ENAMEL 107 



is maintained by the projection of the ridges of enamel. 

 Each plate has its own separate pulp cavity with a distinct 

 persistent pulp, thus differing from the molars of the 

 Elephant, which show a similar disposition of the tissues on 

 the grinding surface, but not growing from a persistent 

 pulp the plates have a common pulp cavity. 



As the incisor teeth of Rodents are continually growing, 

 it can be easily understood how in the case of fracture or 

 dislocation of the exposed part of one tooth the opposing 

 tooth will continue to grow forward following the curve 



FIG. 58. Skull of Rat (Mus decumanus). Dislocation and overgrowth 

 of incisors. The right upper incisor penetrates the skull. (From a specimen 

 lent to the author by Mr. Montagu Hopson. ) 



within its socket, and various malformations may result. 

 Instances are recorded in rats and rabbits where, one incisor 

 not being opposed owing to accident, it has elongated to 

 such an extent that it has penetrated the skull of the 

 animal. 



Fig. 58 shows the effects of fracture and dislocation of the 

 incisors of a rat, photographed from a specimen kindly 

 lent to the author by Mr. Montagu Hopson. 



The course of the enamel prisms in human enamel is 

 difficult to define, and their spiral twistings and longitudinal 

 curvatures render it very difficult to follow their actual 

 direction in any one part, but the general trend of the prisms, 



