TEETH 



493 



become worn out, but in Mammals there are never more 

 than 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 premolars 

 (and in Mammals in which they 

 are present the canines also) have 

 deciduous predecessors, the molars 

 developing behind the premolars 

 and having no predecessors. 



All the teeth are embedded in 

 sockets or alveoli of the jaw-bones, 

 and each contains a pulp-cavity 

 (Fig. 126, 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') remains wide open in 

 each tooth, and the substance of 

 the 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 (ZC) extending into the folds 

 (compare p. 429). 



Fig. 126. — LoiigitudiiKil seclion 

 of a mammalian tooth, semi- 

 diagrammatic. 

 PH. pulp-cavity ; PH' . open- 

 ing of same ; ZB. dentine ; 

 ZC. cement ; ZS. enamel. 

 (From Wiedersheim's Ana- 

 toiuy.) 



