ATTRITION OF THE HYPSODONT TOOTH 157 



in the accompanying table. Yet when mouths are nearing the 

 "gummy" stage the phigs of secondary dentine appear to be the 

 most resistant part of the tooth toward wear. 



Knoop hardness numbers 

 (air-dried teeth) 



Average Range 



Enamel 256 (220-288) 



Dentine 49 (41- 56) 



Secondary dentine 25 (13- 32) 



During the course of this investigation, which involved the ex- 

 amination of thousands of sheep's mouths, dental caries was never 

 detected, though the mouths usually contained much food debris 

 and fibrous portions lodged between the teeth ( Fig. 3 ) . 



The sheep's incisor tapers from the incisal surface and therefore 

 narrows as it wears, which causes gaps to appear between teeth 

 (Figs. 1 and 3). This gives an appearance of spreading, which was 

 not, however, confirmed by measurements of casts of mouths of in- 

 dividual "full-mouthed " ewes taken at different ages. 



Factors Responsible for Wear in Sheep's Teeth 



These may be classified as ( 1 ) managemental ( farming ) prac- 

 tices, (2) anatomical (hereditary) differences in individual mouths, 

 and (3) nutritional factors. 



Managemental Practices 



Changes in farming practices may be of paramount importance in 

 overcoming the problem for individual farmers. Suffice it to say that 

 the type of pasture, as well as its length as controlled by grazing 

 stock, has great influence on rate of wear of sheep's teeth. 



Anatomical (Hereditary) Defects 



Short and long lower jaws* are common faults in sheep. Only 

 after studying photographs of the mouths of over 100 individual 



* The terms undershot and overshot are avoided since they have opposite mean- 

 ings in different countries. 



