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364 DENTAL A^\iT03IY.
The Dental Scteculus and Cement Organ.—So far, no mention has
been made of the development of the dental sacculus. At an early
period in the growth of the dentine papilla a process of the submucous
tissue arises from its base and seems to grow upward on the outside of
both dentine and enamel oi'gans, finally coalescing on top, so as to enclose
the growing tooth-germ in a shut sac, the dental sacculus. AMsether
there is an actual groMth of processes from the base of the dentine bulb,
or whether the adjacent connective tissue is transformed into it, appears
not to have been very accurately determined ; at all events, the con-
nective tissue immediately in contact with the germ soon becomes
distinguishable from that external to it by becoming richer in cells,
vessels, and fibrillar elements. When the sacculus is fully formed, it
is made up of an outer and an inner wall, both richly vascular. The
outer wall becomes the dental periosteum, while in the inner wall,
especially in the vicinity of the roots, osteoblasts appear and are calci-
fied into cementum, as in the formation of ordinary bone-tissue. Its
close application to the surface of the enamel, and ])artial or imperfect
calcification in most teeth, give rise to the membrane of Nasmyth. In
those animals, however, in which coronal cement is ibrmed, such as the
Herbivora, there is developed in connection with the inner wall, between
it and the enamel, a fibro-cartilaginous structure containing character-
istic cartilage-cells. These undergo calcification in a manner not dif-
ferent from that seen in the formation of cartilage bone, and produce the
cementum in the teeth of these animals. It is then known as the
cementum organ.
We have now made clear, we trust, as complete a statement of the
anatomy of a single tooth as is consistent with brevity, but which will
serve as a basis for the comprehension of the more special part of our
subject—viz. the morphology of the teeth in the various subdivisions of
the Vertebrata.
—
The Accessory Oegans the Teeth, their Structure, Devel-
opment, Replacement, and Attachment, in Fishes.
It will be impossible to gain anything like a concise understanding
of the dental organs of this extensive assemblage of vertebrate forms
until we have first briefly outlined their classification. In this I have
followed Prof Gill, believing that his interpre
It is a common practice of naturalists to consider the Vertebrata as
divisible into five classes, as follows : Pisces, or fishes ; Batrachia. or
frogs, salamanders, etc. ; Reptilia, or snakes, turtles, lizards, etc. ; Airs,
or birds ; and Mammalia, or mammals ; but according to Prof. Gill
there are differences quite as great, if not greater, betw'een certain mem-
bers of the old class Pisces as there arc, for example, between some fishes
and frogs. For this reason he divides the permanently gill-bearing ver-
tebrates, or those which aerate the blood throughout the entire life of the
individual by means of specially adapted organs known as *' gills," into
four classes, which he defines as follows :
