrow.—Filling the central cavity of the tubular bones and the cavities of the spongy bone tissue is a tissue largely composed of fat cells. This is the bone marrow. Two varieties are distinguished, the one being red in colour, the other yellow. Red marrow is composed of a number of fat cells lying in a tissue made up of large and small marrow cells and typical giant cells or myeloplaxes (fig. 9). The whole of these elements are supported in a delicate connective tissue. The marrow cells exhibit manifold forms. Some are typical leucocytes and lymphocytes as found in circulating blood. Others named myelocytes are slightly larger than leucocytes, with round or oval nuclei, and a protoplasm containing neutrophile granules. Yet another variety contains large eosinophile granules in the protoplasm. These different types of cell probably develop into leucocytes. The giant cells are large spherical cells with several nuclei.
In addition to fully developed red blood corpuscles there are also present numerous nucleated red blood cells (erythro-blasts or haematoblasts). These are red blood corpuscles in an early stage of formation. They reach the blood after they have lost their nuclei.
|Fig. 9.—Section of Bone Marrow.|
|f, Fat vacuole.||e, Eosinophile cells.|
|my, Myeloplaxes.||r, Red corpuscles.|
|m, Marrow cells.||h, Haematoblasts or erythro-blasts.|
Development of Bone.—The formation of new bone always takes place from connective tissue, but we may distinguish two different modes. In the first the bone is preceded by cartilage (development from cartilage), in the second the bone is laid down directly from a vascular fibrous membrane (development from membrane). The development of bone from cartilage is the more complicated of the two because in it bone formation is taking place in two positions at the same time and in two rather different manners. Thus bone is being laid down from the outside (perichondral formation) from the fibrous membrane surrounding the cartilage, the perichondrium and also within the substance of the cartilage (endochondral formation). Perichondral formation takes place somewhat earlier than endochondral and in the case of a long bone is first observed around the centre of the shaft, i.e. in that portion of the bone which forms the diaphysis. Here the perichondrium is vascular and carries on the surface next to the cartilage an almost continuous layer of typical cells cuboid in shape, the osteoblasts or bone-formers. Calcium salts are deposited in the matrix of the immediately subjacent cartilage and the cell spaces of the cartilage increase in size while the cartilage cells shrink. Further growth of cartilage ceases in this region so that at one time the shaft of the cartilage may appear constricted in the middle. The formation of bone endochondrally is ushered in by the in-growth of blood vessels from the perichondrium. A way through the calcified matrix of the cartilage is made for them by a process of erosion. This is effected by a number of polynucleated giant cells, the osteoclasts, which apply themselves to the matrix and gradually dissolve it away. The enlarged cartilage spaces are thus opened to one another, and soon the only remnants of the matrix consist of a number of irregular trabeculae of calcified matrix. In this way the primary marrow spaces are produced, the whole structure representing the future spongy portion of the bone. The next step in both perichondral and endochondral bone formation consists in the deposition of bone matrix. This is effected by the osteoblasts. In the spongy portion they deposit a layer upon the surfaces of the calcified cartilage matrix, and thus in newly formed bone we find a central framework of cartilage matrix enclosed in a layer of bone matrix (see fig. 10). In the perichondral formation the deposition is effected in the same manner but is not uniformly spread over the whole surface, but trabeculae are formed. These become confluent at places, thus leaving spaces through which blood vessels and osteogenetic tissue pass to reach the interior of the bone. As the deposition of bone matrix proceeds, some of the osteoblasts become included within the matrix. These cease to form fresh matrix and in