to the mechanical conditions of the tooth, but to the nutrition of the
dentine.
Dentine, thus organized, is ‘ unvascular’: the teeth of most
mammals and reptiles, and of a few fishes present this modification
of their main constituent. But the dentine in the teeth of most
fishes, of a few mammals, and of still fewer reptiles, is traversed by
canals containing blood vessels or a vascular pulp; the tooth-
substance, thus modified, I term ‘ vascular dentine.’ Both the
‘ vascular’ and 1 unvascular dentine’ may be present in the same
tooth, as in those of the sloth, the walrus, and the cachalot:
the transition from the vascular dentine to true bone is gradual
and close.
‘ Cement’ always closely corresponds in texture with the
osseous tissue of the same animal, and wherever it occurs of sufficient
thickness, as upon the teeth of the horse, sloth or ruminants;
it is also traversed, like bone, by vascular canals. In reptiles and
mammals, in which the animal basis of the bones of the skeleton is
excavated by minute radiated cells, forming with their contents
the ‘ corpuscles of Purkinje’, these are likewise present, of similar
size and form, in the ‘ cement’, and are its chief characteristic
as a constituent of the tooth. The hardening material of the
cement is partly segregated and combined with the parietes of the
radiated cells and canals, and is partly contained in aggregated
grains in the cells, which are thus rendered opake.
The relative density of the dentine and cement varies according
to the proportion of the earthy material, and chiefly of that part
which is combined with the animal matter in the walls of the cavities,
as compared with the size and number of the cavities themselves.
In the complex grinders of the elephant, the masqued boar and the
INTRODUCTION. V
capibara, the cement, which forms nearly half the mass of the
tooth, wears down sooner than the dentine.
The ‘ enamel’ is the hardest constituent of a tooth, and consequently
the hardest of animal tissues; but it consists, like the
other dental substances, of earthy matter arranged by organic forces
in an animal matrix. Here, however, the earth is mainly contained
in the canals of the animal membrane, and, in mammals and reptiles,
completely fills those canals, which are comparatively wide, whilst
their parietes are of extreme tenuity. The hardening salts of
the enamel are not only present in far greater proportion than in the
other dental tissues, but, in some animals, are peculiarly distinguished
by the presence of fluat of lime.
The absolute and relative size, form, disposition, direction and
intercommunication of the cellular and tubular cavities characterizing
the several tissues of the teeth will be the subjects of the special
descriptions of these organs in the different classes and species of
animals ; but a brief notice of the leading steps to the present
knowledge of the structure peculiar to each tissue may be appropriately
given in this place(l).
In a retrospect of the history of the science of the organization
of animal bodies, anatomy may always be perceived
to have made a marked advance in connection with the progress
of some collateral science. With regard to the hard parts of our
frame in particular, our knowledge of the elementary constitution
of the earthy salts has been due to the refinement of chemical
(1) The review here given of the discovery of the tubular structure of the dentine is
essentially the same as that prefixed to my own observations on the subject communicated to
the British Association in August, 1838.—See Trans, of the Brit. Assoc. 1838, p. 135.