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306 MORPHOLOGY OF THE SKULL. [CHAP.

The basioccipital and basisphenoid are separated by synchondrosis
up to the twentieth year. In the adult large air-cells are found m
the conjoined basi- and presphenoid. A considerable portion of the
nasal septum remains unossified, in front of the mesethmoid. There
is a persistent aliseptal cartilage on either side, antero-inferior to
the nasal bone, and united with the top of the septum ; also a pair of
aliuasal cartilages, each bent on itself, and the two co-applied over
the termination of the septum. Thus the alinasal cartilages bound
the first part of the nasal cavities above, mesially, and externally, but
do not reach to the base of the nostril in either position.

691. The alisphenoid nuclei appear about the eighth week of
foetal life between the foramina rotunda and ovalia, and spread
thence outwards, and also downwards into the external pterygoid
processes. The basisphenoid arises from two granules, lying side by
side in the sella turcica, and uniting about the fourth month. After
their union two other centres appear, forming the lingulsB (basi-
temporals), just outside the carotid grooves. The internal pterygoid
plates (proper pterygoids) arise by separate nuclei in the fourth month.
The alisphenoids are united to the basisphenoid in the first year after
birth. The orbitosphenoids appear as a pair of nuclei, one outside
each optic foramen ; these extend by growth into the orbitosphenoid
cartilages (lesser wings of sphenoid). Another pair of nuclei is found
on the inner side of the optic foramina, and the presphenoid is
formed by their union, or there may be an independent centre. At
birth the alisphenoid^ are only suturally united with the lingulse,
which are still large in comparison with the basisphenoid. The
orbitosphenoids and presphenoid are anchylosed together above the
optic foramina, but the part of the orbitosphenoid beneath the fora-
men abuts against the basisphenoid. Later still, the basisphenoid
becomes larger in proportion to the lingulse, and the posterior clinoid
processes are ossified. The sphenoidal turbinal appears after birth,
applied to the front of the body of the sphenoid (pre- and basi-
sphenoid). Each is in early life a hollow pyramid formed of three
laminse; an inferior, constituting the adult turbinal; an external or
orbital portion, situated in the adult between the orbital plates of
the lateral ethmoid, alisphenoid, frontal, and palatine ; and a superior,
forming the inner wall and roof of the original sphenoidal sinus, be-
coming partially absorbed and partially united to the presphenoid,
which by the enlargement of the nasal cavities and sphenoidal sinuses



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Vin.] THE HUMIK SKULL. 307

Is ultimately reduced to the thin sphenoidal septum and rostrum.
The fff^phenoid is for a year or two broad, and rounded inferiorly ;
it gradually becomes narrower and more prominent

692. Ossi&cation arises in the ectethmoids (orbital plates) in
the fourth or fiflk month, and gradually extends into the turbinal
coils ; during the firs^ year the mesethmoid with the cribriform plate
is ossified by a single nucleus, uniting with the lateral masses about
the beginning of the second year. The ethmoidal cells are developed
in the fourth or fifth year. It is only after birth, and with the
gradual advance to adult years, that the spheno-occipital and spheno*
ethmoidal synchondroses are obliterated ; the occipital mass is com-
pletely united into one bone; the vomer may become anchylosed
with the mesethmoid. The epiotic fenestra existing for a long time
beneath the arch of the superior semicircular canal is finally occupied
by bone.

693. The maxillary, arising in the maxillopalatine process, begins
to ossify immediately after the mandible and the clavicle, from several
nuclei B^clard (Meckel's Archiv, vi. 432) describes centres for the
alveolar arch, the palatal plate, the orbito-malar tract, the nasal and
facial, and the incisor. These are united at the end of ihe third
month of foetal life. The antrum begins as a shallow depression seen
before birth on the inner surface of the bone ; it deepens, extends
outwards, and gradually separates the orbital and palatal portions,
which at birth are close together. The premaxillary, occupying the
nasofrontal process, is distinct, but is covered on its facial aspect by
a process of the maxillary, which unites with it completely before
birth. In all young skulls an incisor fissure is traceable on the
palate between the premaxillary and the maxillary, passing outwards
from the incisor foramen to the front of the canine socket The
palatine arises by one centre ; the vomer by one, which developes two
laminfie embracing the septal cartilage. These two plates undergo
increased union from behind forwards up to puberty, forming a single
median plate, and ultimately leaving only a groove on the anterior
and superior surface, in which the mesetiimoid fits. The nasal and
lachrymal have one centre each; the jugal (malar) has one or two ;
the inferior turbinal one.

694. The squamo-zygomatic portion of thQ temporal arises by
one nucleus, the tympanic by another around the tympanic mcm-

20—2



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308 MORPHOLOGY OF THE SKULL. [CHAP.

brane, and these centres soon unite. The pcriotic capsule ossifies
late, by three centres ; (1) the prootic, forming the roof of the cochlea,
the superior and part of ttie posterior semicircular canals, the in-
ternal auditory meatus, the tegmen tympani, and the upper part of
the girdle of the fenestra ovalis ; thus the prootic gives rise to the
greater part of the petrous portion of the temporal bone, as well as
the upper portion of the mastoid ; (2) the opisthotic, constituting all
of the petrous portion visible on the base of the skull, the floor of the
cochlea, the inyestment of the fenestra rotunda, and half the girdle
of the fenestra ovalis ; it also developes the lamina which completes
the carotid canal, and furnishes the inner part of the floor of the
tympanum ; (3) the epiotic, forming the lower and main part of the
mastoid process.

695. At birth the petromastoid or periotic bone is separated
from the squamosal by cartilage ; bony union takes place in the first
year. The "mastoid" tract is then flat, the glenoid fossa shallow,
the articular eminence of the zygoma scarcely perceptible, the styloid
process cartilaginous. The external auditory meatus is only de-
veloped after birth, by the arching outwards of the united bones,
and the growth of the special tubular part from the external surface
of the tympanic ring. The mastoid process gets prominent in the
second year, but its cells only arise after puberty. A tympanohval
is found at birth and is distinct for a few years after, as a little cylin-
drical plug, in a depression in the hinder border of the tympanic,
just antero-internally to the stylomastoid foramea It is soon anchy-
losed with the periotic mass, and becomes ensheathed by the vaginal
process of the tympanic. The styloid cartilaginous process is con-
tinuous with the tjmpanohyal, begins to ossify before birth, and may
remain unanchylosed with surrounding parts even to middle life.

696. The moieties of the mandible arise at first in fibrous tissue
around the meckelian cartilage. Mr Callender^ has discovered a
mento-meckelian ossification of the cartilage at the symphysis ; and
there appear to be splenial and coronary elements on the inner side,
in addition to the dentary on the outside. The upper end of the
meckelian cartilage becomes ossified as the malleus, and the carti-
lage extends downwards and forwards between the tympanic bone
and the periotic capsule; the processus gracilis of the malleus, lying

1 See Phil, Trans. 1869, p. 163.



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Vin.] THE HUMAN SKULL. 309

in the Glaserian fissure between the squamosal and the tympanic, is
the limit of ossification, the rest of the cartilage becoming gradually
obliterated. The hyoid arch is represented above by tha incus, the
tympanohyal, and the styloid process or stylohyal. The auditory
ossicles are at first altogether outside the tympanic cavity; and as the
latter enlarges, its mucous membrane is reflected around the ossicles.
The stapes is described as having three ossific centres. The hyoid
bone is developed by five centres, for the two pairs of comua and the
body.

697. The dates of appearance of the different osseous centres are
as follows ; sixth or seventh week of gestation, dentary, premaxillar}',
maxillary, frontal ; seventh to eighth week, basioccipital, exoccipital,
supraoccipital, basisphenoid, alisphenoid, presphenoid, squamosal,
parietal, palatine, vomer, nasal, lachrymal, jugal or malar; fourth
month, pterygoid, lateral ethmoid (middle and upper turbinal); fifth
month, inferior turbinal; fifth and sixth months, periotic bones;
eighth month, hyoid (great comua, body, small comua), tympanic,
tympanohyal; stylohyal; first year, sphenoidal turbinal, mesethmoid
and cribriform plate.

698. The union of distinct bones takes place in the following
order: the two basisphenoid centres unite in the fourth month of
foetal life; the pterygoid and the extemal pterygoid plate in the
sixth month ; the presphenoid and basisphenoid in the eighth month.
During the first year after birth the periotic mass unites vnth the
squamosal, the alisphenoid with the basisphenoid, and the moieties of
the lower jaw coalesce at the symphysis. The second year witnesses
the union of the frontals with one another, and of the mesethmoid
with the lateral masses. The exoccipitals coalesce with the supra-
occipital between the second and fourth years, with the basioccipital
from the fifth to the sixth year. The basioccipital and basisphenoid
are anchylosed only after the twentieth year. The styloid ossification
unites with the "temporal" in adult life; the osseous union of the
hyoid comua with the body of the bone does not occur till after
middle age.



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CHAPTER IX.

THE MORPHOLOGY OF THE SKULL.

699. Having now described in a condensed fashion
the more important facts which have been ascertained
respecting the history and structure of the skull in various
forms, we proceed to summarise those facts, and to indicate
some conclusions which appear reasonable. A larger
measure of consideration i^ asked for these conclusions
than if they merely arose from a verification of the facts
which have been detailed; for they have become deve-
loped in the course of many years of exploring work,
in concomitance with a transition from the darkness of
archetypal fancies to the clear light of actual verifiable
history. In many cases the views expressed are based
upon a much more extensive acquaintance with skulls in
all stages of development than has been indicated ; a
mass of detailed information remains for future publi-
cation.

Tlie Cartilaginous Skull

700. In all cages the primary elements of the cartila-
ginous brain-case consist of two pairs, arising beneath the
matrix from which the dura mater developes ; (1) the para-
chordals posteriorly, under the greater part of the hinder
division of the brain ; and (2) the trabeculae beneath the
forebrain. Whatever cartilage appears subsequently in the
brain-case grows nearly always in direct continuity with
these elements, by gradual chondrification of mesoblast
In this way a more or less complete cartilaginous box is
ultimately formed.



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CHAP. IX.] THE CARTILAGINOUS SKULL. 311

701. The notochord is related to the whole length
of the parachordals and to tb^e hinder extremity of the
trabeculse. The stratum in which it originates is imme-
diately below the i^^rrotts axis, and somewhat above that
in which the cartilaginous elements arise. The ante-
rior €iiid of the notochord is often curved upwards towards
tte second brain vesicle ; but in some cases it is turned
downwards again between the trabeculae; and it has
moreover been detected in Elasmobranchs actually bent
double, and running backwards some distance before
teirminating. In other forms the flexure of the noto-
chord is very slight ; it simply bends downwards a little.
In embryo Sharks and Skates from half an inch to an.
inch in length, the notochord is seen to present a number
of headings in its extreme anterior iportion; this feature
is evanescent. In the Fowl at an early stage the cranial
notochord has two constrictions dividing it imperfectly
into three spindle-shaped regions; and this continues
observable for a considerable time. So fai* as we can
make out, there is nothing in the stnicture of the noto-
chord itself which gives us very definite information about
its segmental relations or about the segmentation of the
skull. The point to be remembered is, that the notochord
has a relation to the trabeculae as well as to the para-
chordals.

702. The parachordals become identified sooner or
later with the tube of tissue surrounding the notochord,
which in several cases (Bombinator, Dogfish, &c.) is defi-
nitely chondrified previously to this confluence, and which
is continuous with the cylinder of cartilage surrounding
the rest of the notochord. Each parachordal becomes
coalesced with the trabecula and the ear^capsule of its
own side, and developes more or less of the lateral occi-
pital wall, before uniting with its fellow. In fact, in the
Majority of cases the parachordals during development
recede from one another in their anterior hall* or two-
thirds ; and concurrently with this, the notochord becomes,
by the growth of surrounding parts, apparently retracted



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312 MORPHOLOGY OF T^E SKULL. [CHAP.

in the cranial floor, so as to appear only between the hinder
part of the parachordals. Thus for a time there is consti-
tuted a posterior basicranial fontanelle, or TOembranous
space in the hinder part of the floor of the chondro-
cranium.

703. When the parachordals unite in the region where
the notochord still persists, it is by the growth of car-
tilaginous bridges both over and under it, The bridge
beneath the notochord is very marked and becomes thick ;

, the cartilage is thinner above, and often non-existent for
a long time, so that the notochord lies in a groove on the
basilar plate constituted by the union of the parachordals.
In many cases where a basicranial fontanelle exists, the
cartilages do not approach one another again, and the
fontanelle is only closed by bony growth; but in other
types, especially in Birds, the space is nearly obliterated
by the growth of cartilage, a later bony deposit completing
the work. The whole of the cranial notochord is gradually
aborted in most instances, and its place is occupied by car-
tilage ; but in various forms a remnant is left as a slender
string, embedded in the basioccipital bone or cartilage.

704. The length of the parachordal cartilages com-
pared with other parts varies greatly. In some cases
(Salmon, Fowl) the parachordals stretch anteriorly almost
to the extreme notochordal apex, and posteriorly to a
considerable distance behind the ear-capsules, being at
first much more bulky than the trabeculas. It is note-
worthy that the parachordals extend in the Elasmobranchs
into a very considerable region of the neck. A seg-
mentation takes place afterwards, separating the cranial
from the cervical cartilages at the usual place, and forming
two basal condyles convex backwards. In all other known
forms the hinder extremity of each parachordal primarily
forms a condyle : and thus in every cartilaginous condition
of the vertebrate skull where condyles are formed, there
are two occipital condyles.

705. The parachordal is always bent outwards more

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IX.] THE CARTILAGINOUS SKULL. 313

or less, behind and in front of the ear-capsule, and
partially undergirds it. In Elasmobranchs it grows quite
underneath the otic mass, and is even earned outwards
somewhat beyond it. The paroccipital process in higher
vertebrates is an extension of the parachordal around,
outside, and beyond the ear-capsule. The preauditory
growth of the parachordal is more or less in confluence
with the hinder end of the trabecula.

706. The trabeculae are at first simply solidifications
of tissue, afterwards chondrified, in the sides of the floor
of the first cerebral vesicle, which almost bulges down
between them. The hinder termination of the trabeculae
(Salmon, Frog) is often very slender at first. In
others, as in Urodeles, they are largest behind, and early
embrace the anterior half of the cranial notochord; in
Sharks they are about of equal breadth throughout. In
cases where they are early in contact with the para-
chordals, they lie over them at an angle The two tra-
beculae have always a more or less lyriform appearance,
being approximated in front, beneath the fore part of the
brain-case, and also behind. In the posterior portion of
the intertrabecular space the infundibulum and pituitary
body are found ; but the primary interval between the
trabeculae is related to the whole of the first cerebral
vesicle, and not merely to these bodies.

707. The primitive trabeculae are flat or more or less
rounded in section. The position they occupy in the
head undergoes change in consequence of the mesocephalic
flexure; at first, where early distinct, they are on the
whole on the same level as the parachordals; but the
mesocephalic flexure may bend the former down to an
angle of 120*^ or more with the latter, and in other cases
they occupy this position when first distinguishable. As
growth advances this flexure is entirely lost, and the level
of the trabeculae and the parachordals may again become
approximately identical.

708. It is on the whole in the lower types that the



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314 MORPHOLOGY OF THE SKULL. [CHAP.

trabeculse are perceptible as primitively distinct from the
parachordals ; and this condition would appear to be due
to the mesocephalic flexure. In higher forms (Bird, Pig)
they can at no time be found clearly separated, although
a demarcation may appear to exist as the rudiment of
the posterior clinoid ridge. Where the dimensions of
the trabeculse are least bulky behind, they may barely
embrace the i^ex of the notochord : in every case where
they are distinct they do ^^s. Where tiiey «re of greater
size posteriorly, they are related to a varying length of the
notochord ; in the extreme case (Axolotl), to more than
half the cranial notochord, tapering backwards in close
contact with it, and attaining their greatest width opposite
the notochordal apex.

709. There is a most interesting variation in the
relative size and date of appearance of parachordals and
trabeculae. In some types the parachordals are large and
definite from the first, and the trabeculae disproportionately
small and little solidified (Salmon); in others the trabeculae
are clearly manifest and solid, and have taken up their
definite relations considerably before the parachordals have
appeared at all (Axolotl, Frog). In Elasmobranchs the
two are very similar, but the trabeculae chondrify first,

710. If not existing at the earliest time of solidifi-
cation, a prechordal (postpituitary) bridge is formed as a
meeting-place of trabeculae and parachordals, except where
the parachordals do not extend so far forwards. In most
cases this bridge appears to be wholly or in great part
formed by the trabeculae. Furthermore the posterior
clinoid ridge appears usually to be clearly a trabecular
product, either arising as a transverse ridge on the united
basicranial cartilage (Fowl, Pig), or being produced at first
by the trabeculae overlapping the parachordals.

711. At the broadest region of the trabecula, in front
of the notochordal apex, a small curved tongue or lingula
may be given oflF, remaining free from other parts, and
directed backwards (Fowl); or such a process may be



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I3L] the CARTILAanrOTJS SKULL. 315

large and become united with an antero-extemal process
of the parachordal {Carcharias glaucua).

712. The anterior extension of the trabeculae at their
first appearance varies very greatly with the extent of the
early development of the snout. They always from the
beginning underlie the whole of the anterior part of the
brain-case ; but in some cases (Frogs and certain Newts)
they pass between the nasal sacs to the prenasal region
very early, and throw out comua. In others they very
soon extend to the intemasal region, where the septum
subsequently forms. The trabecular modifications in the
region of the cranial cavity will be first dealt with.

713. The trabeculae underneath the fore-brain, which
are at first separated by an interval, but are closest
together anteriorly, in many forms very early become
approximated all along their length, except where the
pituitary body and the carotid arteries lie between them.
They may be placed quite flat together (Elasmobranchs)
or obliquely at an obtuse or right angle looking down-
wards (Salmon). Coalescence quickly takes place in all
the region of apposition, and a trabecular plate occupies
the whole floor of the brain-case in front, adapting itself
to the shape of the brain, being flat or gently curved, often
rising more or less towards the anterior or ethmoidal
extremity of the brain-case. The prepituitary part of
the trabecular cartilage may develope a distinct anterior
clinoid ridge, and thus a squarish or circular pituitary
fossa may become limited. At a later period in many
cases a thin floor of cartilage, continuous with the trabe-
culae, arises in the bottom of the pituitary fossa, interrupted
only by the perforations for the internal carotid arteries.
The trabecular plate sometimes extends outwards beyond
the cranial cavity, so as to partially support the eyeballs.

714. In Amphibia generally the primary trabeculae
remain apart where tbey underlie the forebrain, but in
the Anura a thin cartilaginous floor unites the bars and
fills up the primordial fontanelle. In Snakes the trabeculae



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316 MORPHOLOGY OF THE SKULL. [CHAP.

are permanently separate as far as the nasal region, and
retain a simple form. In Eels they are separate anteriorly.

715. The most remarkable modification of the cranial
trabecular region is that which gives rise to an interorbital
ssptum in Osseous Fishes, Lizards, and Birds. The roof of
the mouth gradually becomes separated by a considerable
distance from the floor of the cranium, and the deeper
portion of the eyeballs comes to lie between the palate
and the cranial floor. A median membrano-cartilaginous
vertical wall (interorbital septum) separates the orbits
from one another; and the cartilage which it acquires is
usually due to the growth of a median trabecular crest.
Thus the forebrain becomes supported mesially on a wall
which rises high in front of the pituitary region; and
lateral outgrowths from the top of this septum underlie
the sides of the brain. The interorbital septum may be
thick and solid, or thia with some thickening at the base ;
and a greater or less extent of it may never chondrify, or
may become membranous after having once been carti-
laginous.

716. When the trabecula3 primarily extend as two
distinct rods into the internasal region, they are early
coalesced into an internasal plate, which forms the base
of the nasal septum where that is definitely developed.
The same term of internasal plate is also applied when the
internasal chondrification, continuous with the trabeculae,
is single from the first. This plate may be a simple bar
at the base of the internasal region, very like that pro-
duced by the early coalescence of the two trabeculae as
above described, appearing as a continuation from the
level of the cranial floor and sometimes permanently
retaining this form; or it may first arise as a prolonga-
tion of the vertical interorbital septum just described.
In any case, anteriorly to the internasal tract, an azygous
prenasal structure is very often developed; either directed
straight forwards (Skate), or curved downwards as the
axis of the beak (Bird). In the Skate it attains a very
large development forwards as the axis of the rostrum,



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IX.] THE CARTILAGINOUS SKULL. 317

and has a grooved upper surface continuous with the
internasal floor. In the Chick at an early stage, and in
the Tortoise, it is bent underneath the axial parts and
becomes recurrent. Where the adult has a very blunt
snout, the prenasal element is short or almost absent
(Salmon, Axolotl, Frog).

717. The main part of the nasal septum may at first



Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 26 of 31)