62 ELEMENTS OF HISTOLOGY. [Chap. vn. 



to a nucleated protoplasmic remnant, which gradually 

 becomes branched and then represents a bone cell. 

 The osseous matrix is at first a soft fibrillar tissue, 

 but is gradually and uniformly impregnated with 

 lime salts. This impregnation always starts from the 

 centre of ossification. 



76. Wherever absorption of calcified cartilage or of 

 osseous substance is going on, we meet with the multi- 

 nucleated huge protoplasmic cells, called the myelo- 

 plaxes of Robin. Kolliker showed them to be impor- 

 tant for the absorption of bone matrix, and called 

 them therefore Osteoclasts (Fig. 37). For cartilage they 

 may be called Chondroclasts. When concerned in tho 

 absorption we find these myeloplaxes situated in 

 smaller or larger pits, which seem to have been pro- 

 duced by them ; these absorption pits or lacunas on the 

 surface of bones are called Howship's lacunae. They 

 invariably contain numbers of osteoclasts. It can, 

 however, be shown that myeloplaxes are also con- 

 cerned in the formation of bone, by giving origin to a 

 number of new osseous zones with their bone cells. 

 In the earliest stages of development of the foetal jaw 

 this process is seen with great distinctness (Fig. 37). 



77. Dentine forms the chief part of a tooth. It 

 consists of a petrified matrix, in which are numbers of 

 perpendicularly-arranged canals the dentinal tubes 

 containing the dentinal fibres. It is in some respects 

 similar to bone, although differing from it in certain 

 essentials. It is similar, inasmuch as it is developed 

 in like manner by some peculiarly transformed 

 embryonal connective tissue viz., by the tissue of the 

 embryo tooth papilla and inasmuch as cells are con- 

 cerned in the production both of the petrified matrix 

 (impregnated with lime salts), and of the processes of 

 the cells contained in its canals the dentine fibres. 

 The details of structure and distribution will be 

 described in connection with the teeth. 



