DIGESTION. 



229 



a middle membrane formed of a matrix of non-vascular gelatinous tissue 

 containing a hyaline interstitial substance. The enamel is formed by 

 the enamel cells of the inner membrane, by the elongation of their distal 



FIG. 172. Part of section of developing tooth of a young rat, showing the mode of desposition 

 of the dentine. Highly magnified, a, outer layer of fully formed dentine; 6, uncalcified matrix 

 with one or two nodules of calcareous matter near the calcified parts; c, odontoblasts sending pro- 

 cesses into the dentine; d, pulp. The section is stained in carmine, which colors the uncalcified 

 matrix but not the calcified part. (E. A. Schiif er.) 



extremities, and the diroct conversion of these processes into enamel. 



The calcification of the enamel processes or prisms takes place first at the 



periphery, the centre remaining for 



a time transparent. The cells of 



the stratum intermedium are used 



for the regeneration of the enamel 



cells, but these and the middle 



membrane after a time disappear. 



The cells of the outer membrane 



give origin to the cuticle of the 



enamel. 



The cement or crusta petrosa is 

 formed from the tissue of the tooth 

 sac, the structure and function of 

 which are identical with those of 

 the osteogenetic layer of the perios- 

 teum. 



In this manner the first set of 

 teeth, or the milk-teeth, are formed; 

 and each tooth, by degrees develop- 

 ing, presses at length on the wall 

 of the sac inclosing it, and, caus- 

 ing its absorption, is cut, to use a 

 familiar phrase. 



The temporary or milk-teeth, 

 are speedily replaced by the growth 

 of the permanent teeth, which push their way up from beneath them, ab- 

 .sorbing in their progress the whole of the fang of each milk-tooth, and 



FIG. 173. Vertical transverse section of the 

 dental sac, pulp, etc., of a kitten, a, dental pa- 

 pilla or pulp; 6, the cap of dentine formed upon 

 the summit; c, its covering of enamel; d, 

 inner layer of epithelium of the enamel organ ; 

 e, gelatinous tissue; /, outer epithelial layer of 

 the enamel organ; g, inner layer, and ft, outer 

 layer of dental sac. x 14. (Thiersch.) 



