THE DENTAL TISSUES. 41 



in man. Perhaps the most usual condition is that the 

 enamel invests the whole crown of the tooth, stopping short 

 at about the level to which the gum reaches, as in the 

 human and most other mammalian teeth of limited growth. 

 In teeth of persistent growth the enamel extends down into 

 the socket as far as the base of the tooth ; in such cases it 

 may embrace the whole circumference of the dentine, as in 

 the molar teeth of many rodents, or it may be confined to 

 one side only, as in their incisor teeth, where by its greater 

 hardness it serves to constantly preserve a sharp edge as 

 the tooth is worn away. The enamel is believed to be quite 

 absent from many teeth ; thus the subclass Edentata compris- 

 ing sloths, armadillos, and ant-eaters have it not ; the narwal, 

 certain cetaceans, some reptiles, and many fish have none. 



But although it might appear an exceedingly simple 

 matter to determine whether a tooth is or is not coated with 

 enamel, as a matter of fact in practice it is not always easy 

 to be certain upon this point. When the enamel is tolerably 

 thick there is no difficulty in making sections which show 

 it satisfactorily, but when it is very thin it is apt to 

 break off in grinding down the section. And even when it 

 does not, it is in such cases usually quite transparent and 

 structureless, and the outermost layer of the dentine being 

 also clear and structureless, it is very hard to decide whether 

 the appearance of a double boundary line is a mere optical 

 effect due to the thickness of the section, or is indicative of 

 a thin layer of a distinct tissue which might be either 

 enamel or cement um. 



My own investigations upon the development of the teeth 

 of fish and reptiles have led me to suspect that rudimentary 

 layers of enamel exist upon many teeth on which their 

 presence has not been recognised, for I have found that the 

 formative enamel organs occur universally. Upon the teeth 

 of snakes, which were stated by Professor Owen to be com- 

 posed only of dentine and cement, I have endeavoured to 



