THE SKELET.\L SYSTEM 



203 



of the base of the skull develop in a matrix of cartilage. This is known as in- 

 tracartilaginous or endochondral ossification. The flat bones, those of the vault 

 of the cranium and most of the bones of the face, are developed in a matrix of 

 iibrillar connective tissue— inlraniembranous ossification. A form of bone 

 development, similar in character to intramembranous, occurs in connection 

 with both intramembranous ossification and intracartilaginous ossification. 

 This consists in the formation of bone just beneath the perichondrium — sub- 

 perichondrial ossification — or, as with the development of bone perichondrium 

 becomes periosteum — subperiosteal ossification. 



There are thus three forms of bone development to be considered: (i) 

 Intramembranous, (2) intracartilaginous, and (3) subperiosteal. 



Fig. 1 2 1. — -Intramembranous Bone Development. Vertical section through parietal 

 bone of human foetus. X 160. (Technic i, p. 208.) a, Osteoblasts; b, bone trabeculae; 

 c, osteoclasts lying in Howship's lacunae; d, internal periosteum; e, bone cells;/, calcified 

 fibres; g, osteogenetic tissue; h, external periosteum (pericranium.) 



I. Intramembranous Development (Fig. 121). — In intramembranous ossifi- 

 cation the matrix in which the bone is developed is connective tissue. The 

 process of bone formation begins at one or more points in this matrix. These 

 are known as ossification centres. Here some of the bundles of white fibres be- 

 come calcified, i.e., become impregnated with lime salts. There is thus first 

 established a centre or centres of calcification. Between the bundles of calcified 

 fibres the connective tissue is rich in cells and vascular, and from its future role 

 in bone formation is known as osteogenetic tissue (Fig. 121). Along the surfaces 

 of the calcified fibres certain of the osteogenetic cells arrange themselves in a 

 single layer (F'gs. 121 and 122). These are now known as osteoblasts or "bone 



