510 



THE RABBIT 



CHAP. 



-ZC 



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, 

 FIG. 136. Longitudinal section the aperture of the pulp-cavity 



of a mammalian tooth, semi- . * 



diagrammatic. (PH 1 ) remains wide open in each 



PH. pulp-cavity ; PH'. open- 



ing of same ; ZB. dentine ; tOOtll, and the SllbstanCC Of the 



ZC. cement; ZS. enamel. , ,. ., 



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



Anatomy.) ., , ., 



its base as it wears away at the 



other end : in man)' Mammals, however (e.g., dog, cat, 

 man), the aperture becomes narrowed and growtli 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 

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



