THE SKELET.\L SYSTEM 



207 



its anastomosing osseous trabecula;, with their enclosed spaces containing osteo- 

 genetic tissue and marrow, constitute primary spongy hone. 



At either end of the ossification centre the cartilage presents a special struc- 

 ture. Nearest the centre the cell spaces are enlarged, flattened, arranged in 

 rows and contain shrunken cells. Some of the walls break, down and irregular 

 spaces are formed. The ground substance is calcified. Passing away from the 

 ossification centre, the cell spaces become less flattened, still arranged in rows, 

 the contained cells larger, and there is a lesser degree of calcification. This 

 area passes over into an area of hyaline cartilage which blends without distinct 

 demarcation with the ordinary embryonal cartilage of the rest of the shaft. 

 The area of calcified cartilage at either end of the ossification centre is known 

 as the calcification zone and everywhere precedes the formation of true bone 

 (Fig. 12,0. 



3. Subperiosteal or subperichondrial development (Fig. 123) has already 

 been la'rgely described in connection with intramembranous ossification, and 



i h g f 



Fig. 126. — ^Intracartilaginous Bone Development. Same specimen as Fig. 123. 

 (X3Sc), showing bone being deposited around one of the trabeculae of cartilage, a, 

 Blood-vessel; b, bone; c, cartilage remains; d, bone cell; e, cartilage cell space; f, osteo- 

 blasts; g, osteogenetic tissue; A, lamella of bpne;j, connective-tissue cells; ;, cartilage cell. 



differs in no important respect from the latter. It always accompanies one of 

 the other forms of ossification. Bone appears beneath the perichondrium some- 

 what earlier than within the underlying cartilage. Beneath the perichondrium 

 is a layer of richly cellular osteogenetic tissue. The cells of this tissue nearest 

 the cartilage become osteoblasts and arrange themselves in a single layer along 

 its surface. Under their influence bone is laid down on the surface of the carti- 

 lage in the same manner as in intramembranous ossification. 



Intracartilaginous and subperiosteal bone can be easily differentiated by the 

 presence of cartilaginous remains in the former and their absence in the latter. 



All bone is at first of the spongy variety. When this is to be converted into 

 compact bone, there is first absorption of bone by osteoclasts, with increase in 

 size of the marrow spaces and reduction of their walls to thin plates. These 

 spaces are now known as Haversian spaces. 



Within these new bone is deposited. This is done by osteoblasts which lay 



