structural resemblance to bone and is almost identical in chemical
composition : its modifications, which I have called | vasodentine’
and ‘ osteo-dentine,’(l) forming intermediate gradations
between the hard dentine and true bone. True enamel is a tissue
per se ; but in the teeth of Fishes there are several intermediate
stages of gradation which link enamel to dentine, as the dentine
itself, in most Fishes, passes gradually into bone.
Heusinger admits that the relation of the teeth to the corneous
tissue (Horngewebes) is not clearly elucidated in Human Anatomy,
but he affirms that it is most conclusively established in that
of the lower classes of animals. (2) No doubt in tracing the modifications
of the dental system through the Animal Kingdom,
we find true horny productions substituted for teeth in certain
Vertebrate Species, as the Ornithorhynchus, Whale, Tortoise, &c.
(1) These tissues are respectively defined, as follows, in the ‘ Report of the British
Association, 1838.’ “ With respect to the component structures of a tooth, Professor Owen
commenced by observing, that in addition to those usually described and admitted, there were
other substances entering into composition of teeth, and presenting microscopic characters
equally distinct both from ivory, enamel, and cement, and from true bone, and as easily recognisable.
One of these substances was characterized by being traversed throughout by
numerous coarse canals, filled with a highly vascular medulla or pulp, sometimes anastomosing
reticularly,—sometimes diverging, and frequently branching,—sometimes disposed nearly
parallel with one another, and presenting more or fewer dichotomous divisions. The canals in
many cases are surrounded by concentric lamellae, and thus resemble very closely the Haversian
canals of true bone ; but the calcigerous tubes which everywhere radiate from them are
relatively much larger. The highly-organized tooth-substance just described differs from true
osseous substance, and from the cæmentum in the absence of the Purkingian corpuscles or cells.
This structure is exemplified in the teeth of many fishes and some Edentate Mammalia.
Another component substance of tooth more closely resembles true bone and cement, inasmuch
as the Purkingian cells are abundantly scattered through it ; it differs, however, in the greater
number and close parallel arrangement of the medullary canals. This structure is exhibited in
the teeth of the Megatherium, Mylodon, and other extinct Edentata,” p. 137.
(2) System der Histologie, 4to. 1823, p. 160.
So likewise the office of teeth is performed, in the Articulate Classes,
by parts (modified as to form) of the crustaceous and chitinous
integuments. But I know of no transitional or intermediate structures,
such as Heusinger alludes to, between teeth and nails, horns or hair.
The lamellar disposition traceable in the texture of the hardest
dentine, is much more closely similar to that of bone, especially
to the concentric plates surrounding the Haversian Canals, than to
the texture of nails. The structure of the tooth of the Oryc-
teropus, is essentially like that of all true teeth : the apparent
resemblance which it presents to the horn of the Rhinoceros, or
to Baleen, arises from its being compounded of many minute
parallel and elongated denticles.
And the close resemblance in intimate structure and chemical
composition between true teeth and bones being established, it
may be observed that the osseous tissue is not confined to the
endo-skeleton: it is developed largely to form the exo-skeleton
in many Fishes, in the Loricate Reptiles, and even in the Mammalian
Class, as, for example, in the Armadillos (Basypus), where,
to strengthen the integument, bone is substituted for horn, which
forms the scaled armour of the allied Pangolins (Manis.) Now the relation
of the tooth of the Armadillo to that of the Ornithorhynchus is
precisely analogous to that which subsists between the osseous
plates of the Armadillo and the corneous scales of the Pangolin;
but this relation no more establishes identity of tissue or
system of tissues in the one case than in the other.
The general form of the dental matrix and its relation with its
calcified product, bear a close analogy with those of the formative organ
of hairs, bristles, and other productions of the epidermal system.
In these the papilla, or pulp, is developed from the* external
skin; in the teeth from the mucous membrane, or internal skin.
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