analysis : the late improvements in mechanical optics have led
to a resumption of the microscopical observations originally
commenced by Malpighi and Leeuwenhoek ; and the consequent
acquisition of more exact knowledge of the mode in which the
particles of the phosphate of lime and other salts are arranged in
the animal basis or matrix of bone and tooth.
As regards the teeth, the principle of chief import to the physiologist
arises out of the fact, which has been established by microscopic
investigations, that the earthy particles of dentine are not confusedly
blended with the animal basis, and the substance arranged in superimposed
layers; but that these particles are built up, with the animal basis
as a cement, in the form of tubes or hollow columns, in the predetermined
arrangement of which there may he discerned the same relation
to the acquisition of strength and power of resistance in the due
direction, as in the disposition of the columns and beams of a work
of human architecture.
The disposition of the calcareous particles of hone in the
parietes of the Haversian canals, Purkinjian cells and of the tine
tubes which radiate from these cavities, was ascertained before the analogous
conditions of the intimate structure of dentine were discovered.
Until a recent period the analogy of dentine to bone was
supposed to be confined to their chemical constitution, and the
nature of the hardening material ; while the arrangement, as well
as the mode of deposition of the firm tissue, were considered to he
wholly different from that of bone, and the dentine to agree in its
general nature and mode of growth with hair and other extravas-
cular horny parts, with which most teeth closely correspond in their
vital properties.
The structure of a tooth, in fact, was regarded as simply
laminated, and the ivory was described as being formed layer within
layer, deposited by, and moulded upon the formative superficies of
the vascular pulp. The illustrations and supposed proofs of this
structure and mode of growth were derived from the apparently
detached condition of the newly-formed particles of dentine on the
pulp’s surface when exposed by the removal of the calcified part
of the tooth ; from the appearances observed in the teeth of animals
fed alternately with madder and ordinary food, which undoubtedly
illustrate the true progress of dental development; from the illusory
traces of laminated structure observed in vertical sections of teeth
when viewed by the naked eye, or with a low magnifying power ; and
lastly, and chiefly, from the successive hollow cones into which
a tooth is commonly resolved in the process of decomposition.
With regard, however, to the appearances presented by the teeth
of animals under the influence of madder, and to the separation of the
dentine into superimposed lamellae during decomposition, the same
conclusions as to intimate structure and mode of development might
be drawn respecting true bone, which also commonly resolves itself
into concentric lamellae during decomposition, and presents the
same appearance of alternate white and red layers in animals fed
alternately with madder and ordinary food during the progress of its
growth.
The lines running parallel to each other and to the contour of
the crown presented by the cut surfaces of vertical sections of teeth,
especially of the elephant’s tusk, or of the tooth of the cachalot, are
due to a totally different structure from that to which they have been
ascribed. The lamellated arrangement, thus seemingly demonstrated,
is, moreover, far from being a constant appearance; on the contrary,
the superficies of vertically cut or fractured surfaces of the human