Page 350 - My FlipBook

360 DENTAL ANATOMY.

sented by the arrangement of these prisms in the enamel of different
animals, especially of the "gnawing quadrupeds," or rodents.
The prisms, when decalcified and isolated, exhibit slight varicosities
or enlargements, giving them a distinct transversely striated appearance,
not unlike that of voluntary muscular fibres. They are otherwise
structureless. It is maintained by Bodecker that the prisms are not
absolutely in contact, but that minute spaces exist between them which
are filled with active protoplasmic material, which becomes continuous
with that of the dentinal tubuli, thereby furnishing a means of nutrition.
Some investigators admit this interstitial substance, but attribute to it
no greater ftmction than that of simple cementing material, while others,
again, claim that the prisms are in absolute contact, and that no inter-
vening substance is demonstrable. Owing to the disparity in extent
between the outer and inner surface of the enamel, as well as the fact
that the individual prisms do not decrease in size nor branch in their
course outward to the surface, considerable spaces would be left .if it were
not that they are occupied by numerous prisms which do not penetrate
to the dentine. The prisms end in sharp-pointed extremities which are
received into corresponding pits in the enamel cuticle or membrane of
Nasmyth.
Development.—Next in order will be briefly noticed the develop-
ment, so as to complete in this connection an entire statement of the
anatomy of a single tooth. It may be said that although teeth of dif-
ferent types differ to a wonderful degree in their forms, which would
seem to indicate differences quite as great in other respects, yet, in
fact, the ])lan of their development is substantially the same whereyer
found. So far is this true that the description of the embryology of
one tooth will, with little modification, answer fairly well for all teeth.
The more important of these modifications in the details of development
will be discussed in connection ^\ith the teeth of the various subdivis-
ions of the Vertebrata.
We have already stated that the teeth are derived from the lining
membrane of the oral cavity, Avhicli blends with the integument at the
lips. The principal differences between the integument which covers
the surface of the body and the mucous membrane which lines the ali-
mentary canal are those of function and origin, the structure being
essentially the same. In the one the indiyidual cells of the epidermal
layer become devitalized and scale off, Avhile in the other they are
actively engaged in the secretion of mucous, gastric, intestinal, and
other juices during alimentation. The devitalization and consequent
" shedding of the skin " is greater in some forms than in others. In
the frogs and salamanders, for example, the skin is kept constantly
moist by an abundant mucoid secretion, and the epithelium of the integ-
ument may be said to be more " alive " in these animals than in birds,
reptiles, or mammals. The difference in origin consists in the import-
ant fact that the integument is formed from the epiblastic or outermost
layer of primitive embryonic growth, while the mucous membrane of
the alimentary canal is derived from the hypoblastic or innermost layer
of the same. In the early stages of the development of the embryo the
skin is more or less invaginated into the mouth-cavity, and partakes