359 



CHAPTEE V. 



DIGESTIVE SYSTEM OF H^EMATOCRYA. 



69. Dental Tissues.- - A tooth is a hard body attached to the 

 mouth or commencement of the alimentary canal, partially exposed, 

 when developed. Calcified teeth are peculiar to the Vertebrates, and 

 may be defined as bodies primarily, if not permanently, distinct from 

 the skeleton, consisting of a cellular and tubular basis of animal 

 matter containing earthy particles, a fluid, and a vascular pulp. 



In general, the earth is present in such quantity as to render 

 the tooth harder than bone, in which case the animal basis is gela- 

 tinous, as in other hard parts where a great proportion of earth is 

 combined with animal matter. In a very few instances, among the 

 vertebrate animals, the hardening material exists in a much 

 smaller proportion, and the animal basis is albuminous ; the teeth 

 here agree, in both chemical and physical qualities, with horn. 



True teeth consist commonly of two or more tissues, character- 

 ised by the proportions of their earthy and animal constituents, 

 and by the size, form, and direction of the cavities in the animal 

 basis which contain the earth, the fluid, or the vascular pulp. 



The tissue which forms the body of the tooth is called ' dentine,' 

 (dentinum, Lat. ; zahnbein, zahnsubstanz, Germ. ; T*woire, Fr., 

 fig. 236, d). 



The tissue which forms the outer crust of the tooth is called 

 ' cement ' (ccementum, crusta petrosa^ Lat., ib. e). 



The third tissue, when present, is situated between the dentine 

 and cement, and is called ' enamel' (encaustum, adamas, Lat., ib. e). 



6 Dentine ' consists of an organised animal basis and of earthy 

 particles : the basis is disposed in the form of compartments or 

 cells, fig. 237, b, and extremely minute tubes, ib. a : the earthy 

 particles have a twofold arrangement, being either blended with 

 the animal matter of the interspaces and parietes of the tubes, or 

 contained in a minute granular state in their cavities. The density 

 of the dentine arises principally from the proportion of earth in 

 the first of these states of combination. The tubes contain, near the 

 formative pulp, filamentary processes of that part l ; and convey a 



1 CCXLVI. vol. iv. p. 929. 



