494 THE RABBIT chap. 



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 /, c, pin, 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 i'\, c%, pm\, m% = 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 remains uneven. 

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

 voluted 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 sim- 

 pler than the others, and the first lower premolar presents 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. The food taken into the mouth is ground up 



