SUBSTANCE. STRUCTURE. 183 



containing the dental germs is defended by a single external cartila- 

 ginous alveolar ridge : this condition is permanently typified in most 

 existing lizards. Next there is developed an internal alveolar plate, 

 and the sacs and pulps of the teeth sink into a deep but continuous 

 groove, in which traces of transverse partitions soon make their 

 appearance : in the ancient Ichthyosaur the relation of the jaws to 

 the teeth never advanced beyond this stage. Finally, the dental 

 groove is divided by complete partitions, (1) and a separate socket is 

 formed for each tooth, and this stage of development is attained in 

 the highest organized reptiles, as in the crocodile. 



72. Substance. — This may be four-fold, and a single tooth may 

 be composed of dentine, cement, enamel and bone ; but the den- 

 tine and cement are present in the teeth of all reptiles. 



In the Batrachian and Ophidian reptiles a thin layer of cement 

 invests the central body of dentine, and as usual, follows any inflec- 

 tions or sinuosities that may characterise the dentine. Besides the 

 outer coat of cement, which is thickest at the base of the teeth, a 

 generally thin coat of enamel defends the crown of the tooth in most 

 Saurians, and the last remains of the pulp are not unfrequently con- 

 verted into a coarse bone, both in the teeth which are anchylosed to 

 the jaw, and in some teeth, as those of the Ichthyosaur, which 

 remain free. The only modification of the dentine, which could at 

 all entitle it to be regarded in the light of a new or distinct substance, 

 is that which is peculiar, in the present class, to the teeth of the Igua- 

 nodon, and which will be described in the following section. 



73. Structure. — The varieties of dental structure are few in the rep- 

 tiles as compared with either fishes or mammals, and its most compli- 

 cated condition arises from the interblending of the dentinal and other 

 substances rather than from modifications of the tissues themselves. 

 In the teeth of most reptiles the intimate structure of the dentine 

 corresponds with that which has been described as its fourth type or 

 modification in the teeth of fishes, and which is the prevailing structure 

 of mammalian dentine, viz : the radiation of a system of minute cal- 

 cigerous tubes from a single pulp-cavity, at right angles to the external 

 surface of the tooth. The most essential modification of this structure 



(0 At the sixth month. —Edinburgh Medical and Surgical Journal, No. 138. 



