32 



THE CAT. 



[CHAP. ii. 



and canaliculi. It is thinnest towards the cervix of each tooth, and 

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

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



The Enamel is so mineralized a structure that it only contains 

 ahout 3J 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 

 ahout 5 ~ of an inch in diameter. Each rod is attached hy one 

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



extends outwards, its distal part heing 

 at right angles to the external surface 

 of the enamel. 



20. We have seen that hair and claws 

 are epidermic dermal appendages, hut 

 teeth are appendages of the dermis. They 

 are not altogether so, however; for though 

 the dentine is formed hy ossification of a 

 process of the corium, and cement hy 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, hy 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. 



Fig. 15. THIN SECTIOH OF THE 

 ENAMEL AND A PART OF THE 

 DENTINE, 300 DIAMETERS. 



a. External sin-face. 



i. The rods or solid, six-sided A little process from the inflected epi- 

 thelium (or " enamel organ ") which 



prisms. 



d. Tubuli of the dentine, 

 c. Clefts which occasionally 



exist in the deep part 



of the enamel. 



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 



* A substance called ostco-dentinc is 

 sometimes produced by the ossification 

 of the pulp itself. It has vascular quasi 

 Haversiaii canals, surrounded by con- 



centric lamellse, and is so far like bone. 

 On the other hand, tubuli radiate from 

 these canals, which tubuli are larger 

 than the canaliculi of bone. 



