of the teeth having been worn to flat or rounded surfaces, having the same lateral
and longitudinal dimensions as the enlarged bases.
At last the deposit of bone around the bases of the teeth proceeds to such an
extent that the sockets disappear, and the teeth, if left in the mouth, are only in the
fleshy gums ready to drop out. Erom these facts it is evident that important changes
take place in the bases of the teeth after the early closure of the pulp cavity. In
many of the teeth the bases are thrown into strongly marked ridges, which at their
free ends may resemble so many short irregular fangs (see woodcut, fig. 13), which,
as Professor Owen has observed, is an exceptional character in the existing carnivorous
Cetacea.
In studying the foregoing changes in dentition, I had recourse to microscopic
sections to substantiate in the development of the tissues that which was to a certain
extent even patent to ordinary vision.
The earliest stage. examined was that of the foetus, the same figured in the
womb (PI. XXXI). In this the teeth were not erupted, hut the alveolar groove
was almost open and the denticles easily felt by the finger. As I have noted later on,
in the description of the exterior parts of this young animal, and which is shown in
the above figure, a series of skin ridges on the outer labial margins corresponded
outwardly to the non-erupted future teeth. The section, then, which I shall
proceed to describe is taken transversely to one of these ridges, and is drawn on an
'enlarged scale (PI. XXXVI, fig. 4). The fleshy gum on either side of the nascent
tooth is of the dense fibroid character. A thin film of cylindrical epithelium lines the
interior of the dental cavity, particularly its upper portion, or where it is least injured
by the razor in the process of cutting. The tooth itself is of a spear-headed outline,
the base on either side having a deep cleft, which, however, leaves a broad
pedicle of attachment. The summit of the tooth bears a thick cap of enamel
which thins downwards but reaches the very base, and even a thin lamina, of
enamel turns round the comer or angle of the lateral basal cleft. Calcification
has not been completed in the rest of the tooth, but round the edges dentinal
cells are discernible, and more internally the future lines of the tubuli can be distinguished,
besides the larger vascular channels of the dentinal pulp.
No trace of cement or osteodentine is appreciable within the limits of the
tooth substance proper. Below the root of the tooth, however, and in continuity
with it, there are large open spaces, vascular and otherwise, and beyond and all
around open, spongy bone texture. The latter -tissue is barely ossified, though large
Haversian canals in process of formation are abundantly evident.
At a further advanced but still youthful stage, when the teeth are thoroughly
calcified and erupted (PI. XXXVI, fig. 5), not only the apex but the body of the
tooth has a considerable coating of enamel. The dentine then composes the mass
of the tooth, a modicum of cement occupying the lower portion of the fang.
In a perpendicular section of the adult tooth of ,Platam8ta, taken from the
middle of the lower jaw (fig. 6), another stage in the chain of dental growth is met
with; not only is the shape of the tooth altered, but the proportions of the
tissues of the tooth are vastly changed. The enamel is limited to- a thin tiny
cap, and the dentine, with its central pulp cavity, is reduced to about a third or even
less of its original proportions, according to the varying circumstances of age,
position of tooth, &c. The mass of the dental substance has now become osteodentine
or cementai in nature. True bone lacunae and canaliculi freely intermingle
with what was before dental tubuli, and at the fang and the base of the body even
quite supplant dentine.
In another tooth possibly older, but at all events one taken from the lower
jaw behind its middle (PI. XXXVI, fig. 7), apical enamel was barely perceptible,
and the dentinal substance was still further reduced in dimension.
In yet another tooth from the posterior third of the lower jaw (fig. 8) the
substitution of tissues had gone still further, no enamel being left, and pure dentinal
structure was limited to little more than the apex. In fact, the tooth, save a
very small portion, was converted into osteodentine.
In quite old worn-down teeth bony tissue alone remained, or only with difficulty
could traces of dentine tubuli be recognised.
I t is evident, then, that as the bases of the teeth in this genus increase in size,1
it is due to substitution of true osseous tissue for dentine, and that, associated with
this, there is a gradual outward forcing of the teeth which leads to the complete disappearance
in the adult of the elements of the tooth originally exposed above
the gum, the place of which is taken by a tooth of bone, the product of a growing
osseous base, and many times larger than the original tooth which was composed of
all the structures that enter into the formation of Mammalian teeth generally.
The bases of the teeth of Olobicephalus deductor appear also to undergo a
considerable change in size from youth to age, and probably other Cetaceans will be
found to illustrate similar changes.
Pharynx.—In the half-grown individual, the pharynx is defined behind the
epiglottis by the presence of a more or less continuous transverse fold, the folds
behind it being longitudinal; but no such separation between oesophagus and pharynx
occurs in the adult, in which it is thrown into longitudinal folds continuous with
those of the fauces, but much feebler, and prolonged into the oesophagus. In the
semi-adult animal the pharynx has a transverse capacity of 2-50 inches, while in an
adult it measures 5 inches in the same direction. The openings of racemose glands
are numerous on the floor of the cavity and at the base of the epiglottis, but they
are all but absent on its sides and roof.
The tube of the larynx has been twisted to the left side in all the individuals I
have examined, and been firmly grasped by the sphincter of the posterior nares into
which the tube is generally found projecting. In the foetus it is also found occupying
a similar position, and has the same sinistra! asymmetry which is extended to the
tongue in the adult as well as in the young; this organ being more or less tilted up to
the left side, so that the straight course to the oesophagus passes the right side of the
epiglottis, which is doubtless the course followed by the food in the act of swallowing.
1 Owen, Odont., p. 362.