1104 



GENERAL ANATOMY OR HISTOLOGY, 



its surface. This is covered by a very vascular membrane, the pericJiondrium, 

 entirely similar to the embryonic connective tissue already described as constituting 

 the basis of membrane-bone, on the inner surface of -which, that is to say, on the 

 surface in contact with the cartilage, are gathered the formative cells, the osteo- 

 blasts. By the agency of these cells a thin layer of bony tissue is being formed 

 between the perichondrium and the cartilage, by the intramembranous mode of 

 ossification just described. There are then, in this first stage of ossification, t\vo 

 processes going on simultaneously : in the centre of the cartilage the formation 



of a number of oblong spaces, formed 

 withered cartilage-cells, and on the 

 surface of the cartilage the formation 



O 



of a layer of true membrane-bone. 

 The second stage consists in the pro- 

 longation into the cartilage of proc- 

 esses of the deeper or osteogenetic 

 layer of the perichondrium, which has 

 now become periosteum (Fig. 632, ir). 

 The processes consist of blood-vessels 

 and cells osteoblasts or bone-formers, 

 and osteoclasts, or bone-destrovers. 



of calcified matrix and containing the 



FIG. 634. Transverse section from the femur of a 

 human embryo about eleven weeks old. a. A med- 

 ullary sinus cut transversely ; and b, another, long- 

 itudinally, c. Osteoblasts. a. Newly-formed osseous 

 substance of a lighter color, e. That of greater age. 

 /. Lacunse with their cells, g, A cell still united to 

 an osteoblast. 



Fiu. 635. Vertical section from the edge of the 

 ossifying portion of the diaphysis of a metatar- 

 sal bone from a foetal calf. (After Miiller.) a. 

 Ground-mass of the cartilage, b. Of the bone. c. 

 Newly-formed bone-cells in profile, more or less 

 imbedded in intercellular substance, d. Medul- 

 lary canal in process of formation, with vessels 

 and medullary cells, e. f. Bone-cells on their 

 broad aspect, g. Cartilage-capsules arranged in 

 rows, and partly with shrunken cell-bodies. 



The latter are similar to the giant-cells (myelo-plaques) found in marrow, and 

 they excavate passages through the new-formed bony layer by absorption, and 

 pass through it into the calcified matrix (Fig. 632). Wherever these processes 

 come in contact with the calcified walls of the primary areolse they absorb it, 

 and thus cause a fusion of the original cavities and the formation of larger spaces, 

 which are termed the secondary areolce (Sharpey) or medullary spaces (Miiller). 

 In these secondary spaces the original cartilage-cells having disappeared, become 

 filled with embryonic marrow, consisting of osteoblasts and vessels, and derived 

 in the manner described above, from the osteogenetic layer of the periosteum 

 (Fig. 633). 



