Freiburger historische Bestände - digital

D

THE SKULL.

REMARKS ON THE SKULL OF MAMMALS

IN GENERAL.

The form of the mammalian skull is to a conspicuous degree
an outcome of the different development of the organs, which are
placed on or in it, and of the very varied claims, which are laid
on it. It is to an extraordinary degree plastic, freely accomodating
itself to the various requirements. It therefore appears in extremely
different shapes.

The organs, which are determinative of the forming of the
skull, are especially the brain, the teeth, the maxillary muscles,
the eyes, the nasal passages; sometimes also the auricular region.
But still other moments may supervene. If for instance the head
is used as a shovel or fork, with which the animal works in the
soil, we shall see that this may be to a large extent determinative
to the forming of the skull. Also the presence of horns or antlers
is of consequence; etc.

In an animal such as the Dog (comp. PI. 33 fig. 2, PI. 35 fig. 2)
— we have here in the thought dogs of medium size — the facial
part and the brain-case are fairly of the same size, lying in continuation
of each other, the cerebral cavity being rather spacious
and to a large extent determining the posterior part of the skull;
the muse, temporalis is moderately developed; the zygomatic arch
and its anterior continuation, the infraorbital arch, from which the
masseteric muscle arises, are also moderately developed. The molar
series extends somewhat under the orbit.

In many other Mammals we find that the brain makes a more
modest part of the whole. The braincase then is relatively minor
and, to make place for the temporal muscle, crests are developed
on the brain-case above and behind. Didelphys marsupialis (PI. 33
fig. 1, PL 35 fig. 1) for instance presents this image when compared
with dogs of medium size; also large dogs in comparison with
smaller. The facial part is then large in comparison with the
brain-case.

Very commonly such a diminution of the brain and brain-
case is connected with our having to do with an animal of large
size, the brain has as is well known on the whole a relatively
small size in large, a relatively large size in small Mammals. This
is for instance the case with the Hippopotamus (PI. 33 fig. 3, PI. 34
fig. 4, PI. 31 fig. 3) which has a very small brain-case, but provided
with crests; the temporal groove has also a large transversal
diameter, the zygomatic arch lying far off from the brain-case, so
that there is room enough for a large temporal muscle, and also
the zygomatic arch is strongly developed, giving rise to a powerful
masseteric muscle. But in spite of all this the main features
of the cranial form are preserved by Hippopotamus, if we set aside
that the facial part has a dominant development in comparison
with the brain-case.

Also in Mammals with a comparatively large brain the said
type may be preserved, for instance in many bats (PL 33 fig. 9),
the brain of which is very large; also in these animals the characters
of the skull are mainly the same as in the dog.

This type we can take as the starting point for the deviating
forms which are found among the Mammals.

One of these is found in a great number of the Rodents:
Myopotamus, Hydrochoerus a. o. (PL 33 fig. 12, PL 34 fig. 7, 10,11).
In these the figure of the skull has been essentially modified through
the different condition of the temporal groove. To understand what
is the question here, the following should be premised.

By the name temporal groove we indicate in the Mammals
the whole part filled up with the temporal muscle, the part which
is in the human anatomy (for instance by Gegenbaur) called planum
temporale + fossatemporalis. It is laterally arched over by the
zygomatic arch, by which name we design the lateral bony arch
from its posterior origin unto the processus orbitalis, while for the
anterior part of the same arch we use the name infraorbital arch.
Medially the temporal groove is limited by the whole of the large
surface, from which the temporal muscle takes its origin, which
in front extends unto the postorbital or zygomatic process of the
frontal bone. The latter process and the orbital process of the
zygoma laterally mark the limit between the orbit and the temporal
groove; medially the limit is sometimes marked by the
crista pterygoidea situated behind the well known series of cranial
openings (foramen opticum etc.), but often it is not indicated on
the skull at all. The cranial wall of the temporal groove is posteriorly
more or less convex — planum temporale — but is anteriorly
concave; this concave surface medially limits what we
design as the fossa temporalis s. str., which is below the postorbital
process of the frontal smoothly continued in the orbit. In
the starting type the zygomatic arch arises quite posteriorly, close
to or partially above the meatus auditorius.

Now in the above named Rodents the change has taken
place — no doubt in connection with the temporal muscle having
become reduced — that the planum temporale is of a relatively
small size and not convex, but plane or concave, and that the
fossa temporalis s. str. apparently wholly lacks, having been reduced
to a minimum and only appearing as a shallow posterior bay of the
orbit. At the same time the posterior origin of the zygomatic arch
has moved forwards, so that it is situated almost directly below
the processus postorbitalis of the frontal bone, towards which the
squamosal bone extends, even so that the proc. postorb. is partly
formed by the squamosum; the maxillary joint, which is bound
to the posterior end of the zygoma, thus being placed below the