32 



THE GAT. 



[chap. II. 



and canaliciili. It is tlilnnest towards the cervix of cacli tooth, and 

 thickens towards the apex of each fang, and there it may even 

 contain vascular canals like the Haversian canals of bone tissue.* 



The Enamel is so mineralized a structure that it only contains 

 about 3 1 per cent, of animal matter, while it has 90 per cent, of 

 phosphate of lime. It consists of a multitude of slender, solid, 

 undulating, hexagonal rods, closely adjusted to each other, and 

 about 5-yVn of an inch in diameter. Each rod is attached by one 

 end to a minute depression of the surface of the dentine, and thence 



extends outwards, its distal part being 

 at right angles to the external surface 

 of the enamel. 



§ 20. We have seen that hair and claws 

 are epidermic dermal appendages, but 

 teeth are appendages of the dermis. They 

 are not altogether so, however; for though 

 the dentine is formed by ossification of a 

 process of the cerium, and cement by calcifi- 

 cation of the connective tissue surrounding 

 that papilla, yet the enamel has a different, 

 and indeed an epidermal origin. It is formed 

 from a depression of the epithelium of the 

 gum, which dips in till it becomes applied 

 to the apex of the rising dermal papilla, 

 which last is destined, by its calcification, 

 to form the bulk of the tooth. Having 

 thus applied itself to invest the crown of 

 the nascent tooth, it calcifies and so be- 

 comes the enamel. 



Thus each tooth has a double nature. By 

 its dentine and cement it is dermal, but 

 its enamel is a modification of the epi- 

 dermis. 



Each permanent tooth takes its origin 

 in a cavity of the jaw, placed just behind 

 the milk-tooth it is destined to succeed. 

 A little process from the inflected epi- 

 thelium (or " enamel organ ") which 

 forms the enamel of the milk-tooth, is 

 given off to invest the minute papilla 

 which is to grow into the permanent tooth. 

 As the new tooth is formed it rises in the gum, the space inter- 

 vening between it and its successor becoming richly supplied with 

 blood-vessels. The substance of the milk-tooth then becomes 



Fig. 15. — Thin Si:ction oftiik 

 Enamel and a i-Ain' or the 



nCNTINE, 300 DIAMKIKUS. 



a. External .surfaco. 



h. Tlio rods or ssolid, six-sided 



])nsiii.s. 

 (/. Tuliuli ol'tlu! (U'litiiic. 

 c. Clefts wldcli occasionally 



exist ill the ileei> part 



of thu ciuiinel. 



* A sub.staiicc called odco-dcnlinc i.s 

 soniutiincs jirodiiccil by tho o.s.sificatiou 

 of thu pulp itsuir. It lias vascular quasi 

 Haversiau canals, surrounded ■ by con- 



centric laniellie, and is so far like bone. 

 On tlic other hand, tubuli radiate from 

 these canals, which tubuli are larger 

 than the canaliculi of bone. 



