STRUCTURE AND GROWTH OF INCISOR TEETH 85 
The enamel-organ is continuous over the convex labial sur- 
face of the imbedded portion of the tooth but is restricted to 
the most posterior region of the other surfaces, extending only 
1 mm. forward from the basal margin. The enamel-organ 
differs in its structure in three regions of the labial side, and 
may be described separately in these three parts: (1) at the 
basal formative end, (2) near the gingival margin and (3) in 
the long intervening region. In (1) the enamel-organ is being 
constantly regenerated by the addition and growth of new 
cells. In (2) the enamel-organ is undergoing a retrograde proc- 
ess, while (3) represents the region where the enamel-organ 
is at its highest functional development, although its activity 
in increasing the thickness of the enamel is restricted, as noted 
before, to the basal third or less in the upper and to the basal 
half in the lower tooth. 
Considering first the region (3), as shown in figure 26, the 
enamel-organ is conspicuous by reason of its tall ameloblasts 
and the high, narrow papillae. The enamel-organ is described 
in three layers—inner, middle and outer. The inner layer 
consists of the ameloblasts, which measure about 40 wu in height, 
with nuclei situated towards the outer end of the cells. The 
middle is composed of two strata (a) stratum intermedium, 
and (b) enamel pulp. The stratum intermedium is formed 
of 1 or 2 rows of fairly regular cuboida cells resting upon the 
outer ends of the ameloblasts, but the enamel pulp is not now 
recognizable as a distinct layer and exists principally within 
the papillae. The outer layer of the enamel-organ consisted 
originally of a single layer of cells, but these are no longer regu- 
lar in form or arrangement. Together with the remains of 
the enamel pulp, the outer layer forms the papillary elevations, 
60 to 70 » in height. These papillae are surrounded by an abun- 
dant capillary blood supply for the nourishment of the cells 
engaged in the formation of the enamel, and the purpose of 
the elevations is apparently to increase the surface area through 
which absorption may take place from the blood-stream. 
The enamel is in two layers (fig. 26), the rods while travers- 
ing the inner layer being very distinct, and inclining towards 
