STRUCTURE AND GROWTH OF INCISOR TEETH 61 
point, while the posterior end is continually growing back- 
wards and changing its relations. At eruption this condition 
changes, and the posterior extremity becomes practically a 
fixed point from which the whole tooth moves forward. That 
there is, however, a gradual change in the position of the pos- 
terior end of the tooth may be seen in figure 9. As the jaw 
grows, the entire tooth not only grows to keep the same general 
relative position, to surrounding structures, but it may be seen 
that the growing end progresses gradually posteriorly. In 
the full-grown animal this end occupies a distinct outpushing 
of the bone (fig. 1). 
During the post-eruptive period, which continues through- 
out life, this outward growth is continued at a regular rate, and 
at the same rate the outer end has to be worn away. This wear- 
ing-away process would soon result in the pulp becoming exposed 
were not the occlusal end of the pulp-chamber also being con- 
tinually filled in. As may be seen from figure 6 the dentine con- 
tinues to increase in thickness until near the end of the tooth. 
This means that the odontoblasts continue their regular func- 
tional activity until near the end of the tooth. However, the 
final filling-in of the pulp-chamber to form a continuous hard 
occlusal surface is accomplished by the deposition of a hard ma- 
trix between the pulp elements and by the probable calcification 
of the latter. The result is, that as the tooth is worn away, 
the soft pulp never becomes exposed. Although the pulp reaches 
very near to the end of the tooth, a hard substance always fills 
in the end of the pulp-chamber, and so protects the pulp beneath. 
DETAILED DESCRIPTION OF DEVELOPMENT UP TO THE TIME 
OF ERUPTION 
The anlage of the enamel-organ of each incisor arises as an 
epithelial ingrowth, distinct and separate from that for the 
molars. 
In frontal sections of the 14-day fetus, there are slight diffuse 
thickenings of the oral epithelium in the four positions, which 
represent the sites of the future tooth-formations. 
