CH. V.] 



ENAMEL 



55 



that process is complete, entirely hidden by the densely deposited lime 

 salts " (Mummery). 



The tubules of the dentine, the average diameter of which at their 

 inner and larger extremity is ^Yo of an inch, contain fine pro- 

 longations from the tooth-pulp which are processes of the odonto- 

 blasts, the columnar cells lining the pulp-cavity together with nerve- 

 fibrils; the relation of these processes to the tubules in which they 

 lie is precisely similar to that of the processes of the bone-corpuscles 

 to the canaliculi of bone. The outer portion of the dentine, under- 

 lying the cement, and the enamel to a much lesser degree, forms a 

 more or less distinct layer termed the granular or interglobular layer 

 (fig. 66). It is characterised by the presence of a number of irregular 

 minute cavities. The explanation of these will be seen when we 

 study the development of a tooth. 



Enamel. 



Enamel is by far the hardest tissue in the body ; it is composed of 

 the same inorganic compounds that enter into the composition of 

 dentine and bone. According to Tomes, it contains no animal matter 

 at all, and only 2 or 3 per cent, of water. Gelatin is a characteristic 



FIG. 67. Enamel prisms. A, fragments and single prisms of the transversely-striated enamel, isolated 

 by the action of hydrochloric acid. B, surface of a small fragment of enamel, showing the hexa- 

 gonal ends of the fibres with darker centres, x 350. (Kcilliker.) 



product of connective tissue, and enamel is not a connective tissue, 

 but is epithelial in origin. 



Examined under the microscope, enamel is found composed of six- 

 sided prisms (figs. 67, 68) 5 ^ ()() of an inch in diameter, which are set 

 on end on the surface of the dentine, and fit into corresponding 

 depressions in the same, 



