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The morphology of the skull online

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arise double, as in the Pig, the two laminae very early
coalescing. When the nasal organs are widely separated
there are wings of cartilage (nasoseptal laminae), which
arch widely outwards from the basal internasal region
and unite with the olfactory capsules. They may become
so outspread in adult life as to appear continuous with
the internasal plate, only rising gently on each side.

718. Lateral outgrowths always proceed from the
axial cartilages in the nasal region. The olfactory organs
become supported by anterior and posterior growths, the
trabecular comua, and the antorbital or lateral ethmoidal
cartilages. The comua in some types are visible at almost
the earliest stage of the growth of the skull, as outward
curvings of the trabeculae, jutting forwards in front of the
nasal sacs : and they may soon show traces of division into
two lobes, of which one, external, broadens and partly
underlies the nasal sac and defends it anteriorly, while the
other bends inwards and downwards, becoming more or
less recurrent underneath the internasal floor; and finally
these recurrent cartilages, in some Birds, coalesce into
one, quite independently of the prenasal part, which
exists at the same time.

719. The antorbital cartilages in some respects agree
with the cornua, in having a similar relation to the nasal
capsule, in arising early from the trabeculae at their
anterior junction : and they may be almost completely
recurrent (Skate). In Urodeles they arise quite separately
from the ethmoidal angles of the skull. They frequently
acquire a greater vertical depth than the cornua, in
accordance with the height of the corresponding parts;

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and they may become the points of attachment or coa-
lescence of a region of the mandibular arch, or cartilagi-
nous parts developed from it. The antorbital frequently
coalesces with the back of the nasal wall, and in that
circumstance is sometimes carried far outwards by the
growth of the capsule, being disjoined from the axial
part from which it at first arose.

720. We have now to trace the formation of a
cartilaginous brain-case in the tissue surrounding the
primary membranous cranium, in continuity more or less
perfect with the basal structures which have been described.
Separate cartilages are seldom developed in any part of
the cranial boundary, but in many cases it is quite im-
possible to determine whether the diondrification has been
by a conversion of the cells at every point of the tissue,
or by a proliferation of the cells in cartilage already
existing: probably both processes occur. The formation
of lateral walls in cartilage is frequently not so complete
as the furnishing of basal parts: and their extent and
style seem to be very much influenced by the impaction
of the three pairs of sense-capsules. The roof is still less
perfectly chondrified in many types, and has its own special
regions of fenestration.

721. In all cases a cartilaginous occipital ring, bound-
ing the foramen magnum, is formed by the upward growth
of the pai-achordals on either side, meeting above and
coalescing. This ring is perfectly definite and continuous
behind the periotic masses : but is complicated with them
in their hinder region. The occipital ring in most cases is
not limited to the region behind the ear-capsule, but the
cartilage extends above and supero-laterally between the
ear-capsules in the cranial roof, sometimes stretching for-
wards for two-thirds of their length. The occipital invest-
ment may be left sonaewhat imperfect below in the noto-
chordal tract ; but in these cases there is a remnant of the

722. A cranial wall distinct from the ear-capsule
cannot be clearly made out in the auditory region.

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Further forwards there is a cartilagmous wall, inteiTupted
by nerve foramina and by fenestrae. This, wall may be
approximately vertical in lower types; but in ascending
the scale, it becomes more and more tilted outwards and
overhanging, so that ultimately (Pig) parts which were
constituents of the wall in lower types enter into the
composition of the floor of the large cranium, and the
greater part of the wall and roof is unchondrified,
and forms a vast fontanelle in the cartilaginous skull.
It is especially to be noted that the types which pos-
sess an interorbital cartilaginous septum are as a rule
deficient in their cranial walls. The regions of lateral
cartilage will be defined when we come to speak of nerves;
but it must be mentioned here that in some cases {e.g.
Snakes) a small cartilage called alisphenoid arises in the
tract immediately in front of the ear-capsule, in the
cranial wall or external part of the floor; and this may
also appear as a process of basicranial cartilage, not cut
ofl" from it. In front of this a larger wing of cartilage,
the orbitosphenoid, may be developed, also having definite

723. The lateral cranial wall originates in the simpler
types by the growth of a longitudinal crest upon the
trabeculae ; this is most prominent behind, and often early
confluent with the auditory capsule behind and the nasal
in front. Where the cranial region is definitely marked
off (in all but Elasmobranchs), this crest is continued in^
wards in the ethmoidal region, the two crests meeting so
as to form with the floor a trough or barge-like hollow in
which the brain lies : and at the same time the ethmoidal
tract may be confluent with the nasal sacs.

724. The lateral chondrification may become con-
tinuous with a superior (supraorbital) ridge of cartilage
connecting the ethmoidal and nasal regions with the
auditory, and finally the chondrification may proceed in-
wards towards the centre of the cranial roof. The tegmen
cranii (Frogs, Sharks, Raya, &c.) is of varied size, usually
leaving one large fontanelle in front, or two hinder

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small ones in addition (Frog). The growth may take
place chiefly from before backwards, as in the Salmon
and Frog; in the Salmon it almost entirely covers the
brain-case with a thick roof or tegmen, excepting only
two small postero-lateral or parietal fontanelles in which
cartilage is permanently deficient. In Urodeles the growth
of the cranial floor may be arrested early and the tra-
beculae may remain comparatively widely separated, and
finally appear as totally absorbed into the side walls, form-
ing pai't of them ; so that the cranium has two complete side
walls but no cartilaginous roof or floor in the trabecular

The Sense Capsules.

725. The organs of special sense influence the archi-
tecture of the skull in a very marked degree, the eyeballs
being no less potent than the nasal and the auditory
capsules in affecting the external form of the skull and
the style of its intimate structure. The sense-capsules
have in common a more or less complete cartilaginous
investment, and an intimate association with and protec-
tion by the cranial cartilage. They are situated in pairs
at the sides of the skull, and are related to a more or
less extensive fenestration of the cranial cartilage where
they abut upon it. Each capsule receives a special nerve
of sense, besides being supplied with other nerves. The
cartilaginous investment is not perfectly complete in any
case, or if it is complete at any period, does not remain
so, orifices occurring through which the efficient agencies
in exciting sensation may work, or in some cases as the
relic of a primary involution. But the olfactory invest-
ment differs from the other two in being usually like a
cap or dome, more or less widely open below, while they
are more complete, and rather resemble balls.

726. The auditory masses are on the whole of larger
size relatively to the cranium in lower types, and in
ascending we find them more and more subordinated to

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the enlarging brain-case, and inwrought into its general
contour. In the lower groups the height of the ear-
capsule is nearly equal to that of the cranium, and its
breadth may be as great ; and the close impaction of the
organ upon the brain-case does not allow of the formation
of a distinct cranial wall of cartilage at that region, and
leads to a coalescence of cranium and capsule at all the
borders, with marks of distinctness at various points.
Thus we may speak of the lateral tract where there is no
distinct cranial cartilage as constituting a large auditory
fenestra. In higher forms we are not able to perceive
the formation of cranial and auditory cartilages separately
from one another; the growth is continuous from the
first : but the real fenestration may become obvious at a
later stage, after ossification has taken place, by the lack
of anchylosis with other parts throughout the whole or
great part of the borders of the periotic mass,

727. In inany instances a small postero-superior region
of the capsule never chondrifies, or remains unchondrified
till a late period of development, forming an epiotic fe-
nestra. It marks the primary connection of the auditory
mass with the epiblast. Another fenestra is formed in
the Salmon infero-mesially, appearing at first as a space
between the parachordal cartilage and the capsule; but
the former travelling outwards and the capsule lying
over it, and becoming identified with it, the fenestra
travels also outwards, and becomes really an opening in
the lateral aspect and floor of the vestibule, afterwards
closed. Its ultimate position is similar to that in which
the stapes arises in Amphibia. A deficiency occurs in the
cranial surface of the ear-mass in some Teleostean fishes,
so that there is no cartilage at that part between the
membranous labyrinth and the cranial cavity,

728. A remarkable feature about the ear-capsule is
the formation at an early period in nearly all types of an
external more or less horizontal projection, called the
pterotic ridge : it is related to and often contains part of

B. M. 21

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the horizontal semicircular canal. Where there is no
tympanum this ridge overhangs the articulation of the
appendicular elements of the skulL When a tympanic
cavity is developed, this ridge is specialised into a tegmen
tympani, or tympanic roof; and then there is a definite
scooping of the ear-mass beneath the ridge. This cavity
may be still more defined by a posterior boundary and
partial floor derived from the exoccipital cartilage, and
by a cartilaginous floor derived from the ear-cartilage
itself (auditory bulla).

729, The surfaces of articulation and regions of
coalescence of the periotic capsule with appendicular
parts will be mentioned later; but one point must be
referred to here : namely, that a fenestra (ovalis), which
has an intimate relation to the function of hearing, arises
in the part of the capsule adjacent to the tympanum,
either by dehiscence of the cartilage, or by the cutting out
of a small segment, the stapes, which remains in the
hole out of which it had been cut. The stapes is formed
in other cases by the chondrification of the tissue in the
membranous fenestra. In the Urodeles the origin of the
stapes may be described as the cutting ofl^ of an opercular
fold over the fenestral slit; and here the stapes is only
connected by ligament with parts at some distance from
it. But in higher forms the stapes is always closely tied
to, or coalesced with, or originates in continuity with ex-
traneous elements. Another fenestra (rotunda) originates
in the wall of the cochlear rudiment in many Reptiles, and
occurs in a similar situation in all Birds and Mammals.

730. The mobile eyeballs are not amalgamated with
the cranium like the auditory masses ; but the cavities
in which they are lodged show most admirably the plia-
bility of the axial parts, and the manifold shades of adap-
tation which they can assume. Whether the eyes occupy
the sides of the whole fore part of the brain-case, or are
embedded beneath it, or elevated above it (Chimaera),
or carried outwards (Zygaena), the orbits are bounded by
the same cartilaginous structures little modified. In the

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simple lateral position of the eyes (Sharks), the outward
extension of the trabecular base of the cranium provides
a partial floor for the orbit, the projecting supraorbital
tract protects it above, and the antorbital and postorbital
or sphenotic terminations of that tract wall it in before
and behind. The palatal structures and aponeurosis form
the rest of the orbital floor. In the Amphibians, which
have simple lateral orbits, there is little furnishing of
protecting laminae ; but the history of the suborbital car-
tilages in the Anura is of great interest, the subocular
bar being first the mandibular suspensorium and after-
wards the palatopterygoid cartilage. The eyeballs in au
early stage rest almost entirely on cartilage ; but as the
subocular fenestra is gradually widened with the growth
of the head, the eyes come to rest almost exclusively
on membrane. In these types the optic foramen is always
larger, often much larger, than the nerve which passes
through it, the rest of the space being filled by membrane.

731. When the eyeball is more deeply embedded and
more thoroughly protected, we have a modification of the
cranial cartilage which is as if the lower regions of its
side-walls were forced together and converted into a mere
vertical septum. The fore part of the cranial cavity is
elevated upon this septum, the portion of the brain therein
contained becoming small proportionately, and lying be-
tween the upper part of the eyes, or even totally above
them. In this condition, as growth proceeds, a more or
less extensive fenestration of the septum very frequently
occurs, and there may also be (Lizard) fenestration in
the slanting cranial roof of the orbit (orbitosphenoidal
tract), as well as an optic foramen larger than the nerve.

732. In Birds the orbit is protected by the cranial
wall slanting outwards above and behind, by the pro-
minent supraorbital ridge above and behind, and by the
antorbital or postnasal plate of the nasal capsule, which
may be prolonged into an external and inferior process (os
uncinatum of Finches and Parrots). No very definite floor
is developed, the palatine tracts being little specialised


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in relation to the orbit. In the Mammal an additional
ciicumstance influences the position and relation of the
orbits, namely, the extension backwards of the nasal cap-
sules beneath the fore part of the cranium and between
the eyes. In its cartilaginous condition the orbit is roofed
by the orbitosphenoidal tract, and partly floored and walled
by the lateral ethmoidal cartilage. It is subsequently
completed by bones, but may continue to be very largely
supported by membrane beneath. A very considerable por-
tion of the orbit may in these cases be below the level of
the cranial floor.

733. The sclerotic investment of the eyeball is car-
tilaginous in most types except the mammalian. In many
Elasmobranchs and some Osseous Fishes the eyeball is sup-
ported near the entrance of the optic nerve by a movable
cartilaginous rod or pedicle, articulated with the sclerotic
and with the cranial cartilage.

734. In its simple condition in the Elasmobranchs
the nasal capsule is a dome of cartilage widely open below.
Its sensory membrane is produced into many folds, but
these are not supported by cartilage. The capsular carti-
lage does not gtow out from the cranium, but early be-
comes continuous with it behind. More or fewer labial
cartilages become involved in the valvular inferior open-
ing, and may coalesce with the proper nasal investment.
A lateral union of the capsules with the intemasal cartilage
occurs by the intervention of nasoseptal laminae. A fenes-
tra may arise in the inner superior aspect of the capsule,
in addition to the large nerve fenestra more posteriorly,
through wliich the olfactory fibres pass downwards.

735. In the Salmon separate capsular cartilages can-
not be said to exist : there is at an early period a median
internasal tract of cartilage with anterior and posterior
horns, and a floor formed by the conjoined trabeculae. In
the adult the membranous nasal capsule lines a scooped
LoUow on either side of the massive snout cartilage, and
is protected by bones. In most Urodeles there is the

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same massive internasal cartilage and anterior growth
from the comua; but there is more or less of distinc^t
roofing cartilage belonging to the capsule ; no labial car-
tilages here become related to the nasal openings.

736. In the Frog a special vertical nasal septum is
formed. The olfactory sacs rest at first on the anterior
prolongations of the trabeculae. Later, they lie between
them and the distal end of each mandibular suspensorium
— a remarkable condition. Finally, the nasal septum
being formed, a supero-extemal proper nasal roof arises,
and the cornua and labials are dovetailed into the bounda-
ries of the capsules, so as to leave two openings, a superior
and an inferior. The roof bends downwards behind so as
to form a hollow conchuidal recess. In other Anura which
have a nasal septum, the true capsular cartilage may be
very imperfect or almost non-existent, together with a
most extensive adaptation of true labials to the defence
of the sacs (Pipa, Dactylethra). In the Frog we see the
first appearance of cartilaginous laminae within the capsule
at its fore part. Similar laminae occur in many Reptiles.

737. In Birds and Mammals the olfactory capsules
rise to a very high grade of complication. They may
increase in size so as to occupy far the greater part of the
length of the skull; but some considerable part of the
complication connected with them is due to bones which
are not preformed in cartilage ; these need not enter into
our consideration of the capsules proper. Essentially we
may consider the capsules in these classes as consisting
of the median vertical septum, and of outgrowths from
it or arising continuously with it, forming roofs, side-walls,
posterior and partial anterior walls, and more or less of
a floor on the external aspect, by folding in of the side-
wall. In a few forms this floor becomes complete (Fal-
cons). From the inner surfaces of these capsular cartilages
laminae of cartilage proceed inwards towards the septum
and become coiled in very various styles. Three principal
regions are distinguished, aliethmoidal (posterior), aliseptal
(middle), and alinasal (anterior), the latter being related

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to the external or anterior nostril. Turbinal laminae may
arise in all these regions, having special characters of their
own: those of the aliseptal region (inferior turbinal, nasal
turbinal) are often preponderant in size ; those of the ali-
ethmoidal region (middle and upper turbinals) are espe-
cially related to the sense of smell. In Mammals all these
growths are seen at their highest perfection, and in many
cases the nasal sacs burrow backwards beneath the cranial
cavity into the sphenoidal region. Both in the Bird and
in the Mammal there is an adaptation of primordial labial
cartilages to the purposes of the anterior nostril : in the
Pig this orifice is inferior. The posterior nostril is also
provided with special labial cartilages, appearing in the
vomerine region, in Snakes and many .Birds.

738. In the Mammalia the space in which the ol-
factory fibres pass down into the capsules is at fii-st a
large membranous fenestra comparable to that for the
optic nerve of the Frog; the fibres pass in a scattered
fashion through this membrane. During growth carti-
lage appears in the interspaces between the fibres, and forms
the cribriform plate continuous with the top of the hinder
part of the nasal septum. Another fenestration, corre-
sponding to that which occurs in the interorbital septum,
arises in the nasal septum of many Birds, and this may
become very large, aborting the previously existing car-
tilage so much as to produce a large notch, opening below :
this is principally in the aliseptal region. In some cases
there are several fenestrae in the septum.

The Arches.

739. The arches appended to the cranium have cer-
tain resemblances to those of the trunk. They enclose a
cavity, are liable to segmentation, and have very similar
nerve-relations. The non-attachment of the gill-arches to
the cranium or to vertebrae contrasts with the adherence
of ribs to the vertebral column.

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740. All tlie postoral arches may bear gills in some
vertebrates or at some period of development. The man-
dibular and hyoid are highly specialised in regard to
various functions: the last branchials also may be con-
siderably modified. A simple idea of the series may first
be considered, viz. the average branchial arch in most
Fishes. A continuous rod of cartilage originates in each
visceral fold at the sides of the throat, bending more
or less backwards in the pharyngeal roo f, free from the [
cranium and vertebrae, and passing forwards and inwards
below. The inferior ends of each pair of arches usually
become connected by the intervention of a median key-
stone piece or basibranchial, very comparable with the
azygous elements of the sternum. 'By a simple segmenta-
tion these branchial arches are perfected. Four segments
arise, and a more or less perfect jointing allows of mobility
of the parts. The upper and the lower segments are
usually more or less horizontal in position; the two others
are the more important, and are chiefly lateral. The
number of the branchial arches in Fishes varies from five

_t9_jafis:£n ; the hindermost of them is modified, attacheJto
the one before it, and deficient in one or more of its
segments. The basibranchials become attached to one
another in the median ventral line.

741. The hyoid arch in Fishes becomes divided by seg-
mentation into the same number of parts as the branchials,
and there is a basihyal : but the mandibular arch on each
side may be only segmented into two main pieces, a sus-
pensorial and a meckelian. The suspensorial cartilage in
all oviparous vertebrates has a forward prolongation which
enters into the maxillopalatine process of the cheek, and
may be so large as to constitute the entire upper jaw and
enter into no definite union with other parts (Shark) ; or
again, it may unite with a retral antorbital or palatine
growth to form the main palato-quadrate bar (Frog, Sal-
mon). In this case we have a conjunction of evidently
distinct parts. The meckelian cartilage does not undergo
segmentation, nor does it form a basal piece. In some

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cases the right and left cartilages fuse at the symphysis.
The joint between the lower jaw and the suspensoriuni
appears to correspond in position to those between the
epibranchials and ceratobranchials.

742, The relations of the upper end of the mandi-
bular arch to the cranium, and the methods of suspension
of the lower jaw, offer numerous points for consideration.
In certain forms the mandibular arch is directly and
strongly fixed to the cranial floor or wall, or to the ear-
capsule ; and concurrently with these conditions there is a
complete distinction between the hyoid arch and the man-
dibular ; or again, the former is so tied to the latter as
to take a considerable part in the suspension of the lower
jaw. When the mandibular suspensorium entirely or
principally supports the lower jaw, the skull has been
called autostylic^; when it largely shares the work with
the hyoid, the term amphistylic is applied. But in certain
cases the mandibular arch does not retain its primary
relation to the cranium, but becomes distant from it, and
not directly fastened to it; it is firmly tied to the upper
portion of the hyoid arch, which thus becomes the prin-
cipal suspensor of the jaw ; these skulls are hyostylic,

743. The primary relation of the mandibular arch to
the cranium is to its side-wall and base. In the Axolotl
it is at first applied to the trabeculae far forwards, but
soon becomes placed in the usual situation, namely, just
in front of the ear-capsule and near the primary apex of
the trabecula. An ascending process in some forms brings
the arch into relation with the side-wall of the cranium

Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 27 of 31)