78 TEXT-BOOK OF ZOOLOGY 



The incisors must therefore project freely, and therefore between them 

 and the molars there is a wide gap, or diastema. There are consequently 

 no canine teeth. Nor, indeed, does a herbivorous animal need them. 



3. There are five grinding teeth (molars and premolars) on each side 

 of the upper jaw, but the first is rudimentary, and is often last in adult 

 life ; in the lower jaw there are four on each side. Now, in order to be 

 digested, all vegetable substances must first be carefully ground up. 

 How is this effected? If the lower jaw were as firmly articulated as in 

 the carnivora (see cat), this would not be possible. In rodents, there- 

 fore, the articulating head or condyle of the lower jaw is elongated 

 longitudinally, and glides along an equally elongated articular groove. 

 The mandibles in chewing, accordingly, move from behind forwards (sliding 

 movement), and the molars thus act as files, which grate up the food. On 

 account of the postero-anterior movement of the mandibles, the promin- 

 ences of these files, i.e., the enamel ridges (see p. 16), must be placed 



Upper Jaw of Squirrel from Below. t t 



(Natural size. ) Lower Jaw of Squirrel. (Twice natural size. ) 



The socket of the incisor is somewhat broken 

 open in front, indicated by a dotted line 

 behind. 



transversely to the longitudinal axis of the jaws ; and since the osseous 

 portion of the teeth offers less resistance than the enamel ridges, the 

 file-like surfaces remain constantly sharp. 



4. In order to grind up the food, which is taken in very small bites, 

 only short grinding surfaces are required. The head of the squirrel^ 

 accordingly, like that of all rodents, is not nearly as elongated as that 

 of the larger herbivora (see ox, horse). 



5(a). For gnawing especially strong muscles are required, and to work 

 to the best advantage (to shorten the arm of the lever) these muscles, 

 must be inserted as far forward as possible in the lower jaw. 



(6) By this arrangement the mouth opening is much contracted ; on 

 the other hand, 



(c) The upper lip is divided as a protection against injury in gnawing, 

 and for the purpose of exposing the upper incisors (hare-lip). 



6. In order to open a nut or acorn, or to dismember a pine-cone, the 



