INTRODUCTION 5 



between the layers of enamel and dentine, and wearing 

 down more rapidly than either of these, maintains a sharp 

 cutting edge or surface to the tooth, as is most conspicuously 

 seen in the Rodents. 



The calcined portion of the tooth surrounds a cavity 

 occupied by the pulp, which in a foundation of connective- 

 tissue fibres supports the blood-vessels and nerves and the 

 formative cells of the dentine, and persists during the life 

 of the tooth. 



The mammalian tooth is implanted in a socket within 

 that portion of the bone of the jaw which is especially 

 developed to receive it, named the alveolus. The tooth is 

 attached within the socket by a fibrous membrane, the 

 periodontal membrane or ligament, wiiich is continuous with 

 the periosteum of the alveolar bone on its outer aspect. All 

 teeth are not, however, implanted in sockets, being in many 

 animals attached by anchylosis, the tissue of the tooth 

 merging into that of the bone ; but, as shown by C. S. Tomes, 

 this attachment in most cases is not direct to the bone of 

 the jaw, but to a separate process of bone, the bone of 

 attachment, which is analogous to the alveolar process of 

 human teeth. 



Various modifications of these modes of attachment are 

 found in the animal kingdom, which will be considered in 

 another place, and show a remarkable adaptation to the 

 habits and mode of life of the different species. Especially 

 with the class of fishes is this very noticeable, the varieties 

 of adaptive modification being so numerous that a large 

 number have probably not yet been described. 



Teeth are called dermal appendages, and both in structure 

 and development are closely related to the skin. This is 

 especially well seen in the Sharks, where it is very evident 

 that the teeth and the dermal spines are similar structures, 

 the teeth passing by almost imperceptible gradations into 

 the dermal spines and appendages of the skin. The close 

 relation of the teeth and hair is another evidence of their 

 dermal nature. In many instances this interdependence of 

 the teeth and hair has been manifested ; a great excess of 

 hair being accompanied by abnormalities, diminution of 

 number, or absence of teeth*. 



