QUADllUPEDS. 167 



external layer being first formed, and the others deposited 

 beneath, in succession. The socket of the tooth is left at 

 first hollow, owing to the presence of this pulp ; and, where 

 there are more fangs than one, each encloses a portion of 

 the base of the pulp. In proportion as the ivory is secre- 

 ted, the pulp diminishes in size, and at last nearly disappears. 



The capsule with which the pulp is surrounded, is mem- 

 branaceous, and consists of two layers. The central layer 

 secretes the enamel. In some cases, the enamel is spread 

 over a uniform surface ; while, in other cases, it penetrates 

 the body of the tooth, in the form of tortuous bands, ex- 

 tending from the crown to the root, or descending to a li- 

 mited depth into the body In the first and last cases, the 

 enamel is deposited after the ivory has acquired its form, 

 while in those teeth where the enamel extends from the base 

 to the crown, both the ivory and enamel appear to be of 

 nearly contemporaneous formation. In such teeth, exhibit- 

 ed in the rabbit, the growth may be indefinitely extended ; 

 so that the wearing down of the crown of the tooth may be 

 compensated by the increase at the base. Such teeth are 

 of equal thickness throughout ; all the others terminate in 

 narrow fangs. 



Considerable difference of opinion prevails with respect 

 to the formation of the cement. According to M. Tenon, 

 it is to be regarded as originating from the ossification of 

 the membrane of the capsule. Blake viewed it as a de- 

 position from the peripheral surface of the capsule ; while 

 M. CuviER thinks that it is deposited from the same sur- 

 face of the capsule as the enamel. As the cement only oc- 

 curs in those teeth where the walls of the capsule are tor- 

 tuous, and where the enamel penetrates the ivory in bands, 

 the formation of the cement appears to be connected with 

 this complex structure, and to obviate an imperfection which 

 would necessarily result, if the membrane whicli secreted the 



