interspaces are occupied by the calcigerous tubes and cells. The medullary
canals are directly continued from those of the common bone
with which the base of the tooth is anchylosed, or into which it has
been converted. As the medullary canals proceed through the tooth,
they maintain a course more or less parallel, and more or less straight,
or wavy ; but they ramify abundantly, and gradually diminish in
calibre as they approach the surface of the tooth. The illustrations
of this modification of the dental structure in the present work,
are taken from the teeth of the extinct Lamm, Dictyodus, and Sauroce-
phalus, and from those of the recent Sphyroena and Acanthurus. In
the latter genus the dendritic arrangement of the medullary tubes
recognized by Mr. André, has subsequently been figured by V. Born.
V. Born and Retzius have described a similar structure in the teeth
of the wolf-fish (Anarrhichas), of which Mr. Nasmyth has given a
figure in his useful translation of some of the recent treatises on
dental anatomy.
The reticulate medullary tubes pervade the structure of the
teeth of the percoid, sciænojd, cottoid, and gobioid families of fishes ;
and of those of the Capros, Naseus, and other genera of the Theuties of
Cuvier, besides the teeth of the Acanthuri already cited. A similar
reticulate structure is common to the teeth of the Chcetodontes and the
Pleuronectes : in the cycloid fishes, we find it almost universal in the
scomberoid, lucioid, salmonoid, and clupeoid families : it is exchanged
for a higher type of structure in the maxillary teeth of the
lophioid fishes, and in the pharyngeal teeth of the cyprinoids, but
it again reappears in the teeth of the blennioid, gadoid, and mu-
rænoid families ; and the same coarse bone-like structure pervades
the dental plates of the supposed amphibious Lepidosiren.,
The higher type of structure just alluded to is that which
characterises the teeth of most reptiles and mammalia. Here the
dentine consists of a single medullary or pulp canal, and a single
system of calcigerous tubes radiating from the central or sub-central
canal, at right angles to the periphery of the tooth. The teeth
of the extinct sauroid fishes and pycnodonts, the maxillary teeth of
the existing file-fishes (Balistes), and angler (Lophius), the incisors,
canines, and molars of the breams or sparoid fishes, the pharyngeal
pavement-teeth of the wrasse-tribe (Labridoe), the maxillary and
pharyngeal denticles of the scari, and the lamelliform denticles of
the crop-fishes, diodon and tetrodon, likewise the maxillary teeth of
some of the genera of sharks and rays, afford examples of this structure.
I have spoken of it as belonging to a higher type because it is
characteristic of the teeth of the vertebrate animals which are higher
in the scale than fishes ; but if the grade of organization of a tooth
be rated according to the proportion of vascular substance and vital
power which it possesses, then those teeth which most resemble bone
should be regarded as the most highly organized, and such are the
teeth most common in the class of fishes.
As the inorganic calcareous particles are deposited principally
in the parietes of the calcigerous tubes and their terminal ramuli
and cells, it follows that the density of the tooth will increase, and
its vital properties diminish as the calcigerous preponderate over the
medullary tubes, and in proportion as the calcigerous tubes in a given
space are more numerous and minute. But the change from one
modification of dental structure to another, from bone to the densest
ivory, is so gradual, that the physiologist, entertaining a belief in the
inorganic nature of the teeth, would be at a loss where to draw
the line, or to determine where the vital forces ceased their manifestation,
and at which step in the series of tubular structures the
tooth became an inert body. The uniform result of my researches,
on the structure of the teeth in all grades of vertebrate animals,
and in their natural and diseased states, has been a conviction of
the untruthfulness of the terms inert, dead, and unorganized as
applied to the; substance of any tooth whatever. Extra-vascular
undoubtedly is all that portion which consists of the calcigerous
tubes ; the capillary circulation is confined to the pulp or medullary
canals; but since every secretive process and the development
of the primordial cells of every tissue are due to changes produced
in the liquor sanguinis transuded from and beyond the sphere of
the ultimate capillaries, the absence of these vessels in the dense
dental substance is as little conclusive against its vital and organized
nature, as it would be to prove the inert condition of the germinal
membrane of the ovum before the thirtieth hour of incubation.
In no teeth is the dentine rendered so dense as in those of certain