STRUCTURE AND GROWTH OF INCISOR TEETH 59 
enamel. The tubules of the anterior region (a) of the dentine, 
covered by enamel, are more regularly parallel and have finer 
lateral branches than elsewhere. They also seem to taper slightly 
as one follows them towards the enamel. In the dentine not 
covered by enamel (b) the tubules are more sinuous and irregu- 
lar, the irregularities marking the position of origin of the larger 
lateral branches. In all parts at the periphery of the dentine 
the tubules end in a great number of very fine anastomosing 
arching branches. As a consequence of the smaller diameter 
of the little tubules here, a narrow zone at the periphery of the 
dentine has usually a more homogeneous appearance than has 
the remainder.. Towards the anterior end of the tooth, in the 
vicinity of the pulp-chamber, are vascular channels in the form 
of loops within the dentine. The tubules must necessarily take 
a curved course around these vascular channels, and thus the 
position of the vessels is more easily seen. 
In the dentinal tubules Mummery (12), Fritsch (’14) and others 
have demonstrated not only the processes of the odontoblasts, but 
also fine non-medullated nerve fibers. As to why the exposed den- 
tine on the lingual aspect of the teeth is insensitive, there are 
no definite observations to decide. A contributing factor may 
be the compression which the pulp tissues undergo at the an- 
terior end of the pulp-chamber, leading to the physiological 
cutting off of the nerve supply to the dentinal tubules. 
DEVELOPMENT OF THE INCISORS 
The times of the early stages of development of the incisors 
were seen as follows: 
14-day fetus—slight thickening of oral epithelium 
15-day fetus—distinct thickening and growth inwards of oral epithelium 
16-day fetus—dental ledge and beginning of flask-shaped enamel organ 
17-day fetus—dental papilla with crescentic enamel organ capping it 
19-day fetus—both ameloblasts and odontoblasts differentiated 
new-born animal—enamel and dentine formation begun 
8 to 10 days—eruption of the tooth 
Throughout life growth continues, and in the adult animal 
is on the average 2.2 mm. per week in the upper and 2.8 mm. 
per week in the lower incisor. 
JOURNAL OF MORPHOLOGY, VOL. 26, NO. 1 
