THE FORMS, ORIGIN, AND DEVELOPMENT OF THE TEETH. I17 
to be formed upon the surface of the dentine bulb, and the pro- 
gress of calcification is from without inward, from the periphery 
toward the pulp cavity ; so that no portion of the dentine can 
receive any increase externally, but all the additions must be upon 
the interior of the calcified cap. This process goes on until the 
tooth-germ surrounds itself with the dentine, when it is now no 
longer known as the “dentine bulb,” or “ papilla,” or “tooth 
germ,” but as the “tooth pulp” which I have described. But, 
notwithstanding the altered environment of the pulp, it never quite 
loses its formative powers, as may be seen by some illustrations by 
Magitot, a patient French investigator, who shows that the near 
approach of decay may irritate the odontoblasts into renewed 
action, whereby new dentine is put forward over the threatened 
part. As the dentinal fibrils are but prolongations of the proto- 
plasm of the odontoblast or formative cells of the tooth germ, it is 
not difficult to see that when these are reached by the decay pene- 
trating the dentine these cells are set into renewed action. It is 
not to be supposed, however, that no alteration takes place in the 
dentine bulb after the bulk of its work is done, for investigation 
shows that the odontoblast cells somewhat change their outline 
when the day of their activity is over. When the crown of the 
tooth is finished, the neck and root, or roots, begin to form. As 
each portion of the dentine of the root is completed, it is coated 
over with a closely adherent vascular membrane, which presents 
on the inner or dentinal surface a layer of large osteoblast cells. 
It is by the calcification of these cells that the cementum is formed 
in those teeth where it only covers the roots. When the cementum 
has formed, this membrane remains to be now known as the 
alveolar dental periosteum before mentioned. Kolliker thinks, 
what is highly probable, that the cementum is formed in isolated 
scales, which eventually coalesce with one another ; and Magitot 
thinks the formation of the cement in the herbivora is by the ossi- 
fication of cartilage, and not by membraneous action. 
The dentinal fibrils are most interesting as well as important 
structures. Everyone who has had a tooth filled has been feelingly 
persuaded of their existence. In removing the decalcified dentine 
prior to filling up the cavity formed by the decay, the instrument 
cuts across numberless fibrils, each one of which is a centre of 
sensation. All the common talk about the ‘nerve of the tooth 
being exposed,” as if there was one distinct nerve to each tooth, 
and it was the laying of that bare which caused the continuous 
pain we know as tooth-ache, will be seen to be a mistake from 
what I have said of the pulp; and the use of such a phrase as 
“the nerve of the tooth” is unscientific and misleading. In exca- 
vating a cavity in the tooth, while the pain may be keen for the time, 
it will only be continuous until the decay has actually reached the 
