THE DENTAL TISSUES. 43 



indeed of very many reptiles ; in the reptilian class, at all 

 events, it appears to me to be confined to those hi which the 

 teeth are lodged either in sockets or in a deep bony groove, 

 as I am unacquainted with any tooth anchylosed to the jaw 

 in which it exists, unless we are inclined to include under the 

 term cementum the tissue which I have designated " bone 

 of attachment/' (See " Implantation of Teeth.") 



ENAMEL. 



Upon the outer surface of the dentine the enamel forms 

 a cap of a very much harder and denser material. In 

 its most perfect forms it is very far the hardest of all 

 tissues met with in the animal body, and at the same 

 time the poorest in organic matter. In the enamel of a 

 human adult tooth there is as little as 3 1 to 5 per cent, 

 of organic matter, and, judging from its brittleness and 

 transparency, there is probably even less in the enamel of 

 some lower animals ; the lime salts consist of a large quan- 

 tity of phosphate, some carbonate, and a trace of fluoride 

 of calcium ; in addition, there is a little phosphate of 

 magnesium. 



Von Bibra gives two analyses of enamel : 



ADULT ADULT 

 MAN. WOMAN. 



Calcium Phosphate and Fluoride . 89*82 81-63 



Calcium Carbonate .... 4 -37 8-88 



Magnesium Phosphate . . 1*34 2 '55 



Other Salts -88 -97 



Cartilage . . . . . 3'39 5'97 



Fat . -20 a trace 



Organic . . . . 3 -5 9 5 -97 

 Inorganic 96-41 94'03 



The cap of enamel is of varying thickness, being thicker 

 in the neighbourhood of cusps than elsewhere ; in teeth 



