STRUCTURE AND GROWTH OF INCISOR TEETH 5G 
upper and lower teeth. In the former, it is often slightly con- 
cave from side to side, while in the latter it is convex (fig. 5). 
As is shown in figures 1 and 5 the occlusal surface is an elon- 
gated concave area on the lingual aspect of the teeth, and in 
the living animal extends practically to the gingival margin. 
Due to the difference in the curve of the upper and lower teeth, 
the occlusal surface of the lower teeth is always longer than that 
of the upper, and in the mature animal it is usually found to be 
nearly twice as long. 
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Fig. 5 Labial and lingual aspects of the extra-alveolar portions of the (a) 
upper and (b) lower incisors of a 5-month albino rat, showing the occlusal sur- 
faces and incisal edges of the teeth, and the outline of the bony alveolar mar- 
eins.) <2: 
It follows that because these teeth are constantly growing, the 
occlusal surfaces are constantly being worn away. As we shall 
see, when discussing the growth of the teeth, the elongated tem- 
poro-mandibular articulation is important, in allowing the 
teeth to have either the position pictured in figure 1 or to have 
the opposite relation, with the lower teeth outside of the upper. 
Thus the very important factor in the animal’s economy—the 
proper regulation of the length of the opposing incisors—is con- 
trolled by their own inter-action. 
