42 A MANUAL OF DENTAL ANATOMY. 



show that a thin layer of enamel exists, and that there is no 

 ceinentum. The frog has an enamel organ as distinct as 

 that of the snake, but I am hardly positive that there is 

 enamel upon its teeth, although there is an appearance of a 

 thin coat of distinct tissue. I have also demonstrated that 

 the armadillo has an enamel organ, but have failed to discover 

 any enamel or anything like it upon its teeth, and Professor 

 Turner has made a similar observation upon the narwal. 



At all events we may safely say that in these and many 

 other creatures no functional development of enamel takes 

 place : whether it does or does not exist in an extremely 

 thin and rudimentary layer has become a question of much 

 less significance, since I have shown the presence of an 

 enamel organ to be universal at an early stage. 



Hence I feel some hesitation in endorsing Professor Owen's 

 generalisation that the dentine is the most and enamel the 

 least constant of dental tissues ; it is possible that it may be 

 so, but recent researches into the development of teeth have 

 very materially modified the conceptions formed as to the 

 relations of the dental tissues to one another, and must lead 

 us to examine carefully into such deductive statements 

 before accepting them. 



The remaining dental tissue is cementum, which clothes. 

 in a layer of appreciable thickness, the roots of the teeth, 

 and reaches up as far as the enamel, the edge of which it 

 overlaps to a slight extent ; when the cementum is present 

 upon the crown, it occupies a position external to that of 

 the enamel. Cementum occurs universally upon the teeth 

 of mammalia, but it is not always confined to the root of 

 the tooth ; in many teeth of persistent growth it originally 

 invested the whole crown, and after it has been worn from 

 the exposed grinding surface, continues to invest the sides 

 of the tooth. (See the description of the complex teeth of 

 the elephant, cow, horse, &c.) 



It is probably entirely absent from the teeth of snakes, and 



