XT TEETH . 511 



The number of the various teeth in the jaws is con- 

 veniently expressed by a dental formula, in which the 

 kind of tooth (incisor, canine, premolar, or molar) is 

 indicated by the initial letter, i, c, pm, or m ; and the 

 whole formula has the arrangement of four vulgar 

 fractions, in each of which the numerator indicates the 

 number of teeth in the upper, the denominator that of 

 those in the lower jaw, only those of one side being 

 indicated, since the teeth of the right and left sides are 

 always the same. Thus the dental formula of the rabbit 

 is if, CQ, PM*, AH:} = 28. 



The anterior incisors in the upper jaw of the rabbit are 

 long and greatly curved. They are surrounded by enamel, 

 which is much thicker on the anterior surface, where it 

 presents a median groove ; the posterior upper incisors are 

 much smaller and are situated behind the anterior ones. 

 In the lower jaw the single pair of long and curved incisors 

 have no median groove, and they bite between the anterior 

 and posterior upper incisors : owing to the thick layer of 

 enamel anteriorly, they, like the large upper incisors, wear 

 away less quickly in this region, and thus remain sharp, like 

 a chisel, at their biting edges. The premolars and molars, 

 on the other hand, are modified for grinding the food, to do 

 which satisfactorily it is necessary that they should have 

 broad crowns with a surface which wears unevenly. This is 

 effected in most of the cheek-teeth by the enamel becoming 

 involuted along the outer side in a longitudinal direction, 

 so as to form a groove extending into the dentine almost to 

 the other side, the groove becoming filled up with cement. 

 As the enamel is harder than the dentine and cement, it 

 thus gives rise to ridges as the crown wears. The first upper 

 premolar and the last upper and lower molars are simpler 

 than the others, and the first lower premolar has two grooves. 



Connected with the mouth-cavity are several pairs of 

 salivary glands, not present in the other Vertebrates we 

 have examined, the secretion of which saliva contains 

 a ferment called ptyalin, which is capable of converting 

 starch into sugar (compare p. 74). The food taken into 

 the mouth is ground up or masticated and mixed with 



