Various opinions have been entertained in regard to the formation, or growth, of bone. Some anatomists have supposed that all bone is formed in cartilage. But this is not true, for there is an _intra-membranous_, as well as an _intra-cartilaginous_, formation of bone, as may be seen in the development of the cranial bones, where the gradual calcification takes place upon the inner layers of the fibrous coverings. Intra-cartilaginous deposit is found in the vicinity of the blood-vessels, within the cartilaginous canals; also, there are certain points first observed in the shafts of long bones, called _centers of ossification_. These points are no sooner formed than the cartilage corpuscles arrange themselves in concentric zones, and, lying in contact with one another, become very compact. As ossification proceeds, the cup-shaped cavities are converted into closed interstices of bone, with extremely thin lamellæ, or layers. These, however, soon increase in density, and no blood-vessels can be observed within them. [Illustration: Fig. 7. Vertical section of cartilage near the surface of ossification. _1_. Ordinary appearance of the temporary cartilage. _1_'. Portion of the same more highly magnified. _2_. The cells beginning to form into concentric zones. _2_'. Portion more magnified. _3_. The ossification is extending in the inter-cellular spaces, and the rows of cells are seen resting in the cavities so formed, the nuclei being more separated than above. _3_'. Portion of the same more highly magnified.] [Illustration: Fig. 8. Thigh-bone, sawn open lengthwise.] [Illustration: Fig. 9. Lower end of the thigh-bone sawn across, showing its central cavity.] The bony plates form the boundaries of the _Haversian_, or nutritive canals of the bones. In the _second stage of ossification_, the cartilage corpuscles are converted into bone. Becoming flattened against the osseous lamellæ already formed, they crowd upon one another so as to entirely obliterate the lines that distinguish them; and, simultaneously with these changes, a calcareous deposit takes place upon their interior. Bones grow by additions to their ends and surfaces. In the child, their extremities are separated from the body of the bone by layer of cartilage, and the cancellated, or cellular structure, which remains for a time in the interior, represents the early condition of the ossifying substances.