102 COMMON r.RITISH ANIMALS 



lower jaw form as tliey meet a segment of a circle. 

 They grow continuously and are covered with hard 

 enamel on the front surfaces only ; the other parts of 

 the teeth being of softer material wear away more 

 quickly by constant use in gnawing, and thus a sharp 

 cutting edge of enamel is maintained. The colour 

 of the enamel is white, yellow, red, brown, or even 

 black. 



Though the continuous growth of the incisors is a 

 great convenience to the rodents, giving them a 

 power of gnawing through substances that no other 

 animals of their size could penetrate, still this gift 

 may sometimes prove a curse, for, should one of the 

 opposing teeth get broken, the one which worked 

 upon it goes on growing considerably with nothing 

 to wear it down, and curving round may actually 

 penetrate the skull. Thus a rabbit or hare, to whom 

 such an accident has befallen, may die of starvation 

 from inability to nibble its food. 



Everyone should examine a rabbit's or hare's skull 

 to see the working of these powerful chisels, the 

 rodent incisors. Beyond the incisors there is a con- 

 siderable space in the jaw, which is devoid of teeth 

 and over which the skin grows ; then come the cheek 

 or grinding teeth, on which the hard enamel is raised 

 in transverse ridges for grinding up the food. 

 That part of the lower jaw (condyle) which forms 

 the hinge with the upper jaw is elongated from 

 back to front into a long ridge, which enables a 

 rodent to work its lower jaw backwards and forwards, 

 and munch its food with its cheek teeth in the 

 peculiar way we have all noticed when watching 



