to the surrounding parts, inflammation of the pulp and capsule
had ensued, ending in an altered mode of action in the calcifying
processes, which produced the substance, called from its nearer
resemblance to bone | osteo-dentine.’ In this substance is seen the
transition of the normal dentinal tubes into the radiating tubes of
the calcigerous cell; and a well-marked stage in the transformation
of dentine into bone. In ordinary teeth the change in the mode
of conversion of the last part of the pulp is sometimes so gradual as
to render the confluence of the dentine with the osteo-dentine complete
; and the present instance demonstrates the same confluence of
the osteo-dentine with the ivory of the upper part of the tusk
formed before the fracture, and with that at the base formed after
the fracture, when the ordinary processes in the development of
the tusk had been resumed. In the complete resumption of these
processes, after the entire conversion of the matrix at the fractured
part of the tusk into the mass of osteo-dentine which constitutes
the uniting medium, we have a striking example of the truth of
the conversion-theory of dental development. It will scarcely be
contended that the calcified substance which manifests, besides the
complex tortuous tubuli, numerous concentric series of radiated
purkingian cells, and the larger vascular or medullary 'canals, called
‘ Haversian/ in osteology, is the product of a disordered secretion
of a pulp inflamed by injury; we see, on the contrary, evidence of
the conversion of such altered pulp by a series of centripetal
processes of calcification, resulting in the aggregate of subspherical
nodules of the osteo-dentine. Now, if the pulp of a continually
growing tusk were a persistent secreting organ, an accident
occasioning such destruction of it as is manifested by the microscopic
structure of the mass uniting the fractured parts of the Hippopotamus’s
tooth under consideration, must have put an end to the
secreting actions of such matrix, and to the future formation of the
tusk. But here, on the contrary, it is shown that the inflammatory
effects of the injury terminated in the abnormal conversion of the
affected pulp into a bone-like substance, and yet that the subsequent
development of the tusk was effected by the reproduction of a
vascular basis or pulp, undergoing the ordinary processes of calcification
to which the formation of the dentine, enamel and cement is due.
209. Succession.—The true natural affinities of the Hippopotamus
are clearly manifested by the character of its deciduous dentition ;
and, if this be compared with the same dentition in other Ungulata,
it will be seen, by its close correspondence with that of the Hog-
tribe, and more especially with the Phacochere, that the Hippopotamus
is essentially a gigantic Hog. The formula of the teeth
which are shed and replaced is :
t.. 2—-2 ; c. —l - i ; m. —3 - 3 : = wOAm 2—2 * 1—1 3—3 If the simple premolar which is developed anterior to the deciduous
molars, and which has no successor, be regarded from its early
loss in the existing Hippopotamus, as the first of the deciduous
series, we must then reckon, with Cuvier, four milk molars on each
side of both jaws. The incisors in both jaws are simply conical
and subequal, with an entire cap of enamel on the crown. The
deciduous canines scarcely surpass them in size in the upper jaw,
and not at all in the lower; projecting forwards here, from the angles
of the broad and straight symphysis, they appear like an additional
pair of incisors ; and we have seen that the character of equality
of development was retained, with the more typical number of incisors,
by the ancient form of Hippopotamus which formerly inhabited
India. The first true deciduous molar (d 1) has a conical crown
and two fangs in both jaws : that above has also a conical crown
with one strong posterior and two anterior ridges. The second
deciduous molar has a large trilobate crown, the first lobe small
with an anterior basal ridge; the second large, conical, with three
longitudinal indentations ; the third lobe still longer and cleft into
two half cones by an antero-posterior fissure assuming the normal
pattern of the true molars ; the third deciduous molar above more
closely resembles the ordinary upper true molar, but its second pair
of demi-cones is relatively larger. In the lower jaw the last deciduous
molar has a more complex crown than that of any other tooth,
whether of the permanent or deciduous dentition(l); it has three pairs
of demi-cones, progressively increasing in size, from before backwards,
with an anterior and posterior basal ridge and tubercles:
(1) This tooth is described as the first of the four true molars by M. F. Cuvier, loc. cit.
p. 207 j but its true nature was recognised by Baron Cuvier, see ‘ Ossemens Fossiles,’ 4to. i.