STRUCTURE OF TEETH; ENAMEL, DENTINE. 



175 



their length corresponds with the thickness of the layer which they form; 

 and the two surfaces of this layer present the ends of the prisms, which are 

 usually more or less regularly hexagonal. The course of these prisms is 

 generally wavy ; but their curves are for the most part parallel to each other. 

 In the perfect state, the Enamel contains but an extremely minute quantity of 

 animal matter ; but if a young tooth be examined, it is found that, after the 



[Fig, 73. 



'JR. 74. 



The Fibres of the Enamel viewed sideways 

 under a magnifying power of 350 times; 1,1, 

 the enamel fibres ; 2, 2, the transverse stripes 

 upon them.] 



A small portion of fig. 70 covered with turpentine 

 varnish, viewed under a magnifying power of 350 

 times; 1,2,3, are the tubes containing a powdery, 

 lumpy substance. They are regular, and closely un- 

 dulating; but the branches do not appear, because 

 they are penetrated by the varnish.] 



calcareous matter of the tooth has been dissolved away by an acid, there re- 

 mains a set of distinct prismatic cells, which formed (as it were) the moulds 

 in which the mineral substance was deposited.* The Enamel is the least 

 constant of the dental tissues ; being more frequently absent than present in 

 the teeth of Fishes ; being deficient in the whole order of Serpents; and form- 

 ing no part of the teeth of the Edentate and Cetacean Mammals. 



210. The Dentine^ consists of a firm substance, in which mineral matter 

 largely predominates, though to a less degree than in the enamel. It is tra- 



[Fig. 75. 



[Fig. 76. 



A view of the most interior portion of the main 

 tubes of the dental bone in an incisor of a child 

 two years old, close to their commencement in the 

 cavitas pulpi, in order to show their first division.] 



A view of the external portion of the tubes of 

 the same tooth, exhibiting their more minute ra- 

 mifications, which, for the most part, turn towards 

 the crown.] 



\nversed by a vast number of very fine cylindrical branching wavy tubuli ; which 



The Author has discovered a structure precisely resembling this, in the shells of many 

 Mollusca. See Annals of Natural History, December, 1843. 



t A structure exactly resembling Dentine has been found by the Author in the shell of 

 the Crab, especially at the tips of the claws; and a less regular structure of the same kind 

 in the shells of many Mollusca. (Loc. cit.) 



