85
The skull.
Remarks on the skull of Mammals in general.
86
peculiar shape of the facial part described above, but is a consequence
of the foramen magnum being directed downwards, not
hindwards as in other Mammals, which is again in connection
with the fact, that the head in these animals is being carried on
the end of the vertebrate column in a manner different from that
in other Mammals.
A most remarkable cranial type is found in the Toothed Whales
<P1. 33 fig. 16, PL 30 fig. 9—15). By far most characteristic is that
remodelling of the skull which is a consequence of the narial openings
having been moved so far backwards, that the whole of the
greater anterior part of the nasal cavity is at the same time obliterated
1 and the nasal tubes in this way reduced to a pair of vertical
tubes. The result hereof has been that the processus palatini
of the highly developed intermaxillary bones have applied themselves
to the sides of the cartilaginous nasal septum, which they
in connection with the vomer tightly surround (except above); on
the sides of the intermaxillaria the maxillaria have their place.
Of other peculiarities we emphasize the suborbital arch being,
for instance in Phocsena, reduced to a bar of extreme thinness —
a consequence of the reduction of the masseter; further the large
osseous roof extending over the orbits; etc.
Marvellous is also the development which has taken place in
the skull of the Whalebone Whales2. Here the following moments
are decisive: 1. The proportionally enormous facial part with its
mighty apparatus of baleens. 2. The relatively extraordinarily small
brain, a consequence of the enormous size of the animal. The
head is, owing to its large bulk, rather difficult to manage and
the occipital plane, to which the occipital muscles are attached,
is therefore inclined strongly forwards, so that its anterior end
lies (in Baleenoptera rostrata) over the middle of the eye. The
temporal muscle, which shall lift the long mandible, and the insertion
of which to the lever is rather unfavorable, cannot be content
with the ordinary room behind the eye, but its origin extends
forwards above the orbit and has its anterior border at some distance
before the eye3 (in Phocaena the temporal muscle is wholly
situated behind the eye). The cartilaginous nasal septum comports
almost as in the Toothed Whales, but the nasal tubes are more
sloping.
Interesting is also the cranial type found in the Horse, which
is determined by the extraordinary development of the cheek teeth
and of the masseter, the origin of which extends far forwards on
the face, far beyond the orbit and has initiated a mighty inflation
of the air-sinuses around the posterior part of the nasal cavity.
On the other side the temporal muscle is relatively modestly developed
, the brain-case small, its crests little prominent, the whole
brain-case proper appears as all but an appendix to the mighty
facial part.
In Equus the orbit is situated behind the teeth-series (in contradistinction
for instance to the akin Anchitheri am, whose cheekteeth
extend below the orbita). According to W. Kowalevsky 4 the
reason of this should be, that the teeth had in consequence of their
enormous development pushed the orbita backwards. We think
it dubious whether this view hits the right. In the strongly inflated
part above the teeth the orbit certainly might have found
room. Other Mammals with large cheek teeth (Elephas, Hydro-
choerus) have the orbit placed over the teeth. It is rather the
weak development of the temporal muscle in the Horse which
has caused this moving backwards. In Anchitherium the temporal
muscle was, as far as we can judge, relatively much stronger.
The skull of Dipus (PL 33 fig. 10, PL 34 fig. 5) presents an
instance of the influence of the auditory organ on the forming of
the skull. Here the petrosum + tympanicum on each side is a
mighty bony vesicle, which is rather loosely connected with the
rest of the skull; the two vesicles make together, at a rough estimate
, between J and \ of the whole skull.
An easy survey of some principal points of the architecture
of the mammalian skull would seem to us to be gained through
the following consideration (PL 37).
The mammalian skull in the "starting type" (Dog etc.) has
the shape as an antique Amphora, in the neck of which, somewhat
above its nethermost end, a perforated dissepiment (lamina cri-
brosa) has been put in (comp. fig. 1 and 2). The cavity of the
neck above the dissepiment is the nasal cavity 1, the rest of the
cavity of the amphora is the brain-cavity. The handles of the amphora
— the zygomatic-suborbital arch — are at the top attached
farther down on the side, farther from the superior opening, and
at the inferior end nearer to the bottom of the amphora than in
an antique amphora. The inferior part of the neck has a narrowing
, an incurving on each side; here the eyes are placed between
the wall of the vessel and the anterior part of the handles: the orbits.
Below (behind) these the temporal muscle is placed between the
wall of the vessel and the inferior part of the handle: the temporal
groove.
The manner in which the form of the amphora is varied in
different Mammals appears from the schematical horizontal sections
in Plate 37. In some the amphora has been transformed to a very
big-bellied, long-necked bottle, the neck of which is very much narrowed
at the inferior end (Apes, Phoca, fig. 7 and 3). In others
the neck has become very long, the vessel proper not being much
transformed (Hippopotamus). The lower (posterior) insertion of
the handles may be moved forwards and the temporal groove
therewith almost wiped out (some Rodents, fig. 9). Or, the vessel
proper being much shortened and extended to the sides, the temporal
groove may be situated not behind but on the side of the
orbit (Apes, fig. 7). Or, the upper attachment of the handles may
advance more towards the opening of the amphora, the eye therewith
being placed far nearer to the opening, far from the usual
place off the lamina cribrosa (Sirenia, Elephant, fig. 5 and 10).
For the postfoetal development of the mammalian skull it is
generally decisive that in the young animal the brain is relatively
large, the teeth and masticatory muscles relatively weak, which
facts are together highly determinative of the character of the
skull: the brain-case relatively large, the facial part relatively
small, muscular crests not developed, air-sinuses only present to
a small extent. All these peculiarities of the young animal are
then successively wiped out.
THE SKULL OF THE INDIAN ELEPHANT.
The material of skulls of the Indian Elephant, which has been
at our disposal, is the following:
a. A defective skull, c. 16 cm long5,of a fetus. Has been macerated
before we have bought it and the ventral side is rather destroyed
.
b. Skull, 36 cm long, of a young animal, the dentition of which
is as follows: The milk-incisors still present (the roots much ab-
1 Only a small part of it has perhaps been spared in the form of the two flat blind-
sacks, that from the nasal tubes extend forwards, above and closely up to the hind
parts of the intermaxillaries (at least in Phocsena).
2 Comp. Boas, Lehrb. d. Zool. 9. Aufl. p. 696 fig. 666.
3 The whole surface in the cited figure in which the signatures Pa, Fr, Sq are placed
is occupied by the muscle.
4 Monogr. d. Gatt. Anthracotherium u. Versuch einer naturl. Classif. d. fossilen Huf.
thiere. in: Palseontographica 22. Bd. p. 278.
5 The measures have been taken from the hind end of the condyles to the fore
end of intermaxillary bones.
sorbed on the sides), the first cheek-tooth (second milk molar, dp2)
is still present above as well as below, but is rather much worn,
while on the second cheek-tooth (dp3) only the four—five anterior
plates have begun to be worn, the posterior three—four on the
contrary being quite intact; the third cheek-tooth (dp4) lies in
the jaws and is for the greater part composed of plates which are
not yet united. From the data given in Owen's Odontography
(p. 632 ff.) we infer that the animal has been a little more than
1 year old.
b1 and b2. Two skulls of about the age as b —judged from
the grinders — and of similar dimensions as the latter.
c and d are nearly of the same age, both 45—46 cm long,
in both skulls the dp2 has been shed, so that, according to Owen,
1 Here is abstracted from the fact that the neck of our amphora, the nasal cavity,
has an opening on one side — the ventral side — from which a tube is directed downwards
(backwards): the tube which ventrally is limited by the palatine bones, dorsally
by the lamina transversalis (vomer etc).
