Freiburger historische Bestände - digital

83

The skull.

Remarks on the skull of Mammals in general.

84

posterior end of the orbit, while in the starting type it is situated
far behind it. Also the anterior root of the infraorbital arch —
which is in these Rodents, as is well known, perforated by a mighty
foramen infraorbitale, through which part of masseter extends on
the side of maxillare and intermaxillare and takes its origin from
them — has shifted. While in the starting type it has its place
near the hind end of the molar series, it is here connected with
the skull at the fore end of the whole grinder-series. — Other Rodents
take a place halfway between the starting type and the type
just described. So it is in Arctomys (PI. 33 fig. 11, PI. 34 fig. 6
and 9) a. o.: the temporal groove is here rather large, the origins
of the zygoma and of the infraorbital arch are certainly displaced
forward, but by far not so much as in Hydrochoerus etc. (Also
the infraorbital opening is of moderate size).

Another curious type of skulls we find in Sus (PI. 33 fig. 14)
and some of its kindred. To Sus it is characteristic that the dorsal
portion of the hind part of the skull, from the anterior margin
of the orbit and backwards, has been so to say pulled hindwards,
so that the posterior end of the teeth-series ends at some distance
before the orbit (while in the starting type it extends below the
orbit). The anterior part of the skull, from the fore-end unto
the anterior margin of the orbit, is nearly twice as long as the
rest; A most extreme development of this cranial type we find
in Phacochoerus (PI. 33 fig. 15). Here the hind part of the skull
(orbit, temporal fossa etc.) has been pulled so strongly backwards
that the distance from the anterior margin of the orbit to the hind
end of the molar series is all but the same as that from the same
margin to the hind end of the skull. The crista pterygoidea, running
for instance in the Bear from the processus supraorbitalis obliquely
backwards, has in Phacochoerus a direction from above strongly
forwards; the crista is particularly conspicuous and is in a similar
manner as in the Elephant (see below) continued into the processus
pterygoideus of the basisphenoid. In Sus the crista has a direction
almost directly downwards, representing an intermediate state.

The peculiar form of the skull in Sus and Phacochoerus no
doubt has some relation to the head being used for shovelling in
the soil and also for tearing roots out of the soil by means of
the mighty canines1. It is a matter of course that a pulling back
of the eyes and a lengthening of the anterior portion of the skull
is under these circumstances particularly necessary.

In Dicotyles and Porcus, PI. 33 fig. 13, (which latter according
to Brehm2 is not a root-eater) the skull is less transformed than
in Sus. The skull is not so much pulled out posteriorly, the
molar series extends underneath the anterior part of the orbit
and the crista has a slight direction backwards. Still more original
features are found in another of the relatives of the pig, in the
Hippopotamus (PI. 33 fig. 3) already referred to above, in which
the molar series extends underneath the whole of the orbit, where
the crista is directed decidedly backwards and where on the whole
a pulling backwards of the hind part of the skull is out of question.

A diametrically opposed development has taken place with
the skull of the Manatee (PI. 33 fig. 6, PI. 34 fig. 3). Here the long
molar series not only extends backwards below the orbit but below
the whole inferior opening of the temporal fossa. And the displacement
is so marked that the anterior end of the molar series
is placed at some distance behind the anterior margin of the orbit.
The temporal muscle is most powerfully developed and has as it
were pressed the orbit forwards, so that the eyes are not as usual
situated against the sidewalls of the posterior part of the nasal
cavity and the anterior part of the brain-case, but abreast of the
middle of the nasal cavity (or rather nearer the fore- than the
hind-end of it). The zygomatic arch is immensely heavy and
clumsy, as also the inferior jaw. The mighty development of the
muscles of the jaw must be a peculiar adjustment to the herbivorous
habits of the animal; the teeth themselves are not very
large, but, as is well known, the series of teeth in this animal
are continually renewed through a new formation of teeth from
the hind end of the series.

A peculiar forming of the skull we find in the Phocido?, in
a marked degree for instance in Phoca groenlandica (PI. 35 fig. 4).
The eyes are very large and directed rather with the cornea

1 See the figure p. 28 in Brehm, Tierleben, 4. Aufl., Saugetiere, 4. Bd.
s Ibid. p. 31.

upwards (PI. 40 fig. 5) — not so much laterally as in most other
Mammals —, in consequence of which the posterior part of the
nasal cavity lying between the large eyes has been compressed
almost to a thin plate, the upper surface of the skull between
the eyes being reduced to a rounded crest, and the postorbital
process being nearly completely rubbed out (the place of the
process may, however, be determined thereby that it is situated at
the anterior border of the plane of origin of the temporal muscle).
The orbits, the openings of which are in this way directed more
upwards than in other Mammals, have so large a circumference,
that their anterior margin is situated at a small distance from the
hind border of the osseous nasal opening and far in advance of
the infraorbital foramen (while in the starting-skull it is situated
far behind the said foramen). The fossa temporalis s. str. is much
reduced, so that also here (comp. Hydrochoerus etc.) it appears as
a minor appendix to the mighty orbit; the posterior origin of the
zygoma has held its place. Behind the large orbit the brain-case
is suddenly arched outwards: the brain is also large in size and
comparatively broad, the brain-case is so bulky that the origin
of the temporal muscle (in Phoca groenlandica) does not extend
over the whole brain-case, but especially towards the hind end
the upper border of the surface, from which the muscle takes
its origin, is far distant from the corresponding border of the
opposite side.

The skull of the Eared Seals (we have principally examined
Zalophus californianus, PI. 35 fig. 3) forms a connecting link between
the last described cranial type and the starting-skull. The
eyes are certainly also here large, but still there is an upper frontal
surface and a marked postorbital process, and the anterior margin
of the orbit is not placed so near the narial opening as in Phoca.
The fossa temporalis s. str. is also much larger than in Phoca
groenlandica, it is not very far from the conditions in the dog. The
brain-cavity is also not so broad in relation to the length as in
Phoca, but a rather remarkable development has taken place at
the hind inferior part of the orbit, the presphenoid being here
compressed between the two orbits to a quite thin, partly even
defective, vertical bony plate — quite analogous with the inter-
orbital plate in Fishes, Reptilia and Birds.

A similar development of the part of the skull between the
orbits as in the Phocidse we find in the Primates (PI. 33 fig. 4,
PI. 35 fig. 5—11). In these also the cranial surface between the
eyes has been reduced to a rounded rim, which for instance in
the Cynomorphse is very narrow, and the nasal cavity below it
has become very compressed. The eyes should in the Primates
be turned upwards as in thev Seals, if it not were for a modification
of the superior median part of the anterior portion of the skull
which has taken place. The said part (from the posterior margin
of the orbits forwards) has, in stead of forming as usual a horizontal
or feebly sloping continuation of the parietal surface, first
assumed, in the American Apes, an oblique direction from above
downwards, and then in the Catarrhinse the hindmost part of it,
that between the eyes, has taken a direction almost straight downwards
. In this manner the openings of the orbits are directed
quite forwards, while in the Platyrrhinse they are still directed
somewhat upwards. The foremost part of the skull, before the
eyes, has, on the contrary, still a direction obliquely forwards,
even in the Anthropomorphous Apes. It is only within the genus
Homo that this part, the dental series being shortened, also acquires
a direction from above-downwards, which is more marked in the
"higher" than in the lower races of mankind (PI. 35 fig. 10-11);
at the same time the nasals, that form the dorsum of the wall
between the eyes, have become vaulted1 and again confer to this
part a forward direction; in this way the characteristic human
nose arises (fig. 11). In the Apes the nasal bones are always flat,
only in the Gorilla there is a slight indication of a vaulting of
the nasal bones.

As is well known the basis cranii in the Primates makes an,
angle with the hard palate (PL 31 fig. 4), its anterior end being
directed obliquely upwards, its posterior end obliquely downwards
, which is different from the case in most other Mammals.
This has no connection — as might be supposed — with the

1 Comp.FoRSTER, Z.Frage nach d.Bildung d.auss.Nase b. Menschen. in: Arch. f.Anat.
u. Physiol. Jahrg. 1897. Anat. Abt. p. 163.