96 



NORTH AMERICAN FAUNA 74 



and, especially Cj, the cheek teeth are not implanted in a way that seems 

 adaptive to intense vertical pressure, and it is only on these and the more anterior 

 vestigial incisors that intensive abrasion from large, hard objects seems to occur. 

 Significandy, it is in that same area, on the incisive surfaces of the premaxillae 

 and mandible, that the gingiva has become greatly modified to withstand the 

 trauma of compression and other mechanical damage. 



Growth, Attrition, and Abrasion of Cheek Teeth 

 Linear Growth 



Linear growth of the cheek teeth is most rapid in the first 3 or 4 years after 

 birth and practically ceases by the fifth or sixth year (Fig. 64). At that time, the 

 deposition of dentin is essentially completed, even in the largest teeth (Fig. 53). 

 With closure of the apical foramen, all subsequent increments of linear growth 

 are by addition of cementum alone on the root apex. In the males, this 

 apparently takes place on P2 at a rate about equal to that of attrition from the 

 crown, for the tooth does not change appreciably in length after closure of the 

 apical foramen (Fig. 64). However, in the females, the rate of cementum incre- 

 ment on P2 is slightly lower than that of coronal attrition, since the tooth 

 becomes shorter with increasing age. In both sexes, the rate of increment on Cj 

 apparendy is lower than the rate of attrition, for that tooth diminishes in length 

 during the adult years at the rate of about 1 mm per year (Fig. 64). 



The rate of linear increment by apical deposition of cementum on P2 and Cj is 

 negatively correlated with age; at all ages, it tends to be greater on Cj than on P2 

 (Fig. 65). Although has the higher incremental rate, that tooth decreases in 

 length with increasing age, whereas P2, with the lower rate, remains more or less 

 constant in length. As in P, this indicates that the rate of linear decrement from 

 the crown is greater in than in P2. That the decremental rates are not the 

 same in all parts of the mouth is indicated also by measurements of actual 

 amounts deleted from the crowns of and P2 (Table 12). Although these rates 

 did not differ between males and females, they did differ between teeth, that of 

 Cj being nearly twice as high as that of P2. 



Attrition and Abrasion 



The removal of calcified tissue from the crown of each tooth takes place on 

 four facets, only one of which is the product of occlusion (attrition) with the 

 opposing tooth. That one, the occlusal facet (Fig. 66a, h), is clearly distinguished 

 from the other three by its rough, pitted surface, usually concave on the upper 

 teeth and convex on the lower. Ordinarily, it is rather small in area on the teeth 

 of females (5 to 50 % of the clinical crown) but rather large on the more robust 

 teeth of the males (20 to 80 % of the crown) . The pits in its surface are mostly less 

 than 0.5 mm in diameter and occasionally contain grains of sand and finer 

 bottom sediments embedded in the dentin and cementum. I have found such 

 particles common also in other parts of the mouth, as well as in the contents of 

 the digestive tract, into which they apparendy are taken in substantial quan- 

 tities, along with the food (Fay et al. 1977). 



The occlusal facet often is clearly delimited from the rest of the clinical crown 



