94 AKBOREAL MAN 



only is caries of the milk teeth almost the rule, but dental 

 surgeons commonly " stop " milk teeth nowadays, so 

 early is the onset of their decay. The decay of the 

 permanent teeth needs no description, its almost universal 

 occurrence in civilized modern man being familiar to all. 

 But remarkably good sets of permanent teeth are found 

 in skulls of historical date, and in the remains of ancient 

 races extensive caries of the permanent teeth becomes 

 increasingly rare. One definite factor has certainly 

 played a large part in this deterioration of the teeth of 

 modern Man, and this factor is the loss of the reaction to 

 wear and tear the loss of the power of repair. 



When a horse grinds its molars together it wears them 

 down, and the enamel with which the surfaces of the cusps 

 are covered is worn off, and the main substance of the 

 tooth (dentine) is exposed below. Year after year the 

 cusps are worn natter, and an ever-changing pattern is 

 produced, the pattern being made by areas of exposed 

 dentine surrounded by margins of enamel. The dentine 

 which is exposed in this way reacts to the grinding 

 influences, and becomes hardened on its surface, and 

 changed to a condition known as secondary dentine, which 

 provides almost as hard a surface as that of the original 

 enamel. By this process the animal will gradually wear 

 its teeth flat, but these worn and flattened teeth are 

 perfectly sound teeth. The teeth of ancient races and 

 of modern primitive races show well this dentine reaction. 

 Human teeth may be ground down by wear and tear, and 

 react to the grinding influence. But in modern civilized 

 Man the reaction is very much diminished, and in the 

 majority of cases, when the enamel is worn through, the 

 fate of the tooth is sealed, since the dentine, instead of 

 reacting, becomes the site of decay. 



We have seen that in the whole Primate (and the whole 

 arboreal) stock there has been a recession of the jaw. 

 In one way this feature has been carried to very definite 

 human lengths by very definite human methods. When 



