

BONE 



93 



which they are enclosed constituting the lacunae. As the osteogenetic fibers grow 

 out to the periphery they continue to calcify, and give rise to fresh bone spicules. 

 Thus a network of bone is formed, the meshes of which contain the bloodvessels 

 and a delicate connective tissue crowded with osteoblasts. The bony trabeculse 

 thicken by the addition of fresh layers of bone formed by the osteoblasts on their 

 surface, and the meshes are correspondingly encroached upon. Subsequently, 

 successive layers of bony tissue are deposited under the periosteum and around 

 the larger vascular channels which become the Haversian canals, so that the bone 

 increases much in thickness. 



INTERCARTILAGINOUS OSSIFICATION. Just before ossification begins the mass is 

 entirely cartilaginous, and in a long bone, which may be taken as an example, the 

 process commences in the center and proceeds toward the extremities, which for 

 some time remain cartilaginous. Subsequently a similar process commences in 

 one or more places in those extremities and gradually extends through them. 

 The extremities do not, however, become joined to the body of the bone by bony 

 tissue until growth has ceased ; between the body and either extremity a layer of 

 cartilaginous tissue termed the epiphysial cartilage persists for a definite period; 



The first step in the ossification of 

 the cartilage is that the cartilage cells, 

 at the point where ossification is com- 

 mencing and which is termed a center 

 of ossification, enlarge and arrange 

 themselves in rows (Fig. 79). The 

 matrix in which they are imbedded 

 increases in quantity, so that the cells 

 become further separated from each 

 other. A deposit of calcareous material 

 now takes place in this matrix, between 

 the rows of cells, so that they become 

 separated from each other by longi- 

 tudinal columns of calcified matrix, 

 presenting a granular and opaque ap- 

 pearance. Here and there the matrix 

 between two cells of the same row also 

 becomes calcified, and transverse bars 

 of calcified substance stretch across 

 from one calcareous column to another. 

 Thus there are longitudinal groups of 

 the cartilage cells enclosed in oblong 

 cavities, the walls of which are formed 

 of calcified matrix which cuts off all 

 nutrition from the cells; the cells, in 

 consequence, atrophy, leaving spaces 



called the primary areolse. 

 At the same time that this process 

 is going on in the center of the solid 

 bar of cartilage, certain changes are 

 taking place on its surface. This is 



t covered by a very vascular membrane, 

 the perichondrium, entirely similar to the 

 embryonic connective tissue already 

 described as constituting the basis of 

 membrane bone; on the inner surface of this that is to say, on the surface in 

 contact with the cartilage are gathered the formative cells, the osteoblasts. By 

 the agency of these cells a thin layer of bony tissue is formed between the peri- 



FIQ. 79. Section of fetal bone of cat. ir. Irruption 

 of the subperiosteal tissue, p. Fibrous layer of the perios- 

 teum, o. Layer of osteoblasts. im. Subperiosteal bony 

 deposit. (From Quain's "Anatomy," E. A. Schafer.) 



