CHAPTER II 

 ENAMEL 



ENAMEL, the hardest of animal substances, containing less 

 organic matter than any other tissue of the body, enters 

 into the composition of most teeth. 



It either forms the external calcified layer of the crowns 

 of teeth, as in man and many of the Mammalia, or in those 

 teeth which are covered with cement, as in the Herbivora, 

 plays a most important part in maintaining an effective 

 grinding surface. 



This is especially well seen in such compound teeth as 

 those of the Elephant, Wart-hog, and Capybara. The molars 

 of these animals are at first covered with an investing 

 cap of cement, which, when the tooth comes into use, is 

 rapidly worn away, exposing the dentine ; this being more 

 easily abraded than the enamel, the latter is raised into 

 prominent ridges, and serves to maintain a most effectual 

 grinding surface to the tooth. The cutting edges of the 

 incisors of Rodents are maintained in the same manner by 

 the position and unequal wear of the three tissues. 



Enamel may be confined to the tip of the tooth, as in the 

 eel, or invest the whole of the exposed surface, as in man ; 

 it is absent in some fish and in the teeth of the order Edentata 

 (Sloths, Armadillos, &c.), and is not found in many reptiles 

 and cetaceans. 



It is sometimes difficult to determine whether the outer 

 layer of many teeth of fish and reptiles is really enamel. 

 It is translucent, and shows no visible structure, and might 

 be either a thin layer of dentine or of cement, for both 

 dentine and cement in very thin layers often appear 

 structureless and transparent. It has been stated, how- 

 ever, that in all teeth which have been examined an 

 enamel organ is present, as well in those teeth which are 

 provided with enamel as in those in which it is totally 

 absent. It is considered that the presence of the enamel 

 organ, which is produced from the primary inflection of the 



