TEETH. 



[ 756 ] 



TEETH. 



of the ciown of the teeth. It is thickest at ' 

 the opposing surface, decreasing towards 

 the neck, where it terminates. The cuticula 

 is an extremely resistant investment to the 

 exposed portions of the teeth, and wliich 

 disappears when they are mature. It is 

 separable after the action of muriatic acid, 

 and may be tinted with a solution of nitrate 

 of silver, which causes the appearance of 

 figures like large epithelial cells. The 

 enamel has a fibrous aspect, and appears of 

 a bluish-white colour by reflected light, 

 and of a greyish browu by transmitted 



Fig. 735. 



light. It is very brittle, and so hard as to 

 strike fire with steel. It consists of nume- 

 rous solid fibres or prisms (fig. 734), about 

 l-()000 to 1-5000" m breadth, mostly six- 

 sided, more or less wavy, slightly varicose, 

 and transversely striped. These usually 

 extend throughout the thickness of the 

 enamel, and are placed in a direction gene- 

 rally perpendicular to the surfiice of the 

 portions of the ivory which they cover 

 (figs. 726, 731). The form of the 'fibres is 

 best seen by viewing their ends in a trans- 

 verse section (fig. 735). The prisms do not 



Fig. 736. 



^-iSs, 



Fig. 735. Sc.rface of the enamel, with the ends of the enamel-fibres, from the tooth of a calf. Magnified 350 

 diameters. 



Fig. 736. Diagram showing the develojiment of a milk-tooth, and the corresponding permanent tooth, a, furrow ; 

 6, the same with the papilla ; c, the same closing, with the commencement of the reserve cavity ; d, the same, further 

 closing; e, follicle completely formed, with the reserve cavity; y, the reserve cavity receding; g, the same, with a 

 tooth-germ ; h, the alveoli of both capsules formed, the milk-tooth being through the gum ; i, the same further 

 advanced, the neck of the capsule forming a solid cord. 



run exactly parallel with each other, but 

 are arranged in groups or zones, the fibres 

 of which cross each other. The fibres 

 are readily isolated before they have be- 

 come so developed as to be hard, and when 

 very slightly acted upon by muriatic acid. 

 It is doublfid if sometimes the ivory-tubes 

 extend into the enamel. 



Two kinds of dark bands or stripes are 

 seen traversing the enamel (fig. 731). The 

 direction of one of these coincides pretty 

 nearly with that of the fibres, and it arises 

 from the crossing of the zones of fibres, 

 allowing more or less light to pass through, 

 the bauds being light and dark. The other 

 set (fig. 73l,ff) consists of arched, brownish 

 stripes, indicating the laminated structure 



of the enamel. Under the polariscope, a 

 third set becomes visible, arising from the 

 variable inclination of the axes of the fibres 

 to the plane of polarization. 



The en.imel is often traversed by cracks, 

 mostly running parallel with the fibres, and 

 containing air in dry teeth. 



Pulp. — The pulp of the teeth is the vas- 

 cular and nervous matrix of the dentine 

 and the remains of the original tooth-papilla. 

 It contains a few blood-vessels and nerves, 

 being connected with the periosteum and 

 base of the sockets of the jaw. The princi- 

 pal part is made of indistinctly fibrous con- 

 nective tissue, containing numerous cells; 

 and it appears to be quite cavernous from 

 the breaking up of the terminal capillaries. 



