This law is well exemplified in the teeth, although in the comparison
of these organs we are necessarily limited to the range of a single
primary group of animals. We have seen, for example, that the
dentine is scarcely distinguishable from the tissue of the skeleton in
the majority of fishes: but that its peculiarly dense, unvascular
and resisting structure, which is the exceptional condition in fishes,
is its prevalent character in the teeth of the higher vertebrates.
So likewise with the enamel; this substance, which under all
its conditions bears a close analogy with the dentine, i® hardly distinguishable
from that tissue in the teeth of many fishes(l). The fine cal-
cigerous tubes are present in both substances, and undergo similar subdivisions
; the directions only of the trunks and branches being reversed,
agreeably with the contrary course of their respective developments.
The proportion of animal matter is also greater in the enamel
of the teeth of fishes than of the higher vertebrata; and the proportion
of the calcareous salts incorporated with the animal constituent
of the walls of the tubes is greater as compared with the sub-crystalline
part deposited in the tubular cavities. In reptiles, the proportion
of the hardening salts and consequently the density of the enamel are
increased,, but the course, size, and ramification of the calcigerous
tubes still bear considerable analogy to those of the dentine ; and the
prismatic form of the calcigerous tubes, (2) their minute striations,
and the superficial transverse wavy linear ridges, which constitute
the characteristic features of the enamel in the mammalian class, are
not present in that tissue in the cold-blooded vertebrates.
The enamel is the least constant of the dental tissues : it is more
(1) Sargus, PI. 43, fig. 2 ; Phyllodus, PI. 44, fig. 2 ; Scants, PI. 50, PI. 52.
(2) I apply this term to the so-called prismatic fibres of human and other mammalian
enamel for reasons which will appear in the sequel.
frequently absent than present in the teeth of the class of fishes ; it
is wanting in the entire order Ophidia among existing reptiles ; and
it forms no part of the teeth of the Edentata and many Cetacea
among mammals.
The enamel may be distinguished, independently of its microscopic
and structural characters, by its glistening, subtransparent
substance, which is white or bluish-white by reflected light, but of a
gray-brown colour when viewed, under the microscope, by transmitted
light.
The microscopical characters of the enamel have hitherto been
taken from the modification of that tissue in the class Mammalia,
where it presents its most distinctive and consequently highest
condition.
This condition of the enamel, however, like the corresponding
one of the mammalian dentine, in the same degree as it distinguishes
them from the true osseous tissue, and perfects them for their
mechanical applications, removes them from the influence of the
conservative and reparative powers of the living organism. The
mammalian enamel, therefore, once formed and exposed, is least able
to resist vitally the influence of the external decomposing forces ;
but this inferiority is amply compensated by its superior mechanical
endowments. Nevertheless, it undergoes more change, after becoming
exposed by the eruption of the tooth, than does either
the dentine or cement, especially in regard to its original membranous
constituent ; and no true idea of its organic structure can
be obtained except by an examination soon after its formation.
The enamel of the molar tooth of a calf, which has just begun to
appear above the gum, and which can readily be detached from the
dentine, especially near the commencement of the fangs, is resolvable