INTRODUCTION. XXIU 



frequently absent than present in the teeth of the class of fishes ; it 

 is wanting in the entire order Ophidia among existing reptiles ; and 

 it forms no part of the teeth of the Edentata and many Cetacea 

 among mammals. 



The enamel may be distinguished, independently of its micro- 

 scopic and structural characters, by its glistening, subtransparent 

 substance, which is white or bluish-white by reflected light, but of a 

 gray-brown colour when viewed, under the microscope, by transmitted 

 light. 



The microscopical characters of the enamel have hitherto been 

 taken from the modification of that tissue in the class Mammalia, 

 where it presents its most distinctive and consequently highest 

 condition. 



This condition of the enamel, however, like the corresponding 

 one of the mammalian dentine, in the same degree as it distinguishes 

 them from the true osseous tissue, and perfects them for their 

 mechanical applications, removes them from the influence of the 

 conservative and reparative powers of the hving organism. The 

 mammalian enamel, therefore, once formed and exposed, is least able 

 to resist vitally the influence of the external decomposing forces ; 

 but this inferiority is amply compensated by its superior mechanical 

 endowments. Nevertheless, it undergoes more change, after becom- 

 ing exposed by the eruption of the tooth, than does either 

 the dentine or cement, especially in regard to its original mem- 

 branous constituent ; and no true idea of its organic structure can 

 be obtained except by an examination soon after its formation. 



The enamel of the molar tooth of a calf, which has just begun to 

 appear above the gum, and which can readily be detached from the 

 dentine, especially near the commencement of the fangs, is resolvable 



