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The morphology of the skull online

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they may be mentioned first. There are two pairs, the
prootics and exocdpitala (Fig. S7, pro., &.o). The prootic
embraces the trigeminal nerve at its exit from the
cranium, and extends beneath the fore part of the auditory
capsule, and upwards in the side wall of the cranium in
front of it, so as to be sickle-shaped. The exoccipital is
very similar, surrounding the foramen for the vagus nerve,
ossifying the base of the condyle behind and the hinder
part of the periotic mass in front. These bones commence
in the perichondrium, and in the superficial cells of the
cartilage (superficial endostosis) : they proceed to ossify the
cartilage throughout.

363. Two pairs of parostoses arise on the cranial
roof: the anterior (Jrontals) cover the part between the
ear-capsules and the front end of the cranial cavity.
They are much longer than the parietals (pa.) which lie
close together between the fore part of the auditory cap-
sules. Later, the parietal and the frontal of each side begin
to coalesce. The frontals do not quite roof over the great
fontauelle in front ; the parietals seem to lie entirely upon

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cartilage, but the cranial roof is imperfect beneath the
centre of each bone. The nasals (wa.) arise as small
patches over the middle of each nasal capsule, and
become crescentic bones with the concavity forwards and

Fig. 37.

Young Frog, with tail just absorbed ; side view of skull.

Au, auditory capsule ; in front of it, cranial side wall; a.n. anterior
nostril; st. stapes; mck. meckelian cartilage; b.hy. basihyobranchial
plate ; st.hy. stylohyal or ceratohyal ; br. 1, fist branchial arch.

Bones: e,o. exoccipital; pr.o. prootic; pa. parietal; /r. frontal; na,
nasal; pmx. premaxillary; nix. maxillary; pt. pterygoid; tq, squamosal;
qu.ju, quadratojugal ; art, articular; d, dentary.

364. The parasphenoid underlies the cranial floor
from the ethmoidal nearly to the end of the basioccipital
region, being convex and subcarinate below, and scooped
on its upper surface. It is almost as broad as the carti-
laginous basis cranii, to which it is closely applied, tongue-
shaped in its anterior two-thirds, and pointed behind.
In the middle of the periotic region it acquires a basi-
temporal wing on either side, ending sharply near the
foramen ovale. When the parasphenoid has assumed this
complete form, the vomers are only small irregular patches
of bone on the surface of the palate, a little internal to
each inferior nostril

365. The premaxillaries (pmx.) and maxillaries {mx,)
originate before the vomers. The former appear as short
thick bars laid in front of the rhinal processes, with a nasal
process on the upper surface of the face. The premaxil-

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lanes meet in the middle line, but their nasal processes
are separated by a circular membranous space. The
maxillaries are styloid bones continuing the line of the
premaxillaries in the upper lip, so as to form with them
part of a hyperbolic curve. They have an ascending
lamina on the face throughout most of their length, and
end pointedly behind, outside the prepalatine spur of the
palatine cartilage. They appear rather as splints to the
nasal capsule than to the palatine bar. There is a liga-
mentous union in the upper margin of the gape between
the end of the maxillary bone and the quadrate condylar

366. Delicate spicules of bone are found in the
fibrous stroma lining the inner face of the suspensorium
and of the pterygoid cartilage. The pterygoid lamina {pt)
becomes thicker close to the suspensorium, extending
upwards as a needle (which expands to a spatula-form
above) on its inner and anterior edge. There is a distinct
ectosteal {metapterygoid) plate embracing the inner convex
face of the hinder part of the suspensorium. Near the
quadrate condyle a quadrato-jugal (gu, ju.) has appeared,
extending as a small style towards the maxillary.

367. On the outer face of the suspensorium a sqiui-
mosal parostosis early arises, and soon acquires a sigmoid
shape, being broader above and pointed below. Its upper
or supratemporal region becomes transversely extended,
and moulded on the remnant of the cartilaginous orbitar
process. The dentary and articular bones (d., art) arise
similarly at first, as thin fenestrate plates lying in contact
with the meckelian cartilage, or only slightly separated
from it by fibrous stroma. The dentary^ is at first on the
outer surface of the fore half of the mandible, the articular
on the inner and lower surface of the hinder two-thirds,
not however reathing to the angle.

368. The cranial cavity is now very largely bounded
by cartilage, replaced to a slight extent by bone. The
nasal region has taken definite form in continuity with the

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primary trabecular comua, forming the considerable olfac-
tory capsules, and aflfording a basis for the anterior jaw-
bones. Instead of a suspensorial subocular bar, the palato-
pterygoid cartilage is subocular : the suspensorium is still
directed a little forwards, but also passes very largely out-
wards. It has become diflferentiated proximally, giving
off the lamina for the tympanic ring, and forming a new
simple otic process, as well as a rounded condyle on its
inuer cranial process or pedicle; the orbitar lamma is now
insignificant. The transformation of the hyoid and bran-
chial apparatus is very notable, tending to the formation
of a single expanded basal plate, and a rod-like hyoid
style, and to the disappearance of the branchial arches.

369. The bones existing at the close of this stage are
the following: exoccipitals and prootics ; parasphenoid,
parietals and frontals ; nasals, premaxillaries^ maxillaries,
vomers, and rhinals; pterygoids, metapterygoids, qua-
drato-jugals, and squamosals; dentaries and articulars. It
is remarkable how indefinite are the boundaries between
parostoses and ectostoses in the early stages of ossification
in the Frog : the ** membrane "-bones arise for the most
part closely in contact with the face of the cartilage.

Fifth Stage; Frogs of the first summer,

370. The young Frog during the first summer in-
creases in bulk threefold, and at the same time its skull
is greatly developed, especially with regard to ossification.
The cranial cavity becomes less oblong, and broader be-
hind ; the periotic masses are relatively wider and shorter.
The direction of the suspensorium is altered by the
gradual retraction of its lower extremity (Fig. 38).

371. The prootics and exoccipitals extend consider-
ably into the surrounding cartilage, completely encircling
the trigeminal and the vagus nerves respectively. A new
bone, of great interest from its later development, makes
its appearance as a narrow transverse splinter in the
ethmoidal region immediately constituting the front wall

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of the cranium. This bone, arising as a true ethmoid,
has to be subsequently denominated sphenethmoid {eth.) ;
it possesses both ectosteal and endosteal portions, formed
synchronously ; at present no trace of it can be seen from
the sides or from below.

Fig. S8.

Young Frog, near end of first summer ; upper view of skull, with left
mandible removed, and the right extended outwards.

h.o. basioocipital tract ; «.o. supraoccipital tract ; /o. frontal fontanelle;
e.n. external nostril; internal to it, intemasal plate; a.t. tympanic

Bones : e.o. exoccipital ; pr.o, prootic, partly overlapped by p. parietal;
/. frontal ; eth, rudiment of sphenethmoid ; na, nasal ; pmx, pre-
maxillary ; mx, maxillary ; pg, pterygoid, partly ensheathing the reduced
cartilage ; q.j. quadratojugal ; sq. squamosal ; ar» articular ; d, dentary ;
m,mk. mento-meckelian.

372. Other regions of the cranial cartilage begin to
have their peripheral cells directly calcified : but this
bony matter is not deposited in tesserae as in the Elasmo-
branchs, nor throughout distinct morphological regions;
the osseous granules do tend, however, to be aggregated
where in another type a proper bone-territory would be
established. The epiotic, supraoccipital, pterotic, basi-
sphenoidal, and upper and lower nasal regions are those
in which this superficial endostosis can be traced. The

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bony deposit is in very small grains, in semilunes ex-
tending half round a ceil, and in separate and connected

373. The frontal and parietal of the same side still
retain a partial suture between them: the parietal over-
laps the ascending part of the prootic ; the frontal extends
to the fore end of the great cranial fontanelle (Jo.), just
behind the commencing sphenethmoid ; the parietofrontals
of opposite sides do not yet meet by a considerable space.
The nasals {na) are much altered in shape; the upper
part is now a broad subquadrate plate, with a small
spur running downwards and outwards on the ethmoidal
cartilage ; the two nasals are widely separated from one
another. The rhinal bone is seen as a notched and
grooved plate lying on the floor of each external nostril .

374. The premaxillaries and maxillaries (pmx,, nuc.)
are much advanced, and dentigerous. Their nasal processes
are considerably enlarged : those of the premaxillaries are
completely apposed, and extend pretty evenly to the
level of each external nostril ; those of the maxillaries
rise to the outer boundary of the nostrils, and are higher
behind, towards the ethmoidal spur of the nasals. Behind
this region the nasal plate rapidly sinks, and the maxil-
lary is a mere style passing backwards to within a short
distance of the quadratojugal. The palatal laminae of
the premaxillaries and maxillaries are of less extent than
their nasal processes; they are thin shelves which die
away behind. The parasphenoid is broader in front and
much more dense than formerly; it does not reach re-
latively so far back as it did, being rather emarginate
than pointed. Its basitemporal wings are wider and
broader, corresponding with the increased treadth of the
auditory region ; their hinder edge is conterminous with
the fore margin of the exoccipitals. The vomers have
developed into very irregular triradiate bones, mainly
related to the inner margin of the inferior nostrils, but
sending a spur backwards and inwards towards the para-

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375. The great subocular space between the cranial
wall and the palatopterygoid bar is now nearly oblong.
The prepalatine spur at the antero-external angle is
strongly marked. Beneath the transverse part of the
cartilage between this region and the ethmopalatine junc-
tion a delicate slightly-curved narrow palatine bone is
applied. It rather overlaps, but does not touch the styli-
form ectosteal pterygoid {pg.) which lies on the inner face
of the whole longitudinal part of the cartilage, and tends
to ensheath it above and below. Posteriorly the pterygoid
sends oflf at an obtuse angle a spur which underlies two-
thirds of the suspensorium : above, it becomes applied
to the small metapterygoid and partly fused with it.

376. During the first summer the suspensorium, which
was still turned forwards when the young frog left the
water, becomes progressively retracted so as to form more
than a right angle with the fore part of the skull. The
pedicle where it had coalesced with the cranial cartilage
is converted into fibrous tissue, and the boss below is a
distinct broad condyle articulating with the antero-lateral
angle of the auditory cartilage below. The secondary
otic process has completely fused with the supero-lateral
edge of the auditory capsule, and overhangs the tympanic

377. The meckelian and labial cartilages are replaced
at the symphysis by a small solid mento-meckelian bone
(m. mh) on each side; and the very delicate dentary, not
half the length of the ramus, has coalesced with it. The
main part of the bar lies in a trough-like ectosteal bone,
the articular (an).

378. The stages by which the parts of the middle
ear, so late in their development, become manifest, may
here be recounted*. In tadpoles we have described merely
a simple stapes, having between it and the continuous
skin a small space, a relic of the first visceral cleft. In

1 See Huxley ; " On Bepresentatives of MaUeus and Incus," P. Z. S,
1869, pp. 391 -407.

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young frogs that have taken to the land, a definite tract of
granular tissue can be traced from the stapes forwards and
outwards to the upper part of the suspensorium beneath
the otic process ; the facial nerve passes over this tract.
In the course of rather more than a month a delicate
clubbed rod of cartilage is formed in this tissue, its clubbed
end fitting into the narrow anterior part of the fenestra
ovalis, which has not become completely filled by the
stapes. The cartilage cells in the pointed anterior end of
this rudimentary columella pass insensibly into fusiform
connective-tissue cells.

Fig. 39.

Adnlt Frog, side yiew of auditory region, with semioirotdar canals and
parts of middle ear displayed.

oc,c, occipital condyle ; ep, epiotic region ; a,8C,y hac, p.9C. anterior,
horizontal, and posterior semicircular canals; 5. trigeminal foramen;
2. optic foramen, surrounded by a ring of membrane ; it. stapes ; itM,
interstapedial ; m,8L mediostapedial (bone); eat extrastapedial;
Buprastapedial ; it.K stylohyal cartilage cut off short.

Bones : e,o. exoccipital ; pr.o, prootio; p. parietal; /. frontal; pa.».
parasphenoid; mM, mediostapedial.

379. About two months later, towards autumn, the
columella is complete (see Fig. 39). The posterior ex-
tremity is segmented off as an oval mass of cartilage
about half the size of the stapes; this is the inter-
stapedial (it, at). The middle third, or mediostapedial
(m. st,), is more slender, and is invested by a bony sheath.
The anterior third curves outwards, applying itself to the

B. M. 11

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back of the upper part of the suspensorium; this extrc^
stapedial (e. sL) is broader and flatter than the middle
part, and becomes involved in the fibres of the tympanic
membrane. At its proximal end the extrastapedial gives
ofif a small rounded cartilaginous process upwards and
backwards at an acute angle with the middle piece; this
suprastapedial («. st) is attached by ligament to the side
of the auditory capsule,

380. By this time the little cartilage which was de-
tached from the postero-superior angle of the suspensorium
forms a tympanic annulus, constituting more than three-
fourths of a circle. It is a broad band, thick on the
exterior, with a thin inner edge. Its extremities Ue up-
wards, and are strongly attached to the outer edge of the
supratemporal portion of the squamosal, hiding the upper
half of the descending part of that bone. The circular
space thus enclosed is filled by a thick web of fibrous
tissue, to the inner face of which the extrastapedial cartilage
is attached. The external skin is continuous over this
layer of fibrous tissue (the rudimentary tympanic mem-
brane), dipping down to it somewhat,

3S1. The lower part of the hyoid arch is a styloid
cartilage loosely connected to the skull and suspensorium
by ligaments. Ventrally it sends forward a little hypo-
hyal lobe, and is fused with the broad shield-shaped basi-
branchial plate. Posteriorly this plate bears remains of
the hypobranchial portions of two branchial arches : the
anterior are slender and unossified; the posterior pair are
ossified, and embrace the larynx. The other regions of
the branchial arches have disappeared

382. The principal points to be borne in mind with
respect to this period, in addition to the growth of bones
previously existing and the moulding of the skull into its
adult form, are the development of the ethmoid and mento-
meckelian bones, the retraction of the suspensorium and
of the lower jaw, the completion of the columella and the
tympanic annulus, and the disappearance of the branchial

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arches. The facial nerve passes over the columella in the
same position as it passes over the upper member of the
hjoid arch where that is early and fully developed.

The Skull of the AduU Frog.

383. The skull of the frog is a flat semielliptical
structure, broadest in the hinder region, where it is
formed of continuous cartilage and bone. In front of
this expansion, which occupies about one-fifth of the
length of the skull, the narrow axial parts enclosing

Fig. 40.

Adult Frog : npper view of sknU with investiiig bones and lower jaw

f,m, foramen magnnm ; oc,c, occipital condyle ; parietal fonta-
nelle ; f,fo. frontal fontaneUe ; p.rh, prorhinal cart&age ; e.n, external
nostril ; al,n. alinasal cartilage ; u,V, modified upper labial ; p,pa, pre-
palatine spur.

Bones: e,o, ezoccipital; pr.o. prootio ; ipMh. sphenethmoid ; pa,
palatine ; pg, pterygoid ; q.j, quadrato-jugal.

the brain are separated from the lateral boundaries for
about half the length of the head, by a large ovoid tract,


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occupied by the eyeballs and by muscle. In the anterior
part of the skull the aiis and the lateral bars again
unite in a precranial expansion. The external bars we
have mentioned are carried at a level below that of the
cranial floor.

384. When the investing bones of the skull are
stripped off, the underlying cartilage and its proper bones
are seen to present a somewhat different appearance.
The continuous elliptical outline is lost, and the nasal
cartilages appear as a very distinct mass attached to the
axial or proper cranial region.

385. The cranial cavity is enclosed by continuous
cartilage and cartilage-bone, except where nervous struc-
tures pass out of it, and where the roof presents mem-
branous fontanelles. Behind, there is an occipital region
mostly posterior to the periotic masses. The auditory
region is broad, and there is no distinction between
periotic and cranial cartilage. The preauditory part of
the cranial cavity is three-fifths of its length.

386. The brain-case is nearly twice as deep behind
as it is in front (Fig. 41), and considerably wider above
than below. Consequently its side walls in front of
the ear-masses slope outwards as they ascend. With
these peculiarities, the cranial cavity of the frog is of
a very smooth and symmetrical type, without marked
prominences or irregularities. The base is very flat
beneath, and emarginate in the middle line behind. The
condyles, largely ossified by the exoccipital bones, stand
out on either side above the base (Fig. 41, occ. c), having
about half the height of the skull. The exoccipitals
extend nearly to the middle line of the cranial floor, and
approach it almost as closely above; the supraoccipital
region has a thin film of superficial endostosis.

387. The exoccipitals have a large extension into
the auditory masses, which are flattened and broad, with
a curved outline externally. They are very consider-
ably unossified, possessing indefinite films of superficial

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endostosis, and a pair of large anterior bones, the prootics
(pr.o.). These occupy the antero-intemal regions of the
periotic masses, are deeply notched below where the
trigeminal and facial nerves pass out in front of the
auditory tract, and extend forwards above these nerves
to ossify a considerable part of the alisphenoidal region.
The anterior canal and the fore end of the horizontal
are especially girt by the prootic; while the exoccipital
is related to the posterior canal, and the hinder end of
the* horizontal. There is a pterotic ridge or tegmen
tympani, external to the horizontal canal, and overhanging
the stapes and tympanum. The exoccipitals contain the
foramina for the glossopharyngeal and vaffus nerves, at
the junction of their auditory with their occipital regions.

388. The continuous cranial floor in front of the
exoccipitals has some slight endostosis in its substance,
but not suflBcient to be denominated a bone. The car-
tilaginous side walls of the skull present nothing remark-

7 S
Adult Frog ; median longitudinal section of skull, lower jaw remoyed.

occ.e. occipital condyle ; t,n. nasal septum ; 1, olfactory, 2, optio,
5, trigeminal, 7, facial, 8, glossopharyngeal and yagus foramina.

Bones : e.o. exoccipital ; pr.o, prootic ; p. parietal ; /. ^ntal ; ip.eth.
sphenethmoid ; pmx, premazillary ; j7a.<. parasphenoid.

able, except the very posterior position of the foramina
for the optic nerves: two-fifths of the length of the
cranial cavity is in front of them. The fontanelles on
its upper surface consist of a pair of small oval parietal
openings (Fig. 40, p, fo.) in the region between the prootic
bones ; and of one frontal (/. fo.) much larger, oblong in
outline, with a triangular spur of cartilage projecting
forwards from the posterior boundary. The cartilaginous

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roof between the frontal and the parietal fontanelles is
feebly ossified by endostosis, a rudiment of a supra-
sphenoidal bone.

389. The floor, side walls, and roof of the cranial
cavity are ossified anteriorly by one bone, the sphen-
ethmoid (Figs. 40, 41, sp. eth). This bone extends
further backwards in the side walls than in either the
floor or the roof of the cranium. Below, it has a nearly
transverse posterior margin; in the roof it is concave
behind, embracing the fore end of the frontal fontanelle.
The olfactory crura pass through its cranial concavity
to gain the nasal cavities ; a slight vertical ridge projects
between the crura into the cranium, like a rudimentary
crista galli. The sphenethmoid also extends forwards
above and below in the ethmoidal region, aborting the
cartilage, and ending by a blunt median projection in
each case; which point corresponds to the fore end of
a median vertical plate connecting the upper and lower
laminae : this is a true perpendicular ethmoidal (mes-
ethmoid) plate (Fig. 41).

Fig. 42.

Adult Frog ; upper view of skull, with lower jaw remoyed.
«.o. exoccipital; p. parietal;/, frontal; e. sphenethmoid; n. nasal;
pm, premaxillary ; rnx. maxiUary; g. quadrato-jugal; pt, pterygoid; »ttf.
suspensorium ; j).o. prootio; ojp. opisthotic.

390. The investing bones of the cranium proper are
very simple. There is a pair of parieto-frontals, joined

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by suture, above, and a parasphenoid below. The parieto-
irontals extend from the sphenethmoid, whose lateral
regions they partially cover, to the exoccipitals. 1 hey
diverge from each other behind, exposing the cartilaginous
cranial roof, and also in front. They are thick and
smooth, and dip towards their sagittal suture : their
parietal portions are conterminous with the prootics, and
in front of them the bones dip over the edges of the
cranial roof, investing its upper side walls for a small

391. The parasphenoid underlies the whole cranial
floor (separated from it by a perichondrial membrane)
except in the occipital region, even sending a small spur
backward between the exoccipitals. Its oblong basi-
temporal wings are very wide, undergirding most of the
width of the auditory masses, and lying between the
prootics and the exoccipitals. In front of these wings
the bone is very regularly linear, and subcarinate.

392. Beyond the sphenethmoid the whole of the
precranial cartilages are unossified. A rather low septum
(8. n. Fig. 41), thicker above, nms forwards to the end
of the snout. The nasal floor formed by the coalescence
of the hinder half of the trabecular comua is a broad
thin flap, separated by a notch from the antorbital or
ethmopalatine bar, extending outwards. Anteriorly the
cartilage is expanded outwards and curved round the
inner nostril on its outer side, so as nearly to enclose it.
The broad anterior end of the comu has sent forward in
the middle a slender prorhinal process, and at its outer
angle is bilobate (Fig. 40. p. rh.).

393. The roof of the capsule is broadest at its
posterior region, where it has completely coalesced with
the ethmopalatine bar. Both above and below, the
cartilage is partially hardened by endosteal bone : one
more notable patch than the rest is in the ectethmoid

Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 14 of 31)