STRUCTURE AND GROWTH OF INCISOR TEETH 93 
layer of the enamel-organ, and the cells of the enamel pulp. 
Between these elevations are numerous capillaries which in- 
sure a rich blood supply to the enamel-forming cells. 
There are three layers in the functional enamel-organ—inner, 
middle and outer. The inner is constituted of the tall amelo- 
blasts, and the middle is made up of two divisions, (a) stratum 
intermedium and (b) enamel pulp. The latter unites with the 
single layer of cuboidal cells which compose the outer layer, 
to form the epithelial papillae (fig. 26). 
The apex of the primitive tooth is formed of a variety of 
secondary dentine—‘osteo-dentine’ of Tomes—which is softer 
than true dentine, and differs in its structural arrangement 
(fig. 20). After eruption, this terminal portion of osteodentine 
is soon worn away by attrition, and the typical occlusal surface 
is developed, as seen at 19 or 21 days. At 21 and 23 days the 
first two molars erupt in both upper and lower jaws, and from 
now on the animal is able to secure food for itself, and if neces- 
sary can maintain an independent existence. 
As the tooth continues to be worn away there is a provision 
for the continual filling in of the apex of the pulp-chamber by 
the formation of what may also be called osteodentine. This 
is a form of secondary dentine, containing, when first formed, 
cells and blood-vessels. This is always at a little distance, 
about 1 mm., from the occlusal surface, but as any part of the 
tooth, in its outward progression, approaches the occlusal sur- 
face, the soft elements disappear within the osteodentine, and 
the latter forms a hard continuous surface with the adjoining 
true dentine. The position of this osteodentine is marked as 
a line on the occlusal surface of the teeth (fig. 5). 
Prior to eruption there develops around the apex of the tooth, 
as it lies in contact with the surface epithelium, a thickened 
ring of stratified epithelium. This ring of tissue is pierced by 
the apex of the tooth at eruption, and would seem to have the 
function of serving as a resistant margin for the soft tissues, 
and of preventing other tissues being carried along with the 
erupting tooth. 
