Freiburger historische Bestände - digital

95

The skull.

The skull of the Indian Elephant.

96

Besides developing in height, as the years go the skull is also
broadening remarkably at its posterior end. If we compare the
two skulls represented in Plate 24 respectively of the skull b and
k it appears that the hind part of the skull of the older has in
comparison with the anterior parts (for instance the part in which
the molars are implanted) become much broadened; but this
broadening is not the result of an even uniform enlargement of
the elements of which the posterior segment of the skull is composed
, but some elements have only been little, others enormously
augmented. This is most apparent in the part of the occiput, the
ventral wall of which is composed of the basioccipitale, the tym-
panicum and the squamosum. As is well-known there is in the
Elephant a peculiar continuation of the bony auditory meatus
formed by a rim-formed postglenoidal outgrowth from the squamosal
bone which ventrally meets a posttympanal outgrowth from
the same bone. This additional auditory passage, which partially
surrounds1 the convolute cartilago annularis, is in the young Elephant
still quite short, but increases with the years enormously
in length, and while in the delineated young Elephant (b) it only
makes a small part of the distance from the middle line to the
lateral border, in older Elephants it makes one half or more.
Contemporarily the articular surface (for the mandible) which
lies before it has become a corresponding increase in the transverse
direction of the skull. These parts having thus become
extraordinarily increased, the tympanicum (and still more the pe-
trosum inclosed by the tympanicum) and the basioccipitale have
only grown very little in the transverse direction.

Those parts of the skull situated laterally to that part of the
surface, in which the ligamentum nuchas is inserted, are also
gradually mightily developed. While that surface (where the ligament
inserts) in the skull b is very feebly hollowed out in the
occipital plane, it is gradually very much deepened, the lateral
parts being strongly arched.

The whole peculiar development of the skull described in the
preceding is intimately connected with an overwhelming pneu-
matisation of the cranial wall. All the extraordinarily developed
parts are filled with air. The pneumatized parts are generally
composed of two rather thin surface-plates, an external and an
internal, between which goes a system of thin plates at right
angles to the surface.

At little near the whole brain-cavity is surrounded by bones
filled with air. The inferior part of the wall lining the groove,
in which the ligamentum nuchse is inserted, is not pneumatic and
this is apparently generally the case with the part of the cranial
wall lying below that groove unto the foramen magnum; in our
oldest Elephant (Chang) the latter (PI. 26 fig. 1) is quite a thin
bony plate, on the thinnest spot hardly 4 mm thick, in d (PI. 27
fig. 2) it is thicker, at least 8 mm. Still thinner is the bottom
of the nuchal-ligament-groove (the inferior part of which as said
before is not pneumatized); it is in Chang scarcely 1 mm thick,
while in d, where the whole bottom of the groove belongs to the
not-pneumatized part, it is in the thinnest spot 3—4 mm thick.
Compare also the sagittal sections PL 27 fig. 1 (new-born), fig. 3,
PL 19 etc.

Non pneumatic is further the hind end of the basioccipital
— in b the whole basioccipital — and the lamina cribrosa of the
ethmoid. All the rest of the cranial wall is pneumatized.

Although the basisphenoid is to a large extent filled by the
processus molaris, there still has been room in it for air-sinuses,
which in the skull g and in Chang, but not in the young Elephant
d, extend into the anterior part of the basioccipital. Also
the presphenoid is amply pneumatized. In these bones in the
basis cranii there is in older specimens more bony substance in
proportion to the air-sinuses than in the dorsal cranial wall: that
basal cranial part lying centrally in the whole skeleton of the head
(the heavy mandible included) is likely to have a greater strength
than the upper wall. In the young Elephant d this difference is
not apparent; but at this age the air-sinuses of the basis cranii
are not yet much developed.

1 Boas, Ohrknorpel u. auss. Ohr d. Saugetiere. Kopenhagen 1912, p. 193.

The lateral walls of the brain-case (exoccipitale, squamosum,
parts of parietale and frontale) are also much pneumatized; in
older specimens the pneumatized lateral walls are of an enormous
thickness; the great breadth of the hind parts of the skull
and the enormous length of the bony auditory meatus is associated
herewith.

An immensely thick layer of pneumatized bony substance
forms the dorsal wall of the brain-case in old Elephants. Already
in the fetal skull (a) the nasal bones (which for descriptive reasons
are included here) are pneumatized and on the border of
the horizontal and the vertical part of the frontal (the eyebrow)
there is also a little part which is pneumatized; but otherwise
there is in this phase no pneumatization of the dorsal wall of the
skull, no more than of other parts of the skull. In the new-born
(PL 18, PL 27 fig. 1) the air-sinuses extend dorsally through the
whole frontal bone and further into the anterior part of the
parietal; but it is still quite a narrow cavity. In c they already
extend into the whole dorsal wall and have increased essentially
in height, so that the air-filled space on the middle is c. 4 cm
high. In Chang the air-space in the dorsal wall is 21—22 high
on its lowest point; in comparison herewith it may be noted, that
the brain-cavity, where it is highest, is in the same Elephant 15
cm high.

Also the facial part of the skull is already very early pneumatized
. In the fetal skull a the upper part of the intermaxillary
and the adjoining part of the maxillary bone are pneumatized
. Later on the pneumatisation extends over the whole of
the two bones. In the palatal roof beneath the fissura palatina
there is as in the basis cranii relatively much bony substance,
while the upper parts are more brittle.

The wall of the tympanicum is likewise rather much pneumatized
; but against what is the case as to the other air-sinuses
those of the tympanicum are evaginations from the tympanic cavity.
From the latter further air-sinuses extend into the exoccipitale.

As we have seen, the incongruity, which in the Elephant arises
with the age, between the size of the brain and that of the rest
of the head, is adjusted through the development of the large
air-sinuses of the cranial wall.

It is of some interest to see how this incongruity is adjusted
in a Fish, in which the brain is also relatively small in older specimens
but where a development of similar air-sinuses could hardly
be imagined, at all events is not effectuated.

In order to illustrate the matter by an example we have made
a comparison between the head of a small and that of a large Cod
(Gadus morrhua). Externally they are very similar, also the skulls
are at all events at a more hasty examination copies of another,
proportionate in size to that of the heads. But the proportion of
the brains is another: while the length of the whole head and also
the length of the skull is 1\ times as great in the large specimen
as in the small, the brain of the large Cod is only \\ times as
long as that of the small. Here the matter is adjusted in this
way that the jelly-like connective tissue, which is an indifferent
filling-up substance, is considerably more developed in the large
than in the small fish; the connective substance here plays the
same part as the air-sinuses in the Elephant.



Cod

Ratio



small
mm

large
mm



The whole head from the hind margin of the
basis cranii unto the anterior end of the
snout (soft parts included).................

74

186

2.5

The skull from the hind margin of the basis
cranii unto the anterior end...............

63

160

2.5

Greatest height of the skull...................

18—19

44-45

2.4

The brain from the anterior end of the prosencephalon
to the hind end of the cerebellum
.....................................

18

27

1.5

As to the closure of the cranial sutures we make the following
remarks.

The maxillary bone is only late united with the neighbouring
bones; still in the rather old skull k the sutures are — as far as
can be decided without bursting the skull — still open. On the
contrary the sutures between the intermaxillary and the nasal
and between the latter and the frontal have already begun obliterating
in g, while they are open in /.

The frontal is early united with the parietal. In b and bx
the suture between the frontals and the parietals are wholly open,