

DEVELOPMENT AND RENEWAL OF TEETH. . 193 



has acquired its full development ; and the process of budding then 

 ceases in Man, being limited to a single act of reproduction in the 

 case of the ordinary Milk-teeth, and to a double one in that of the first 

 permanent Molar. In many animals of the lower classes, however, the 

 process goes on through the whole of life without any limit ; the newly- 

 formed teeth, however, usually taking the places of those of the previous 

 set, and not being developed at their sides like the second and third 

 permanent molars of Man. By a process of this kind, the continual 

 renewal of the Teeth takes place in those Reptiles and Fishes, whose 

 dentition goes on to the saccular stage ; in those at which it stops at the 

 papillary, the successive teeth are formed from new and independent 

 papillae. The analogy between the continued succession of teeth in the 

 lower Vertebrata, by the gemmiparous reproduction of their capsules, 

 and the development of the capsules of the permanent teeth of Man 

 from those of the temporary set, is made further evident by the fact, 

 that a third set occasionally makes its appearance in persons advanced 

 in life ; the development of which would not be intelligible, if we could 

 not refer it to the continuance of the same process in the other capsules, 

 as that which regularly takes place to a limited extent in the permanent 

 molars of Man, and which goes on without limit through the whole lives 

 of the lower Vertebrata. 



325. The chief exception to the rule, that no Reptiles or Fishes have 

 permanent teeth, is found in the curious Dicynodon ; an extinct Reptile 

 which had two large tusks growing from persistent pulps, like those of 

 the Elephant, the front teeth of the Rodentia, and the grinders of the 

 Edentata. In such teeth, the base of the pulp remains unconverted, 

 and a new development of cells is continually taking place in that situa- 

 tion ; these new cells are in their turn converted into dentine, in con- 

 tinuity with that previously formed ; and thus the tooth or tusk is con- 

 tinually lengthening at its base, in a degree which compensates for its 

 usual wear at its summit. If anything should prevent that wear, 

 as when the opposite tooth has been broken off, there is an absolute 

 increase in the length of the tooth, from the continued growth at its 

 base ; which may become a source of great inconvenience to the animal. 

 There is nothing, in the Human subject, at all analogous to this mode 

 of development from persistent pulps ; the process being checked by the 

 closure of the root around the base of the pulp, which obstructs the 

 supply of blood it receives. 



326. The following table shows the usual periods at which the diffe- 

 rent teeth of the two sets first show themselves above the gum. It must 

 be borne in mind, however, that these periods are subject to very great 



and that the average alone can therefore be expressed. 



TEMPORARY OR DECIDUOUS TEETH. PERMANENT TEETH. 



Months. Year8 ' 



Central Incisors, ... 7 First Molar, . 6 to 



Lateral Incisors, . . .810 Central Incisors, 



Anterior Molars, . . .12 13 Lateral Incisors, 



Canines, 14 20 First Bicuspid, 



Posterior Molars, . . .18 36 Second Bicuspid, 





13 



Canines, 

 Second Molars, 

 Third Molars, 



7 8 



8-9 

 910 

 10 11 

 12 -.12* 

 12 14 

 16 30 



