THE TEETH 



351 



same, except that the cement and enamel dip down into two or moi'e cavities 

 instead of one, and are also reflected in a sinuous manner upon the sides. 

 This inequality in the hardness of the component parts of these teeth causes 

 them to wear away with different degrees of rapidity, and thus leaves a 

 rough surface, wliich materially aids in grinding down the hard grain 

 which forms a large portion of the animal's food. In the upper jaw, the 

 table presented by each molar tooth is much larger than those of the 

 corresponding lower teeth, and thei-efore it is easy to distinguish the one 

 from the other. 



Fia. 4.— Section of Incisob. 



e. Cement on external surface. 



c. Cement reflected within the cavity. 

 e. Enamel also reflected. 



d. Dentine. 



». Tartar, coloured black by decomposition of 

 food, contained within the cavity. 



Fio. 5. 

 Section of a Portion of Incisor (magnified). 



c. Cement. 



c. Enamel. d. Dentine. 



The temporary or milk incisors differ in shape a good deal from the 

 permanent set. The milk teeth are altogether much smaller, but especially 

 in the neck, which is constricted in them, whilst in the permanent set, 

 which go on growing as they wear out, the diameter is nearly the same 

 throughout. The former are also whiter in colour, and have grooves or 

 indentations on their outer surfaces, running towards the gum. Lastly, 

 the mark on the table is much slighter than in the pei^manent teeth. 

 The temporary molars are not distinguishable from the permanent teeth of 

 that class. 



As A CONSEQUENCE OF THIS ARRANGEMENT OP PARTS, the teeth, as they 

 wear down, present a different appearance according to the extent to which 

 their attrition has reached. On this fact is founded a means of arriving at 



