BONE (AS. ban, Ger. Rein, bone, leg: cf. Icel. befall, straight). The hard material of the skeletons or frameworks of mammalian animals, reptiles, most fishes, and birds. in the embryo, it is preceded by cartilage (q.v.). which con sists of cells massed together, except in the flat bones, as those of the skull and shoulder-blade, of which the primary foundations are to a great extent of fibrous tissue. Points centres of ossitieation form, the cells alter their form and arrangement, and a deposit of earthy materials, phosphate and of lime, takes pimp, rendering the former flexible substance rigid. By soaking a bone in dilute mineral acid, we can dissolve these earthy matters and render it again flexible, ossein• or bone cartilage, alone remaining. On the other hand, if we expose it to intense heat, the animal matter (gelatin) is burned, and though the bone retains at first its form• a slight touch will cause its now un earthy matter to crumble. away. We see, in the ill-nourished children of large towns, too many examples of how necessary is a proper relation of these two elements of bone to each other. In the disease called rickets (q.v.). the earthy matter is deficient, and the too flexi ble leg-bones bend under the weight of the trunk. In the aged person, again, the bone substance becomes more densely packed with earthy matter and becomes brittle, rendering the bones peculiarly liable to fractures. Ani mals lower in tile settle than vertebrates have, in place of bone, mineral eoneretions or incrustations, principally consisting of ear hmate of lime. Besides constituting the skele ton, bone occurs in plates in certain animals, as in turtles' integument, the heart of certain ruminants, the tongue of certain fishes, etc.
The specific gravity of bone is between 1.87 and 1.97. It is twice as strong as oak, and a. cubic inch of it will support 5000 pounds.
The bones of the skeleton are classified ac cording to their shapes—viz., as long bones (e.g. the thigh-bone and arm-bone), flat bones (as the shoulder-blade and skull-bones), short and irregular bones (as those of the wrist or vertebnr). The substance of bones is arranged differently in different parts—either bard and close, which is called the condensed substance, or loose and reetieulated. called the cancellated structure. Long bones have a shaft of hard substance. terminating at each end in soft or caneellated structure; in the latter situations, the bone is more expanded and rounded off to enter into the formation of a joint. Irregular
bones consist of a shell of condensed tissue, in closing a mass of eancellated structure, and are smoothed off into surfaces adapted to those of the adjoining bones. Flat bones consist of two layers of hard tissue, with an intermediate can eellated structure. Anatomists class as mixed bones those which are both long and flat, as the ribs, the breast-bone. and the lower jaw.
IlisToLocv. Bone belongs under the general histological classification of connective tissue, its hardness being due to the deposition in its intercellular substance of calcareous matter. Bone may be divided into two varieties—com pact and spongy. The former makes Up the hard outer shell of the various bones, while the latter is found in the interior. Bones are cov ered externally by the periosteum (q.v.). They are largely supplied with blood-vessels. which arc carried into them from the periosteum, the larg est being those which enter the en nevi hated ends of the long bones. The long bones are hollow, the central portion being filled with marrow. This ventral canal is lined by a membrane which receives a special artery for the supply of the adjacent compact tissue. This vessel enters the bone rather above the mid-point, and divides into two branches—an ascending and a descend ing—which subdivide and anastomose with the already mentioned as entering the ean eellated hone. After the arteries enter the com pact tissue of hone they run in small capillary canals, which permeate the bone and anastomose, forming oldong loops or meshes. The veins of bone are also contained in these eanals. and larger than the arteries. These capillary canals in which the. vessels run an. named 'Havel-slam,' after their discoverer, (lepton Havers, an old English anatomist. They vary in diameter from to .a of an inch. They take a longitudinal direction, and if a transverse section is exam ined under a mieroscope it appears pierced with holes, which arc the Ilaversian canals cut across. Each canal is surrounded by its own layers of bone, forming a sort of thick-walled cylinder with the Ilaversian canal as the centre.. Be tween the layers are seen numbers of minute spaces arranged concentrically around the Ha.