510 



THE RABBIT 



CHAP. 



the teeth are continually renewed as they become worn 

 out, but in Mammals there are only two functional sets, 

 which are known respectively as the deciduous or " milk " 

 teeth, and the successional or permanent teeth : certain 

 of the former may even be absorbed before birth, as is 

 the case with the incisors of the 

 rabbit. The incisors and pre- 

 molars (and in Mammals in which 

 they are present the canines also) 

 have deciduous predecessors, the 

 molars developing behind the 

 premolars and having no pre- 

 decessors. 



All the teeth are embedded in 

 deep sockets or alveoli of the jaw- 

 bones, and each contains a pulp- 

 cavity (Fig. 136, PH) extending 

 into it from the base and 

 containing blood - vessels and 

 nerves. In the case of the rabbit, 

 the aperture of the pulp-cavity 

 (PH 1 ) remains wide open in each 



H. pulp-cavity ; PH'. open- , ,, n ,, -, , ,, 



ing of same ; ZB. dentine ; tOOth, and the SUbstanCC Of the 

 ZC. cement ; ZS. enamel. . , . -\-i i i i j 



(From Wiedersheim's Comp. tOOth IS Continually added to at 



its base as it wears away at the 



other end : in many Mammals, however (e.g., dog, cat, 

 man), the aperture becomes narrowed and growth ceases 

 after a time, the base of the tooth forming one or more 

 roots or fangs. The main substance of each tooth is 

 formed of dentine (ZB), into which the pulp-cavity 

 extends for a considerable distance and round which 

 the enamel (ZS) forms an external layer, which may 

 become more or less folded inwards (as in the cheek- 

 teeth and front upper incisors of the rabbit), the cement 

 extending into the folds (compare pp. 445 and 511). 



FIG. 136. Longitudinal section 

 of a mammalian tooth, semi- 

 diagrammatic. 

 PH. 



