108 



ANATOMY OE VERTEBRATES. 



27.5 



Section of lower jaw, with four alveoli and teeth, of the black 

 Alligator, v. 



the previously formed tooth on the one side, and of the alveolar 



process on the other, is 

 represented in the ex- 

 posed alveolus of fig. 

 275, the tooth a hav- 

 ing been displaced and 

 turned round to show 

 the effects of the stimu- 

 lus of the pressure. The 

 size of the perforation 

 in the tooth, and of the 

 depression in the jaw, 

 proves them to have 

 been, in great part, 

 caused by the soft ma- 

 trix, which must have 

 produced its effect not 

 by mere mechanical 

 force. The resistance 

 of the wall of the pulp- 

 cavity having been thus overcome, the growing tooth and its 

 matrix recede from the temporary alveolar depression, and sink 

 into the substance of the pulp contained in the cavity of the 

 fully formed tooth. As the new tooth, ib. c, grows, the pulp of the 

 old one is removed ; the old tooth itself is next attacked, and, 

 the crown being undermined by the absorption of the inner sur- 

 face of its base, may be broken off by a slight external force, 

 when the point of the new tooth is exposed. The frail remains 

 of the old tooth are sometimes lifted off the socket upon the 

 crown of the new one, as in fig. 275, b, when they are speedily 

 removed by the action of the jaws. 



No sooner has the young tooth penetrated the interior of the 

 old one than another germ begins to be developed from the angle 

 between the base of the young tooth and the inner alveolar 

 process, or in the same relative position as that in which its 

 immediate predecessor began to rise, and the processes of succes- 

 sion and displacement are carried on, uninterruptedly, throughout 

 the long life of these cold-blooded carnivorous Reptiles. 



From the period of exclusion from the egg, the teeth of the 

 Crocodile succeed each other in the vertical direction ; none are 

 added from behind forward, like the true molars in Mammalia. 

 It follows, therefore, that the number of the teeth of the Croco- 

 dile is as great when it first sees the light as when it has acquired 



