156 c. r. barnicoat 



General 



The eiglit incisors ("teeth") of sheep (actually six incisors, the 

 two corner teeth being canines) are confined to the mandible and 

 bite against a pad in the upper jaw. The molars ("grinders"), twelve 

 in each jaw, are mainly used in rumination and are not of particular 

 interest to this discussion. 



Deciduous teeth are shed at 12 to 15 months, after which four 

 pairs of permanent incisors erupt in due order when the sheep is 

 about 15, 24, 33, and 48 months of age. It is then "fresh full- 

 mouthed." The first pair (centrals) often contain as much tooth 

 substance as the rest, and their rate of wear controls that of the 

 others. 



The hypsodont tooth consists of a crown covered with enamel, 

 and when it is first cut only about half of the crown appears above 

 the gum ( Fig. 1 ) . The remainder of the crown erupts as the animal 

 grows and matures, mostly during the summer and autumn. 



If the tooth is not subject to wear, it retains its "chisel" edge, and 

 may attain a length of 20 mm or even more when the animal is 5 

 years old.* Slight wear gives teeth with flat biting surfaces, which 

 are preferred by most fanners. When wear is excessive, however, 

 the unerupted crown responds by emerging from the gum more 

 rapidlv than normal. When all the crown has appeared the tooth 

 may wear eventually to a "gummy" state. On the other hand, teeth 

 of older sheep not fully erupted ma\^ actually lengthen when they 

 are transferred from sparse to generous conditions of feeding. 



Histological studies show that in the earliest stages of wear, when 

 the thin layer of enamel on the incisal surface has just worn down, 

 dead tracts in the dentine appear between it and the apex of the 

 pulp chamber, and the deposition of secondary dentine is initiated. 

 Sheep's teeth have a surprising facility for forming secondary den- 

 tine, which plugs the pulp canal as it is paid out of the gum with 

 the tooth as it wears (Figs. 2 and 3). Incisal surfaces of older sheep 

 may consist entirely of secondary dentine. The secondary dentine is 

 relatively soft, according to the Tukon Hardness Tester, as shown 



* Hill country sheep are graded and sorted when 5 years old, the condition of the 

 mouth being an important factor. 



