312 



THE TEETH. 



present, when examined with a high magnifying power, a tesselated 

 a]3pearance (fig. 213, b). 



Fig. 213. 



213. — Enamel Fibres (from Kolliker). 350 Diameters. 



■ A, fragments and single fibres of the enamel, isolated bytlie action of hydrochloric acid. 

 B, surface of a small fragment of enamel, showing the hexagonal ends of the fibres. 



When submitted to the action of dihite acids, the enamel is ahnost 

 entirely dissolved, and leaves scarcely any discerniljle traces of animal 

 matter. By the action of an acid, the enamel of newly formed or still 

 growing teeth may be broken up, and its structural elements more 

 easily distinguished. 



It is further found, on treatment with acid, that a very thin mem- 

 brane called by Kolliker " cuticle of the enamel," — and by Busk and 

 Huxley " Nasmytli's memdrane " entirely covers the enamel of unworn 

 teeth upon its outer surface (fig. 212, a). This membrane forms a pro- 

 tective covering to the enamel. It is of an epithelial and horny 

 nature, and obstinately withstands prolonged boiling with water as 

 well as the action of acids and other re-agents. 



The crusta petrosa or cement (fig. 201, 3) is the third sub- 

 stance which enters into the formation of the teeth. This is a layer 

 of true bone, slightly modified in structure, and investing that part of 

 the dentine which is not protected by the enamel. It covers the whole 

 fang, towards the lower end of which it becomes gradually thicker, 

 and is specially developed at the apex, and along the grooves of the 

 compound fangs. As life advances, the cement generally grows thicker, 

 especially near the point of the fang, where it sometimes blocks up the 

 orifice leading to the pulp-cavity. 



The crusta petrosa is lamellar in structure, and contains lacuna) and 

 canaliculi resembling those of bone but larger and more irregular (fig. 

 208, 1). In the deeper layers of the cement the fine canaliculi sometimes 

 anastomose with some of the terminal tubules of the subjacent dentine, 

 as already stated. Wliere the cement is very thick it may contain vascu- 

 lar canals, analogous to the Haversian canals of bone. On the deciduous 

 teeth the cement is thinner, and contains fewer cells. It has been 

 shoAvn by Sharpey that perforating fibres, similar to those of ordinary 



