304 DEVELOPMENT. 



The structure of the enamel presents fewer varieties than in 

 the lower classes of Vertebrata, and in all the Mammalian genera 

 in which it exists, it manifests its most distinctive and elaborate 

 condition. It consists of more or less curved or wavy prismatic 

 fibres, averaging about ^th of an inch in diameter, and having a 

 general direction vertical to the plane of the dentine ; some of the 

 fibres extend through the thickness of the enamel, others are 

 shorter and are w^edged in their interspaces ; the fibres of the thick 

 enamel of the molars of the large Pachyderms are most curved 

 and interwoven ; they are so arranged, however, as always to have 

 one end directed towards the outer surface of the tooth, and the 

 other end to that by which the enamel is attached to the dentine : 

 the outer, or peripheral end of the fibre is sensibly larger than the 

 inner one. In Man and Quadrumana the fibres are transversely 

 striated : I have not detected this character in the Herbivorous 

 Quadrupeds. The fine horizontal lines on the exterior surface of 

 the enamel(l) are usually most conspicuous in the teeth of 

 Mammals. 



130, Development. — The teeth in this, as in the foregoing classes, 

 are formed by superaddition of the hardening salts to pre-existing 

 moulds of animal pulp or membrane, organized so as to insure the 

 arrangement of the earthly particles according to that pattern which 

 characterizes each constituent texture of the tooth. The complexity 

 of the primordial basis, or matrix, corresponds, therefore, with that 

 of the fully-formed tooth, and is least remarkable in those conical 

 teeth which consist only of dentine and cement. The primary 

 pulp, which first appears as a papilla rising from the free surface 

 of the alveolar gum, is the part of the matrix which by its calci- 

 fication constitutes the dentine ; it sinks into a cell and becomes 

 surrounded by a closed capsule, in every Mammiferous species, at 

 an early stage of the formation of the tooth ; and, as the cement 

 is the result of the ossification of the capsule, every tooth must 

 be covered by a layer of that substance. In those teeth which 

 possess enamel, the mould or pulp of that constituent is developed 



(1) Introduction, p. xxvi. 



