STRUCTURE AND GROWTH OF INCISOR TEETH ay 
below, is confined to the outermost part of the fibrous layer. 
There appears to be no Nasmyth’s membrane over the enamel, 
which means that there has been a complete transformation 
of the enamel matrix into enamel rods. The pigment extends 
- about two-thirds of the total length of the upper tooth, and about 
one-half of the total length of the lower tooth, and hence it 
follows that the deposition of enamel is completed within the 
basal third of the upper and the basal half of the lower tooth. 
By examining cross-sections of the tooth at different regions 
(fig. 6) it would seem that the full thickness of the enamel is 
attained within even a smaller area at the basal end of the tooth. 
The arrangement of the enamel over the labial aspect of the 
upper and lower teeth is shown in figure 4, drawn from cross- 
sections of the teeth of a 5-month animal. In both teeth the 
sections were made just posterior to the alveolar border. In 
both upper and lower teeth the enamel is thickest over the 
labial aspect, and is continued over the adjacent mesial and 
lateral surfaces. In both, the enamel is continued farther on 
the lateral than on the mesial surfaces, and relatively far- 
ther on the lateral surface in the lower than in the up- 
per tooth. In the upper tooth the enamel has a_ flattened 
external surface labially, while in the lower it has a rounded 
contour. In the upper there is a distinct labio-mesial and a 
labio-lateral angle, the enamel being somewhat thicker at the 
former. In the lower there is a labio-mesial angle, though 
less prominent than in the upper, and the labio-lateral angle 
is practically absent. 
In a 5-month animal the thickness of the enamel and its 
constituent layers was measured in the mid-line of the teeth, 
as follows: 
Upper Lowe 
Me M 
oO valet nicki esis cae Men bat eee cso vt aso teed ndteaee ae 100-110 140-150 
Oudersilbromsela vers his el ate Ok oe 5 vs «id coe baoete oe 30-40 20-30 
Pigmented portion of outer fibrous layer.............8-10-12 6-8 
ihiner lesatiorta Tavern c2 jo. 0 64 cs vecinidy Jeialie lew see 70 120-125 
It will be observed, however, in figure 4 that the enamel is 
not thickest in the mid-line of the upper tooth, but at the lateral 
and mesial angles. While the enamel of the upper tooth meas- 
