STRUCTURE AND GROWTH OF INCISOR TEETH 55 
MICROSCOPIC STRUCTURE OF ENAMEL AND DENTINE 
Sections of enamel show two layers; an outer thin and an 
inner thicker layer, as noted by Owen (’40-’45, p. 399). The 
enamel rods run in different directions in the two layers as fully 
described by J. Tomes in 1850. In the inner layer the enamel 
rods appear to run in two sets, obliquely to one another, while 
in the outer layer the rods are all parallel. The outer layer has 
also been called the fibrous layer, and in its superficial part 
is situated the yellow or orange pigment which gives the color 
to the enamel. 
Figures 7 and 8 show the arrangement of the enamel rods 
in the two layers. In the inner or plexiform layer, when exam- 
ined in cross section, the alternating series of enamel rods decus- 
sate, forming an angle varying between 70 and 90°. In longi- 
tudinal sections (fig. 26) these rods are slightly S-shaped, running 
outwards from the enamel-dentine surface at an angle of 50 
to 54°, and inclining towards the anterior end of the tooth. 
Figure 8 is from a ground-section in which the enamel was 
broken during the process of preparation, and the broken edge 
shows distinctly the two sets of rods running at nearly right 
angles to each other. Under high magnification the rods are 
slightly notched. 
In cross-sections of the outer fibrous layer, the rods are paral- 
lel and form in the mid-line of the tooth an angle of 90° with 
the outer surface. As one proceeds away from the mid-line of 
the tooth, whether mesially or laterally, the general tendency 
of the long axis of the rods as they pass from the dentine junc- 
tion to the periphery, is to incline in the direction away from 
the mid-line of the tooth. The ameloblasts usually form an 
obtuse angle with the rods of the outer layer and seldom coincide 
in direction with them (fig. 7). In longitudinal sections the 
rods of the outer layer are not usually so distinctly seen as in 
cross-sections. In favorable longitudinal sections, however, 
they are seen to run quite obliquely, inclining towards the apex 
of the tooth, and forming an angle of 20 to 25° with the plane of 
the enamel-dentine junction. The pigment, as will be seen 
