THE SKELETAL SYSTEM 



205 



which they have hollowed out in the bone. Between the outer surface of the 

 bone and the pericranium is a layer of osteogendic tissue, the innermost cells of 

 which are arranged as osteoblasts along the outermost osseous lamellae. Here 

 they are constantly adding new bone beneath the pericranium. This new bone 

 is laid down, not in flat, evenly disposed layers, but in the form of anastomos- 

 ing trabecular enclosing marrow spaces. __ ^ — --, 



It is thus seen that subperiosteal bone, like in- v 



tramembranous, is at first of the spongy variety, 

 and that with the development of the cranium the 

 original intramembranous bone is entirely ab- 

 sorbed, together with much of the subperiosteal. 



2. Intracartilaginous Development.^In this 

 form of ossification an embryonal type of hyaline 

 cartilage precedes the formation of bone, the carti- 

 lage corresponding more or less closely in shape to 

 the future bone (Fig. 123). Covering the surface 

 of the cartilage is a membrane of fibrillar connec- 

 tive tissue, ih.c perichondrium or primary periosteum. 



In most of the long bones the earliest changes 

 take place wiihin the cartilage at about the centre 

 of the shaft (Fig. 123). Here the cartilage cells in- 

 crease in size and in number in such a way that 

 several enlarged cartilage cells come to lie in a 

 single enlarged cell space, and the cartilage as- 

 sumes the character of hyaline cartilage. The cell 

 groups next arrange themselves in rows or columns, 

 which at first extend outward in a radial manner 

 from a common centre, but later lie in the long axis 

 of the bone. During these changes in the cells there 

 is an increase in the intercellular matrix and a de- 

 posit there of calcium salts. In this way the car- 

 tilage becomes calcified, the area involved being 

 known as the calcification centre. Further growth 

 of cartilage at the calcification centre now ceases 





P'ic. 123. — Intracartilagin- 

 ous Bone Development. 

 Longitudinal section of one of 

 the bones of embryo sheep's 

 and, as growth of cartilage at the ends of the bone ' foot, showing ossification cen- 



continues, the central portion of the shaft appears *^'"^- , ^3°-. ,^^ ?'ui^' §' 

 ' , , ^ ... 20Q.) a, Periosteum; 6, blood- 



constricted. The changes up to this point seem, to vessels; c, subperiosteal bone; 

 be preparatory to actual bone formation. d, intracartilaginous bone; e, 



OssificaUon prefer begins by blood-vessels from ^f^^d;,S^;_ 

 the periosteum^ pushing their way into the calci- calcification zone, 

 fied cartilage at the calcification centre, carrying 



with them some of the osteogenetic tissue from beneath the periosteum. These 

 blood-vessels wdth tlieir accompanying osteogenetic tissue are known as perios- 

 teal buds (Fig. 124). Osteoblasts now develop from the osteogenetic tissue 

 and appear to dissolve the calcified cartilage from in front of the advancing 



iThe term "periosteum" is admissible from the fact that the first bone actually 

 formed is beneath the perichondrium, which thus becomes converted into periosteum 



