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HAND-BOOK OF PHYSIOLOGY. 



deeper layer of odontoblasts, which are wedged in between the cells of 

 the superficial layer (Fig. 64) which form the tubules only. 



Since the papillae are to form the main portion of each tooth, i.e., the 

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

 to form. As the dentine increases in thickness, the papillae 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 fang. The shape of the crown of the tooth is 

 taken by the corresponding papilla, and that of the single or double fang 



by the subsequent constriction be- 

 low the crown, or by division of the 

 lower part of the papilla. 



The enamel cap is found later on 

 to consist (Fig. 65) of three parts: 

 (a) an inner membrane, composed 

 of a layer of columnar epithelium in 

 contact with the dentine, called ena- 

 mel cells, and outside of these one or 

 more layers of small polyhedral nu- 

 cleated cells (stratum intermedium 

 of Hannover); (V) an outer mem- 

 brane of several layers of epithelium; 

 (c) a middle membrane formed of a 

 matrix of non-vascular, gelatinous 

 tissue, containing a hyaline intersti- 

 tial substance. The enamel is formed 

 by the enamel cells of the inner 

 membrane, by the elongation of 

 their distal extremities, and the di- 

 rect conversion of these processes 

 into enamel. The calcification of 

 the enamel processes or prisms takes 

 place first at the periphery, the cen- 

 tre remaining for a time transparent. The cells of the stratum interme- 

 dium 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 periosteum. 



In this manner the first set of teeth, or the milk-teeth, are formed; 



and each tooth, by degrees developing, presses at length on the wall of the 



sac enclosing it, and, causing its absorption, is cut, to use a familiar phrase. 



The temporary or milk-teeth have only a very limited term of existence. 



FIG. 65. 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 ; gr, inner layer, and h, outer layer 

 of dental sac. X 14. (Thiersch.) 



