XXVI INTRODUCTION. 



gous condition may be discerned in the slightly augmented diameter of 

 the enamel- fibres at their peripheral, as compared with their central 

 extremities. When the extremities of the human enamel -fibres are 

 examined with a magnifying power of 300 linear dimensions, by 

 reflected light, they are seen to be co-adapted, like the cells of a 

 honey-comb, and like these to be, for the most part, hexagonal. 



The external surface of the enamel is marked by fine transverse 

 lines or ridges, of which Retzius counted twenty-four in the vertical 

 extent of one tenth of an English inch of the crown of a human 

 incisor ; these lines are parallel and wavy, and, like the analogous 

 markings on the surface of shells, indicate the successive formation 

 of the belts of enamel-fibres that encircle the crown of the tooth. 

 These lines may be traced around the whole crown, but are very 

 faint upon its inner or posterior surface. 



Retzius cites Leeuwenhoek as the discoverer of these superficial 

 transverse lines of the enamel : but the older observer supposed them 

 to be indicative of the intervals between the successive movements 

 in the cutting of the tooth through the gum. 



The enamel by virtue of its physical qualities of density and 

 durability forms the chief mechanical defence of the tooth, and is 

 consequently limited, in most simple teeth, to the exterior surface of 

 the exposed portion of the dentine, forming the ' crown ' of the tooth. 



It sometimes forms only a partial investment of the crown, 

 as in the molar teeth of the iguanodon, the canine teeth 

 of the hog and hippopotamus, and the incisors of the Rodentia. 

 In these the enamel is placed only on the front of the tooth, 

 but is continued along a great part of the inserted base, which is 

 never contracted into one, or divided into more fangs ; so that 

 the character of the crown of the tooth is maintained throughout 



