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The morphology of the skull online

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somewhat withdrawn from the middle line, leaving a

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lanceolate chink in which the notochord lies : this is the
posterior basicranial fontanelle {p.b.f. Fig. 62).

529. The occipital ring of cartilage is complete ; the
foramen magnum is obliquely directed backwards (/. m.
Fig. 59), and the supraoccipital region is almost vertical;
the latter is emarginate in the middle line, both in front
and behind. A pair of triangular exoccipitals (e.o,) are
ossifying ; they extend from the lateral margin and base
of the foramen magnum forwards to the foramen for the
vagus nerve. The exoccipital region sends a lamina of
cartilage (tj^mpanic wing of the exoccipital) around the side
and base of the hinder part of the ear-capsule. This wing
is the posterior boundary of a scooped antero-inferior
hollow in the periotic mass, which is to become the tym-
panic cavity (Fig. 59, ty,) ; at present it is filled by a soft
flocculent stroma, soon to be absorbed. There is but a
slight roof to this cavity (tegmen tympani or pterotic ridge).

530. Mesiad of this tympanic hollow, and extending
in front of it, is a thick rounded boss of cartilage, protruding
both within the cranial cavity and on its inferior aspect,
and containing the finger-like cochlear process of the ear-
cavity {cL Fig. 62). These cochlear protuberances are
remarkably near to the median line of the cranial floor, at
the sides of the middle region of the notochord. The
whole of the periotic capsule is unossified at present.

531. The circumpituitary or basisphenoidal region is
perforated by a nearly circular pituitary hole, into which
the pituitary body passes, and through which the internal
carotid arteries enter the cranial cavity. The axial carti-
lage is narrow in the posterior clinoid region, then a little
expanded outwards and much extended vertically; and
projecting from the circumpituitary cartilage antero-
laterally is a curved process or lingula of cartilage,
extending downwards and backwards for a short distance.
From the hinder half of the basisphenoidal region a
considerable elongated cUisphenoidal lamina arises {a.s.
Fig. 59), passing outwards and backwards on either side
in the cranial floor, and ascending a little into the side

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wall ; coalescing at its extremity with the periotic mass, but
leaving an elongated space unfilled by cartilage between
the two tracts. The cranial surface of the alisphenoid
conforms closely with the concave curvature of the hinder
part of the cranium (see Fig. 62).

532. In the cartilage of the antero-lateral boundary
of the pituitary fossa is a pair of small bony centres;
they are continuous, however, with the parosteal ossifica-
tion of the rostrum to be hereafter mentioned. Just in
front of these basisphenoidal centres, where the compression
of the basicranial cartilage to form the interorbital septum
begins, is a pair of small distinct cartilaginous plates,
aflfording articulating surfaces for similar cartilages deve-
loped on the pterygoid bones (indicated in Fig. 59). The
interorbital septum is now of much greater depth, carrying
the fore part of the brain up to a high leveL The pre-
nasal continuation of the nasal septum is thickened above,
and almost straight below, though slightly downbent at its
extremity (p.n. Fig. 59). The trabecular cornua have
become lost in the lower anterior part of the alinasal

533. The frontal, parietal, and nasal bones have
appeared; they need not be described in the present stage.
The bones of the floor of the cranium and of the palate
must be noticed more fully, because of the great interest
of their history. The subcranial region which in the Frog
is ossified by the basitemporal wings of the parasphenoid,
is here supplied with a pair of distinct and largfe basi-
temporal bones {b.t Fig. 59), which extend from near the
median line, beneath the cochlea, and so far outwards as
to constitute a floor for the tympanic cavity : their anterior
limit is near the fore margin of the alisphenoid cartilage.
These ossifications arise in a thick web of fibrous tissue in
the hinder part of the palate; and the matrix is abundant
in the middle line, extending forwards to the bone next to
be described. The Eustachian tubes run forwards and
inwards above the anterior edge of these bones, and meet
in the middle line, beneath the pituitary fossa.

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Fig. 69.



Embryo Chiok, middle of second week of incnbation; under view of
sknll, with arches removed.

ne. notochord; f.m, foramen magnum; «.o. supraoccipital tract;
9, foramen for hypoglossal nerve; p.8.c, posterior semicircular canal;
8, foramen for glossopharyngeal and vagus nerves ; ty, tympanum ;
i.e. internal carotid artery passing above the basitemporal bone ; a.8,
alisphenoid cartilage; no', notochord diagrammatically seen in the
posterior basicranial fontanelle ; p.n. prenas^ cartilage.

Bones : e.o. exoccipital ; b. t. basitemporal ; r.b.8. rostrum ; at its
hinder end on either side the basisphenoid centres ; pa, palatine ; pg,
pterygoid; px, premaxillary; mx. maxillary; pmx. maxillopalatine plate
of maxillary ; j, jugal ; qj, quadrato-jugal ; q, quadrate.

534. The interorbital septum rests upon a grooved
fusiform rod of bone {r,b.$.), representing the anterior
part of the parasphenoid of the Salmon, the Axolotl, and

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yii.] THE fowl: third stage. 233

the Frog, and the whole of that of the Snake and the
Lizard. This ossification in Birds receives the special
name of rostrum ; its posterior extremity, at the anterior
part of the pituitary space, has crept into the perichon-*
drium and the superjacent cartilage on either side.

535. The premaxillaries (pa?.) are formed as double
laminae (nasal and palatal), united at the outer edge:
above, they nearly meet by their nasal processes, although
the prenasal cartilage protrudes between them in front:
below, they lie outside it. The posterior extremity of
each is forked, the outer being the dentary process, and
the inner the palatal. From each premaxillary along the
margin of the upper jaw to the quadrate condyle there are
found three subequal styliform parostoses, niaxiUary (mx.),
jvgal (j.)y and quadrato-jugal (qj.). The maxillary at its
anterior third sends inwards an earshaped process over the
palatine bone; this is the maxillopalatine process {pmx,),

536. The palatine bones {pa. Fig. 59) are flat,
pointed in front, where they pass between the palatal pro-
cesses of the premaxillaries and the prenasal cartilage ;
flattened behind and somewhat arcuate outwards, leaving
a pair of long palatonasal spaces between them and the
nasal septum and the rostrum. Posteriorly they lie broadly
against the rostrum, and then turn a little outwards, be-
coming pointed where they articulate with the pterygoids.
The latter (pg,) are sigmoid clubs, flattened in front where
they articulate with the cranial axis and the points of the
palatines; and subcylindrical behind, articulating by a
cupped face with a small knob on the quadrate cartilage,
just above its principal condyle. The articulation of the
pterygoid with the basis cranii is by the intervention of
a distinct plate of cartilage playing against the basi-
pterygoid cranial plate formerly mentioned (§ 532). The
tissue in which the palatines and pterygoids are formed does
not become true hyaline cartilage before being ossified,
although fast tending towards that condition ; and more
or less cartilage may appear in diflferent parts of these
tracts in various birds.

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537. The ascending portion of the quadrate cartilage
is enveloped in an ectosteal sheath (g.): the mandible is
becoming covered with its splints, the dentary, splenialy
suranguktr, and angular pairs (Fig. 63, p. 242) : there is
no coronoid. The stapes or columella is fully formed, but
not ossified. The two small ceratohyals have united in
the greater part of their extent, forming a small arrow-
head-shaped lingual cartilage. Shaftbones are appearing
in the ceratobranchials.

538. This stage is marked by the more perfect mould-
ing of the occipital region, and the gi^eat development of
the nasal parts. The constriction of the notochord and
the ossification surrounding its posterior portion are of
great interest: The tympanic cavity is plainly indicated,
and the Eustachian tubes acquire a notable position above
a pair of bones, the basitemporals. Attention should be
especially directed to the origin of these bones and of the
rostrum. Few ectosteal centres have appeared ; exocci-
pitals, basisphenoids, quadrates, and ceratobranchials. The
principal membrane-bones of the roof of the skull, the
palate, and the jaws are now found ; but the vomer is not
yet in existence.

Fourth Stage : Embryo at end of second and heginning
of third week of incubation : head about an inch long,

539. The chondrocranium is rapidly becoming ossi-
fied, its general form remaining substantially the same.
The notochordal ossification has extended on each side
into the parachordal cartilage, but the condyle is still
unossified. The basioccipital bone thus originating is
broadest in the middle, and is most prominent above
along the notochordal line (b.o. Fig. 62). The posterior
basicranial fontanelle (p-bf) has become a wide space
between the clinoid wall and the basioccipital. The
exoccipitals (e.o.) have increased in size, and possess a pro-
cess directed outwards in the tympanic wing ; and paired

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supraoccipitals (Fig. 60, s.o,) have appeared, ossifying
most of the cartilage above the foramen magnum.

Fig. 60.

Embryo Chick, end of second week of incabation; posterior view of


o.c, occipital condyle; nc. indicates the superior position of the
notochordal remnant ; p.8.c. posterior semicircular canal ; h.8. horizontal
semicircular canal; t.eo. tympanic wing of exoccipital cartilage; sp.o,
sphenotic process ; fo. supracranial fontanelle.

Bones : e.o, exoccipital ; s.o. supraoccipital ; sq. squamosal ; /. frontal ;
p, parietal.

540. The anterior part of the otic cartilage, in the
cochlear region and below the anterior canal, is ossified
as prootic (pr.o. Fig. 61), and perforated by the auditory
nerve. The outer and hinder view of the much tilted
auditory mass shows nothing but cartilage; a small
opisthotic osseous wedge, however, has appeared in the
edge of the capsule adjacent to the exoccipital (op.).
Already the supraoccipital is ossifying the cartilage around
the posterior part of the anterior canal : and the exoccipital
is enclosing the posterior canal below.

541. The rostrum, the basisphenoidal ossifications
connected with it, and the basitemporals are gradually
becoming one bone, together with another considerable
pair of osseous tracts {pretemporal vrings) developed in
stroma laid down upon the lingulae formerly mentioned
(§ 531), and extending outwards and backwards to the
upper extremity of the quadrate along the anterior
margin of the basitemporal {^p,tp. Fig. 65, p. 246). The

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basitemporals, now uniting, include between them and
the floor, of the skull, a space in which the Eustachian
tube and the carotid artery of either side run forwards

Fig. 61.

Embryo Chick, end of Becpnd week of incubation ; inner view of
hinder part of cranium, with median section of basilar and supraoccipital

o,c. occipital condyle ; h.o. basioccipital ; pr,o. prootio ; a,8C. anterior
semicircular canal ; «.o. supraoccipital ; op. opisthotic ; e.o. exoccipital ;
7, foramen for facial nerve ; 8, foramen for glossopharyngeal and vagus ;
9, foramen for hypoglossal.

and inwards. The pretemporal wings and the basi-
temporal plate meet at an angle, and enclose a considerable
cavity anteriorly, continuous with the tympanic; these
are the anterior tympanic recesses.

542. The proper basi sphenoidal ossification has ex-
tended in an annular form round the pituitary space
(Fig. 62), and into the clinoid wall and the margin of the
posterior fontanelle. The alisphenoid cartilage {a,s.
Fig. 62) has become complicated in its relationships,
being united with the fore part of the ear-mass above
and below (the main part of the trigeminal nerve issuing
between them) ; and also continuous by its base with the
posterior clinoid wall and the side of the pituitary region.
It has an external and posterior sphenotic process (sp.o.),
and is also considerably curved forwards. A membranous
fontanelle {a.s.f.) arises in its centre ; and both in front
and behind this, distinct ossific centres appear in the
cartilage : these afterwards unite to form one alisphetioid

543. The remainder of the chondrocranium in front
of the pituitary body is unossified. The anterior part of

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the brain is borae laterally upon membrane and covered
by membrane and membrane-bones (see Fig. 63, p. 242) ;
it is supported mesially by the elevated cartilaginous inter-
orbital septum, which has a convex superior edge and a
concave posterior margin, on either side of which the optic
nerves diverge. The septum has a somewhat thickened
lower edge where it rests upon the rostrum. The
continuity of the interorbital with the nasal septum is
interrupted to a great extent by the development of a
vertical hourglass-shaped cramo/aciaZ/ene^^ra (see Fig. 66,
p. 250, in front of m.e.) in the cartilage just in front of the
termination of the parasphenoidal rostrum. At the same
spot the cartilage is expanded into an oval plate at the
base, corresponding to the primary breadth of the coalesced

544. There is another fenestra of great interest in
the posterior and upper region of the interorbital septum ;
it is as large as the last-named, and may be termed inter-
orbital fenestra; its backward termination comes very
near to the limit of the cartilage {i.o.f. Fig. 63). Yet
one other feature of interest must be mentioned; the
median cartilage at the extreme anterior end of the brain-
cavity sends backwards and upwards a sharp spike in its
roof, the sole representative in the Fowl of the ichthyic
tegmen : close beneath it the olfactory nerves emerge.

545. There is nothing especially to be noted about
the proper nasal septum, except that it is grooved in front
of the craniofacial fenestra for a branch of the orbito-
nasal nerve, which passes from the upper wall of the orbit
over the lateral ethmoidal plate into the nasal roof, and
then bends downwards and inwards in front of the fenestra,
grooving the septum. When it has reached nearly the
bottom of the septum, the nerve passes under a small
cartilaginous bridge, and turns forwards along the base
of the septum to its peripheral distribution. The almost
cylindrical prenasal cartilage, to which the septum slopes,
is at its highest relative development {p.n. Fig. 62).

546. The top of the ethmoidal and nasal septum is



continuous with various outgrowths on either side, arching
over and extending into the different regions of the laby-
rinth. The nasal roof is complete on each side along its

Fig. 62.

Embryo Cluck,' end of second week of incubation ; upper view of
skull, the brain and parostoses being removed.

o.c, occipital condyle; a.8x, anterior semicircular canal; t.e.o,
tympanic wing of exoccipital ; cl. cochlea ; p.b.f, posterior basicranial
fontanelle; pts, pituitary space; a,cL anterior clinoid ridge; p.el.
posterior clinoid ridge, outer part ; 5, trigeminal foramen ; 5a. foramen
for its orbitonasal branch; as.f, alisphenoid fenestra; »p.o. sphenotic
process ; p.8. presphenoidal region, or top of the interorbital septum, the
great depth of which is not perceptible ; eth, ethmoidal region ; al.e.
aliethmoidal cartilage ; al.8. aliseptal cartilage ; al.n, ahnasal cartilage ;
p,n. prenasal cartilage.

Bones: b,o. basioccipital; e.o, exoccipital; s.o. supraoedpital ; pr,a*
prootic ; a.8, alisphenoid ; around ^£.s. basisphenoid.

whole length (Fig. 62), but the side-wall growing from it
is deficient in certain regions, exposing externally the
superior or proper upper turbinal coil and the inferior or
lower turbinal of Anthropotomy. There is no "middle
turbinal." The lateral ethmoidal region furnishes a large
vertical subquadrate antorbital plate, separating the orbit
from the nasal cavity. The distribution of the olfactory

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VII.] THE fowl: foubth stage. 239

nerve is over a bag-like involution of the aliethmoidal
roof, continuous behind with the antorbital wall. The
inferior turbinal coil is united with the postero-inferior
angle of the same wall, but is elevated anteriorly. Further
forward, in the alinasal region {al.n,) there is a curiously
sinuous wall, with an alinasal turbinal depending from
the roof and curved in correspondence with the sinuosity
of the wall. The nasal opening in the cartilage is a
long notch passing backwards and upwards, and rounded
behind, the alinasal cartilage being enfolded around it
everywhere and passing into the alinasal turbinal (see
Figs. 69—71, p. 252).

547. The cranial cavity is now partially invested by
membrane-bones above, but they are principally lateral,
leaving a large median fontanel! e. The parietals {p.
Fig. 60) are directly in front of the ear-capsules and the
supraoccipital moieties ; they are expanded below, where
they are overlapped by the squamosals, and narrower
above. The squamosals {sq.) are broad squarish plates
overlying the lower part of the parietals and frontals, and
reaching down to the postfrontal or sphenotic process of the
alisphenoid and the top of the quadrateJ The frontals
(/.) are thin hollow shells of bone, covering a lar^e part
cif the crown of the head ; they are bent suddenly inwards
along the upper edge of the orbit, forming a considerable
orbital plate in each case, but leaving a large membranous
space between it and the interorbital septum (Fig. 64).
They become sharp-pointed anteriorly over the ethmoidal

548. The nasals are flat narrow plates lying over the
frontals behind, and on the aliseptal region of the nasal
cartilage ; they fork anteriorly behind the alinasal region,
sending an upper process along the nasal process of the
premaxillary, and a lower process down to the maxillary
{na. Fig. 64). Outside the hinder part of the nasals is a
rather broad supraorbital ossification {Ich, Fig. 64), with a
downward imperforate spur occupying the lachrymal region
and applied to the outer edge of the antorbital plate.

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549. The premaxillaries and the rest of the bones of
the upper jaw and palate, and also the quadrate and the
lower jaw, are further advanced in development, becoming
more solidified, and perfecting the various relations already
described. A new bone has arisen in the palate, viz. the
vomer {v. Fig. 65, p. 246), which is however but a very small
style, lying under the nasal septum, behind the points of
maxillopalatine processes of the maxillaries.

550. In several respects the skull at this period
reminds the skilled observer of conditions found in the
Tinamous. Cartilage-bones have increased in number;
there are paired supraoccipitals, prootics, opisthotics, and
alisphenoids. The remarkable manner in which the basis
cranii is compounded will attract attention ; the fenestra-
tion of the orbitonasal septum is of scarcely inferior
interest. The cartilages of the nasal labyrinth are now
modelled as to their main outlines. The supraorbital and
the vomerine ossifications are the principal new formations
in membrane.

Fifth Stage : The Chick about the second day after

551. The skull of the ripe chick is interesting in
many respects; it is profitable for comparison with the
skulls of the Fish, the Reptile, and the immature Mammal.
Looked at generally, it comes much closer to the nearest
congeners of the typical Fowl than does that of the adult
bird. The drawings of this form might, with very little
modification, serve as diagrams to illustrate the structure
of the skull in any Bird above the Struthionidae; compared
with the Aves Praecoces, the skull is like theirs at the
same period ; but it corresponds to that of nestlings of the
Aves Altrices at about the ejid of the first week after

552. The period of a week has sufficed for very great
changes in chondrous and fibrous tracts ; and now is the
best time for catching the true form of many of the osseous

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territories, although some have already lost their distinct-
ness, whilst others have not yet appeared. The ectosteal
tracts have set up endostosis in the cartilage lying between
their inner and outer plates, so that they are being
enlarged intrinsically as well as from the immediately
overlying (perichondrial) fibrous tissue. Save in the
instance of the early-grafted parasphenoidal elements, the
parosteal tracts or splint-bones are still altogether free
from union with the endoskeletal parts, whether bony or
cartilaginous. The hyaline cartilage furnishing the inter-
spaces and headlands of the ectosteal plates has become
very dense through the abundance of the cheese-like inter-
cellular substance ; it is semitransparent when thick, and
for the most part of a lilac colour. The splint-bones are
still fibrous, but are beginning to become smooth through
the continual ossification of aponeurotic layers. As a
correlate of this exogenous growth, the first deposit of
bony matter is being absorbed in many places, so as to
form diploe; this process is most advanced in the basi-
temporals and squamosals.

553. The extensive occipital plane, swelling back-
wards above, is largely ossified, although there are con-
siderable chondrous tracts remaining. The basioccipital
extends into the occipital condyle, and it is considerably
underfloored by the basitemporal plate in front (Fig. 63).
The exoccipitals ossify the lower half of the sides of
the foramen magnum. They are very irregular in shape,
extending considerably into the ear- cartilage : they are
perforated by the vagus and hypoglossal nerves. The
supraoccipital centres have coalesced almost completely to
form a large bone bounding the upper half of the foramen
magnum, which is pointed above ; superiorly, the margin
of the bone is curved like a fan, and abuts on the

554, Looking at the internal surface of the cranium,
it will be seen that nearly all of the arch of the anterior
semicircular canal {a.8.c.) lies in the supraoccipital {s.c),
being supero-posterior in position, owing to the backward-


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tilting of the whole auditory capsule. Nearly the whole
of the posterior canal and part of the horizontal lie in the

555. Below and in front of the supraoccipital, the
prootic (pr.o.) ossifies most of the remainder of the
auditory mass : but there is a small epiotic lying attached
to the prootic i^ the anterior part of the recess for the

Fig. 63.

Chick two days old; median longitudinal section of skull, the brain
being removed.

a.8.c. anterior semicircular canal in supraoccipital bone ; i.e. foramen
for internal carotid artery ; basipterygoid process ; jp.«. presphenoidal
region; i.o.f. interorbital fenestra; i.t.h. inferior turbinal ; n.n. nasal
nerve passing downwards and forwards ; s.n. nasal septum ; pn. prenasal
cartilage; 2, optic foramen; 5, trigeminal foramen; 7, foramen for facial
nerve ; 8, foramen for glossopharyngeal and vagus nerves ; irik. meckelian
cartilage, which is also indicated in outline beneath the splenial bone ;
p.a.p. posterior angular process.

Bones: h.o. basioccipital ; e,o. exoccipital; s.o. supraoccipital; op.
opisthotic ; pr.o. prootic ; h.t. basitemporaJ conjoined with basisphenoid ;
r.h.8. rostrum; p. parietal ; sq. squamosal ; /. frontal ; a.s. alispheno'd;
eth. ethmoid ; p.e. mesethmoid ; px. premaxillary ; n.px. its nasal
process ; d.px. its palatal plate ; ar. articular ; i.a.p. its internal angular
process ; a. angular ; su. surangular ; sp. splenial ; d. dentary.

flocculus. Between the prootic and the exoccipital is a.
wedge of bone, the opisthotic, visible internally {op.) : the
vagus nerve (8) enters its foramen between this and the

Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 20 of 31)