When the skull undergoes complete
ossification, osseous matter is thrown down at not fewer
than three points in the middle of its cartilaginous floor. The
ossific deposit, nearest the occipital foramen, becomes the basioccipital
bone; that which takes place in the floor of the pituitary
fossa becomes the hasisphenoid
; that which appears in
the reunited trabeculae, in front of the fossa, gives rise to the presphenoid.
Again, in front of, and outside, the cranial cavity,
may be represented by one or more distinct
An ossific centre may appear in the cartilage on each side
of the occipital foramen, and give rise to the ex-occipital
above it, to form the supra-occipital
. The four occipital elements,
uniting together more or less closely, compose the occipital
of the skull.
In front of the auditory capsules and of the exit of the
third division of the fifth nerve, a centre of ossification may
appear on each side and give rise to the alisphenoid
normally, becomes united below with the basisphenoid.
In front of, or above, the exits of the optic nerves, the arbitosphenoidal
ossifications may appear and unite below
with the presphenoid.
In front of the occipital segment, the roof of the skull is
formed by membrane; and the bones which complete the two
segments of which the basisphenoid and presphenoid form the
basal parts, are membrane bones, and are disposed in two
pairs. The posterior are the parietals
, the anterior the frontals
and the segments which they complete are respectively
. Thus the walls of the cranial cavity
in the typical ossified skull are divisible into three segments.
I. Occipital, II. Parietal, III. Frontal
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- the parts of which
are arranged with reference to one another, the sensory organs
and the exits of the first, second, fifth, and tenth pairs of
cranial nerves (I., II., V., and X.), in the manner shown in the
diagram (The names of the purely membrane bones in this diagram are in large
capitals, as PARIETAL; while those of the bones which are preformed in
cartilage are in smaller type, as BASISPHENOID.) on the preceding page.
The cartilaginous cases of the organs of hearing, or the periotic capsules
, are, as has been said, incorporated with the
skull between the ex-occipitals and the alisphenoids - or, in
other words, between the opcipital and the parietal segments
of the skull. Each of them may have three principal ossifications
of its own. The one in front is the prootic; the one
behind and below, the opisthotic
; and the one which lies
above, and externally, the epiotic
. The last is in especial relation
with the posterior vertical semicircular canal; the first
with the anterior vertical semicircular canal, between which,
and the exit of the third division of the fifth nerve, it lies.
These three ossifications may coalesce into one, as when they
constitute the petrosal
parts of the temporal bone
of human anatomy; or the epiotic, or the opisthotio, or both,
may coalesce with the adjacent supra-occipital and ex-occipitals,
leaving the prootio distinct.
The prootic is, in fact, one
of the most constant bones of the skull in the lower Vertebrata
though it is commonly mistaken, on the one hand for the
alisphenoid, and on the other for the entire petro-mastoid.
Sometimes a fourth, pterotic
ossification, is added to the three
already mentioned. It lies on the upper and outer part of the
ear-capsule between the prootic and the epiotic (see the figure
of the cartilaginous cranium of the Pike, infra
In some Vertebrata
the base of the skull exhibits a long
and distinct splint-like membrane bone (Bones may bo formed in two ways. They may be preceded by cartilage,
and the ossiflo deposit in the place of the future bone may at first be deposited
iu the matrix of that cartilage, or the ossific deposit may take place, from the
first, in indifferent, or rudimentary connective, tissue. In this case the bone
is not prefigured by cartilage. In the skulls of Elasmobraneh fishes, and in
the sternum and epicoracoid of Lizards, the bony matter is simply ossified cartilage,
or cartilage bone. The parietal or frontal bones, on the other hand
are always devoid of cartilaginous rudiments, or, in other words, are membrane
In the higher Vertebrata
the cartilage bones rarely, if ever, remain as such:
but the primitive ossified cartilage bcoomea, in great measure absorbed and
replaced by membrane bone, derived from the perichondrium.) - the parasphenoid
, which underlies it from the basi-occipital to the pre-sphenoidal
region. In ordinary fishes and Amphibia
, this bone appears
to replace the basisphenoid and presphenoid functionally,
while in the higher Vertebrata
it becomes confounded with
the basisphenoid. The Vomer
is a similar, splint-like, single
or double, membrane bone, which, in like manner, underlies
the ethmoid region of the skull.
In addition to the bones already mentioned, a prefrontal
bone may be developed in the prefrontal region of the nasal
capsule, and bound the exit of the olfactory nerve externally.
bone may appear behind the orbit above the
alisphenoid. Sometimes it seems to be a mere dismemberment
of that bone; but, in most cases, the bone so named is a
distinct membrane bone.
Furthermore, on the outer and upper surface of the auditory
capsule a membrane bone, the squamosal
, is very commonly
developed; and another pair of splint-bones, the nasals
cover the upper part of the ethmovomerine chambers, in
which the olfactory organs are lodged.