272 COMPARATIVE ANATOMY OP THE TEETH. 



to be made, the law of fixed limitation in the growth of all 

 organs would seem to have some exceptions. The rodentia, 

 pachydermata, and cetacece, are cited as instances of unlim- 

 ited growth of the teeth, and such indefinite growth as 

 is confined, almost in all cases, to the incisors or ca- 

 nines. The cause of this constant growth of these teeth 

 during life, is thus given by M. Blandin: "These teeth," 

 he says, "are classed with those which have no roots; their 

 internal cavity is conical in form. The pulp, conical also, 

 rests, by a large base, upon the bottom of the alveolus, 

 whence it receives its vessels and nerves, and not through 

 the medium of a pedicle. 



"In consequence of this arrangement, it is evident that 

 the bony matter can never surround the pulp in such 

 a manner as to interrupt its functions ; and there is no 

 reason," he then a'dds, "why it should not continue to 

 secrete this substance during life." 



The order in which the teeth of mammalia make their 

 appearance, is the same as in man, i. e. from before to the 

 back part of the mouth. The number of dentitions in some 

 of the mammalia is not limited as in man. The elephant, 

 it is found, has its molars renewed as many as eight times. 



This frequent renewal of the teeth, it seems, is regulated 

 in proportion to the life of the animal, as to whether long 

 or short, and there are but two ways in which the animal 

 can be kept supplied with teeth, either to replace by a new 

 set when the present ones are lost, or by constantly adding 

 new matter to the base of those already formed, as fast as 

 they are destroyed by friction on their upper surface. The 

 teeth are liable to be ivorn away from friction, some more 

 than others, as the back teeth of the elephant are not 

 so much worn as the front. The kind of food seems to 

 exert a special influence in this wearing away ; those that 

 live on grass and nuts, as the gramenivora and rodentia, have 

 distinct lines on their cutting and grinding surfaces, and 

 the direction of these lines indicates the direction of the 

 teeth in the mastication of the food. In the ruminantia the 

 lines are transverse, showing that the friction is from side 



