THE STRUCTURE OF THE ELEMENTARY TISSUES. 79 



dentine, each of them early takes the shape of the crown of the tooth 

 to which it corresponds. As the dentine increases in thickness the 

 p.ipillae diminish, and at last when the tooth is cut only a small amount 

 of the papilla remains as the dental pulp, and is supplied by vessels and 

 nerves which enter at the end of the root. The shape of the crown of 

 the tooth is taken by the corresponding papilla, and that of the single 

 or double root by the subsequent constriction below the crown, or by 

 division of the lower part of the papilla. The number of roots being 

 foreshadowed by the number of arteries going to the papilla. The roots 



Fig. 80. Vertical transverse section of the dental sac, pulp, etc., of a kitten, a, Dental papilla 

 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 h, outer layer of dental sac. X 14. (Thiersch.) 



are not completely formed at the time of the eruption of the teeth, but 

 subsequently. 



The enamel cap is found later on to consist (fig. 80, d, ,/) of four 

 parts: (1) an inner membrane, composed of a layer of columnar epithe- 

 lium in contact with the dentine, called enamel cells; (2) outside of 

 these one or more layers of small polyhedral nucleated cells (stratum in- 

 termedium of Hannover); (3) an outer membrane of several layers of 

 epithelium; (4) a middle membrane formed of a matrix of non-vascular 

 gelatinous tissue, containing stellate cells. The enamel is formed by the 

 enamel cells of the inner membrane, by the deposit of a keratin-like 

 substance, which subsequently undergoes calcification and forms the first 

 layer. Other layers are formed in the sam manner, the cells retiring 



