TEETH. 



213 



The root of the tooth embedded in the gums is provided at its 

 lower end with a small opening, and this leads into the pulp- 

 cavity (Fig. 173, PH 1 , PH). Into its interior, vessels and nerves 

 extend. 



Zf 



FIG. 173. SEMIDIAGRAMMATIC FIGURE OF A LONGITUDINAL SECTION THROUGH A 



TOOTH. 



ZS, enamel ; ZB, dentine (ivory) ; ZC, cement ; PH\ aperture of the 



pulp-cavity (PH). 



While in Vertebrates below Mammals a succession of teeth 

 takes place throughout life, in the latter group this as a rule 

 occurs only once, that is, the first or so-called milk dentition is 

 only replaced once by a second stronger and more fully-developed 

 permanent dentition. In certain Mammals, such as the 

 Cetacea and Edentata (with the exception of Dasypus peba), 

 there is no succession, and they are therefore distinguished as 

 Monophyodonts from other Mammals, or Diphyodonts. The 

 teeth of Edentates are without enamel. 



In Rliinoloplms (Cheiroptera), some Rodents, and Sirenia, the milk-teeth 

 never cut the gum, and become entirely absorbed before birth. In Rodents 

 various conditions of the milk dentition are seen : in the Rabbit they cor- 

 respond in number with their successors, 1 though the incisors disappear before 

 birth, the outer upper and the lower one being quite rudimentary. In others, 

 and in some Insectivores, no milk teeth at all are known : the Hedgehog has 

 a complete milk dentition, while no milk-teeth are known in the Shrew. In 

 Marsupials and Guinea-pigs only one milk molar is present. All these facts 

 indicate that a gradual reduction of the milk dentition is taking place. 



In those cases where the teeth are similar in foim throughout, 

 as, for instance, in existing toothed Whales, we have ahomodont 

 as opposed toaheterodont dentition. In the latter case the teeth 



1 That is, with the incisors and premolars of the adult. 



