ERUPTION OF PERMANENT TEETH. 



181 



After four years (Fig. 169, Y) there is a continuous but 

 very irregular wearing of the teeth, which is governed by the 

 nature of the food. Those animals which are fed on fine 

 herbage and those which are stall-fed level off the tables 

 evenly until but stumps remain ; but in all sheep there is always 

 more rapid wearing of the pinchers and first intermediate teeth, 

 which gives the plane of the tables of the arch a concave form. 



Fig. 

 rV, three years. V, four years, a, external face. &, internal face. 



Those animals fed on the hill-sides and where shrubs and 

 undergrowth are abundant use their teeth much more rapidly, 

 and wear notches between them like the markings on a horse 

 which cribs on a vertical object ; these notches are called, very 

 appropriately, by the French agriculturists, swalloW'tails (queue 

 dliirmideUe), 



