DIGESTIVE SYSTEM. 



169 



direct attachment to the bony skeleton. The 

 hinge-joint is merely a modification of the fibrous 

 attachment, and is found in many fishes, reaching a 

 high degree of development in the poison fangs of 

 snakes. The hinge is composed of connective-tissue 

 elements. In snakes the fang has a muscular at- 

 tachment by which the reptile is able to erect the 

 fang. By ankylosis is meant a direct calcified union 

 with the bone of the jaw. Such teeth have no 

 flattened base, but a calcified pulp which binds them 

 firmly to the bony 

 skeleton of the 

 mouth. Ankylosis 

 is confined to the 

 teeth of certain 

 fishes. 



The development 

 of a socket is associ- 

 ated with large teeth 

 and a consequent 

 strong attachment. 

 The evolution of a 



socket is well represented phylogenetically in 

 reptiles where Wiedersheim makes three classes: 

 (i) pleurodont dentition (lacertilia) , where "the 

 teeth are situated upon a ledge on the inner side 

 of the lower jaw, with which they become fused 

 basally;" (2) acrodont dentition (chameleon,) where 

 "they lie on the free upper border of the jaw;" 

 (3) thecodont dentition (crocodiles), where "they are 

 lodged in alveoli." In man all the teeth are im- 

 bedded in well-developed alveoli of the jaw-bones. 



Fig. 122. Diagram illustrating the de- 

 velopment of a socket, a, Pleurodont 

 dentition; b, acrodont dentition; c t the- 

 codont dentition. 



