THE TEETH OF MAMMALS. 289 



to a lower level is well and simply illustrated by the 

 " mark " of the incisor teeth of a horse. 



In an uncut, and therefore perfectly unworn tooth, such 

 as is represented in the figure, the condition of the apex 

 may be compared to the finger of a glove, the tip of which 



FIG. 118 . 



has been pushed in or invaginated. The depression so 

 formed is, like the rest of the surface, coated with enamel, 

 and with a thin layer of cementum. 



When the tooth is worn down to a considerable extent, 

 we have a field of dentine, in the centre of which is an 

 oval ring of enamel; within this a space filled with the 

 debris of v food, &c. This constitutes the mark (see next 

 page), and as the tooth becomes further worn down, below 

 the level of the bottom of the pit, the mark disappears, 

 and a plain area of dentine results. 



JSTot only may inflections of the surface and of the enamel 

 take place from the grinding surface, but they also abun- 

 dantly occur upon the sides of the tooth. The inflection 

 of the surface, which in the incisors of the horse is of the 

 simplest possible form, may be cruciform, or variously 

 waved and broken up, thus leading to all sorts of com- 

 plications of surface. As the tooth becomes worn, the 

 longitudinal inflections, running in from the sides, may also 

 be oblique, or variously waved, or they may extend through 

 the entire width of the tooth, thus cutting it into a series 



( J ) Apex of crown of an upper incisor of a Horse, not yet completely 

 formed. 



