THE CONNECTIVE TISSUES. 



meshes of the tissue contain great numbers of soft, plastic connective- 

 tisjfue elements, the marrow- cells ; many of these, in actively- 

 growing bone, become the osteoblasts. In yellow marrow the 

 majority of the marrow-cells have undergone transformation into 

 fat-cells. Additional huge, irregular, multinucleated, protoplasmic 

 masses are occasionally encountered; these are the giant-cells, or 

 myeloplaxes (Robin), and are of interest as being elements es- 

 pecially concerned in the absorption of osseous tissue, being iden- 

 tical with the osteoclasts (Kolliker). These cells, with their nuclei, 

 offer an example of what formerly was described as the endogenous 

 mode of cell-formation. 



Dentine is analogous to bone, although differing in details of 

 arrangement, since it is derived from embryonal connective tissue. 

 The matrix becomes calcified, and contains, embedded within the 

 ground-substance, numerous long, parallel, partly-branched tubes, 

 the dentinal tubules. These correspond with the lacunae of bone, 

 enclosing in some places delicate processes, the dentinal fibres. 

 A more extended account of the structure and development of den- 

 tine will be found in connection with the structure of the teeth. 



Development of Bone. With the exception of the bones of 

 the vault of the cranium, of the face, and of part of the lower jaw, 

 the skeleton is mapped out, in its foetal condition, by solid cartilages 

 which correspond in form more or less 

 closely with the future bones. The 

 primary embryonal cartilage is of 

 the hyaline variety, being extremely 

 rich in cells, many of which are engaged 

 in division; the cell-groups are separated 

 by a relatively small amount of inter- 

 cellular substance, and the outer surface 

 of these solid cartilages is closely in- 

 vested by an important membrane, the 

 primary periosteum. 



When bone is formed at the centres 

 of ossification within the cartilage, it is 

 termed endochondral bone ; when 

 formed directly from and beneath the 

 periosteum, periosteal bone. While quite complicated in its 

 sequence of changes, it must be remembered that endochondral 

 development results in the formation of structures which are largely 

 temporary, and which finally, for the most part, suffer absorption. 

 The permanent bones of the skeleton are, chiefly, the products of 



FIG. 59. 



n 



Primary embryonal cartilage repre- 

 senting one of the carpal bones : p, 

 perichondrium, or primary periosteum ; 

 n, nutrient canals extending from the 

 periphery. 



