xi 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 /, 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 /f, c^, pm\, ;// = 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 pre- 

 molars 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 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 (compare p. 74). The food taken into the 

 mouth is ground up or masticated and mixed with the saliva 



