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108 DENTAL HISTOLOGY AND OPERATIVE DENTISTRY.

the calcium salts in the development of the tissue. They record the
growth of (Miamel of the crown as a whole, as each line was at one time
tlie surface of the enamel cap. These stratifications, or, better, incre-
mental lines, are shown in Figs, 89-91.
At the tim(> the rod at .1 (Fig. 90) was comphitely formed the rod at
R was just beginning to form at its dentinal end. From this it would

Fig. 90.

Incisor tip sliowing stratification or incremental lines. Rods at A were fully formed at the time
the rods at B were beginning to form. (About 50 X.)

seem that any structural defect due to imperfect development would not
follow the direction of the enamel rods from the surface to the dentin,
but would follow the stratification lines ; and if these structural defects
influenced the penetration of caries, we should expect to have the direc-
tion of penetration modified. Fig. 89 shows a structural defect in the
enamel over a cusp following the stratification band, and it will be
noticed also that there is a structural defect in the dentin at a corre-
sponding position.