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The morphology of the skull online

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exoccipital. On the outer aspect of the skull the opisthotic
can be seen separating the fenestra ovalis from the fenestra
rotunda, also growing round the latter. It does not reach
the posterior occipital wall. The prootic has a scooped
anterior crest, continued on to the ali- and basisphenoid,
and helping to form the resting place for the midbrain :

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the crest is notched in front for the trigeminal nerve. The
large backwardly-projecting plate of the prootic is pierced
about its middle by the facial and auditory nerves (7).

556. Immediately in front of the basioccipital and
as far as the rostrum the cranial floor is a thick mass of
bone, for the coalesced basitemporals have united with
the outspread pretemporal wings, and a section shows
that their diploe is continuous (Fig. 63, b.t). The basi-
temporal plate presents a pair of large convexities behind,
and its anterior very contracted extremity is notched be-
neath the junction of the Eustachian tubes, which run
forwards in its substance, as before described (§541, p. 28.".).
Immediately in front of this region are the two oval carti-
laginous basipterygoid articular plates, looking outwards
and downwards on the side of the basisphenoid, which is
slightly pedunculate on either aspect to carry them. The
alisphenoid (a.«.) which arose from two centres is now one
bone on each side, distinct from the basisphenoid, and
retaining its fenestra; it forms a slightly bulging wall
to the lower part of the back of the orbit, and is almost
transverse in position: it is applied to the prootic behind,
and its upper surface articulates with the large orbital
plate of the frontal.

557. The rounded rostrum (r.6.5.), grooved above
for the interorbital cartilage, is still perfectly distinct,
reaching to the cranio-facial fenestra, where it curves up-
wards in front of the base of the ethmoidal cartilage. The
latter is now completely separated from the nasal septum
below — the fenestra has become a large notch. The inter-
orbital septum and fenestra remain the same as before,
but the anterior (ethmoidal) portion of the perpendicular
plate is ossified by a double ectosteal lamina (mesethmoid),
not reaching to the bottom of the septum. This bone
(p»e.) is thick and solid anteriorfy where it bounds the
cranio-facial notch, and at its upper angle; behind, it does
not nearly reach the interorbital fenestra.

558. From the upper edge of the mesethmoid the
cartilage curves down laterally and inwards to form the


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swollen upper turbinal; the antorbital wall is as before.
The nasal septum has a posterior angulated margin sepa-
rated by a considerable distance from the fore end of the
mesethmoid; but above, there is continuous cartilage,
forming a permanent isthmus. The septum sinks antero-
int'eriorly into the prenasal cartilage (jp.M.)» which within
the last three days has become reduced to about a fifth of
its former bulk and half its length: this condition is
retained till about the middle of the first winter. The
nasal septum and turbinal coils are unossified.

Fig. 64.

Chick two days old ; external lateral view of skull.

ox. occipital condyle ; p.s, presphenoid region ; in front of it, inter-
orbital fenestra ; p.p. antorbital plate ; al.n. alinasal cartilage ; a.n.
external nostril; II, optic foramen; V. trigeminal foramen: art. articular
surface of lower jaw.

Bones : e.o. exoccipital ; s.o. supraoccipital ; sq. squamosal ; pa,
parietal; fr. frontal; al.8. alisphenoid ; h.s. basisphenoid ; m.eth. mes-
ethmoid ; Ich. lachrymal ; na. nasal ; pmx. premaxiUary ; mx. maxillary ;
pi. palatine; pt. pterygoid; ju. jugal; g.jw. quadrato-jugal ; st, stapes;
qn. quadrate ; ang. angular ; 8.ang. surangular ; d, dentary.

559. The roof- and wall-bones of the cranium, the
frontals and parietals, have grown rapidly, obliterating
much of the great upper fontanelle. The parietals (Fig.
64, pa.) are narrow transversely-placed oblong plates,
fitting accurately to the posterior margin of the frontals

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(which they partially overlap), and the anterior margin of
the supraoccipital. The parietals are flanked below by a
pair of bones larger than themselves, but more irregular,
the squamosals (sq.) More than half the squamosal can
be seen on the inner surface of the cranium, between the
oblique lower edge of the parietal and the outer (hinder)
edge of the alisphenoid {sq. Fig. 63); externally it clamps
the sphenotic cartilaginous process of the alisphenoid, and
overarches the quadrate and the tympanic cavity.

560. The frontals (fr.) are the largest cranial bones;
they are very convex above, and their extensive incurved
orbital plate is bounded by a sharp semicircular supra-
orbital ridge. The line formed by this ridge is continued
farther down in front by the suture of the nasal with the
lachrymal, and behind by the junction of the squamosal
with the sphenotic process, thus bounding the very regu-
lar socket for the large eyeball. A membranous region
remains in the cranial floor between the edge of the orbi-
tal plate of the frontal, the alisphenoid, and the inter-
orbital septum, from the ethmoidal region to the common
optic foramen. The frontals are approximated anteriorly
to the coronal (parieto-frontal) suture, and then again
diverge and become pointed in front, leaving the top of
the mesethmoid uncovered, — a persistent condition.

561. The broad posterior edges of the nasals (na.)
cover the pointed anterior extremities of the frontals:
they are twisted upon themselves, and extend downwards
and outwards and then forwards below the alinasal carti-
lages (aZ. n,) ; they also send a sharp fork above the same,
by the side of the nasal processes of the premaxillary. The
supraorbital plate of the lachrymal (IcL) is large and oval;
its anterior portion remains slender.

562. The premaxillaries (pmoc.) have coalesced into a
single strong triangular bone, which has a solid rostral
portion in front of the nasal septum and equal to it in
length. Its nasal processes are united where they pass
between the nares, but permanently distinct behind
and above, where they reach the mesethmoid. These

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processes are longer than the inferior sharp dentary
margin (Fig. 65, d,px.), which is one-fourth longer than the
palatal processes (p.pos.). Each palatal process passes
under the apex of the palatine bone. The axial part of
the premaxillary is grooved below, and the hinder half
(rf the groove is filled by the small prenasal cartilage
(p,n. Fig. 63).

563. The slender maxillary (Figs. 64, 65) now rises
in front into the angle between the descending crus of

Fig. 65.

Chick two days old ; under view of skull, with lower jaw removed ; the
upper bones are shown in outline on either side of the interorbital

OCX. occipital condyle ; f.m, foramen magnum ; eu, anterior opening
of Eustachian tubes after traversing basitemporal bone ; pn. prenasiU
cartilage ; pf. sphenotic process ; ty. tympanic cavity ; 8, foramen for
glossopharyngeal and vagus nerves ; 9, foramen for hypoglossal nerve.

Boues: b.o. basioccipital ; e.o, exoccipital; s.o. supraoccipital ; b,t
basitemporal ; p.t.p. pretemporal wing of basisphenoid ; qu. quadrate ;
sq. squamosal; a. 8. alisphenoid; v. vomer; pa. palatine; pg. pterygoid;
d.px. dentary process of premaxillary; p.px. palatal process of pre-
maxillary ; m, maxillary ; mx.p. maxiUopalatine plate of nuoiUary ;
j. jugal ; q.j. quadratojugal ; I. lachrymal ; /. frontal.

the nasal bone and the hinder edge of the dentary process
of the premaxillary. The maxiUopalatine plates (mx.p.

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Fig. 65) are broader and reach nearly to the mid line,
being separated partly by the nasal septum and partly by
the small vomer, which is rounded in front, and split for a
short distance behind. The forks of the vomer (v.) articu-
late with the inner and anterior points of the inner plates
of the palatine bones, which lie side by side mesial ly,
nearly concealing the rostrum. Behind these plates the
palatine bones (pa. Fig. 65) are thickish angular rods,
both together having an elongated lyriform shape. The
hinder extremities are curved slightly outwards for arti-
culation with the pterygoids. The latter (pg. Fig. 65)
are short stout mallet-shaped bones, the handles being
turned outwards and a little backwards to articulate by a
cup with a little ball on the quadrate (§ 536), and also
sending up above this a small epipterygoid plate. The
head of the mallet is notched in front to receive the
rounded posterior end of the palatine. The inner face of
the head has a distinct oval plate of cartilage upon it,
exactly corresponding to and articulating with the basi-
ptei-ygoid plate on the basisphenoid.

564 The quadrate (qu. Figs. 64, 65) has the apex of
its orbital spur unossified; a similar condition is persistent
in the Chelonia. The otic process has two cartilaginous
surfaces close together ; a rounded upper and outer head
fitting in a cupped space on the squamosal, and a smaller
one gliding on a cartilaginous facet of the prootic. There
is a slight projection on the front of the otic process, but
there is no postero-internal head as in most Birds, the
relation to the tympanic wing of the exoccipital being
lost (§ 521). The shaft of the quadrate is short, and
bears a rounded condyle, almost transverse, and divided
into a larger outer and a lesser inner portion. The quad-
rate is connected with the maxillary by the intervention
#f slender jugal and quadrato-jugal bones (j., g./.), the
latter being bent inwards to clasp the side of the quad-
rate above the condyle. The base and shaft of the stapes
are ossified, and its outer end is a triradiate cartilage-.
The tympanic cavity will be described in the adult.

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565. A new bone has appeared in the mandible — a
rudiraent of the articular {art Figs. 63, 64); it projects
inwards from the condylar region as an internal angular
process, tipped with cartilage [i.a.p. Fig. 63). The posterior
angular process (p.a.p.), and the shaft of the meckelian
cartilage, which is three-fourths of the length of the jaw,
are unossified. The dentaries of opposite sides have coa-
lesced; they lie outside the cartilage and partly embrace
it; their length is three-fourths of that of the jaw, and they
are forked behind. On the inner face of the cartilage is a
long- oblong splenial (sp.), leaving both the point and the
hinder part of the cartilage bare. The angular below
and tlie surangular above the posterior part of the carti-
lage leave its inner face considerably exposed, but they
cover it externally, and meet.

566. Ossification has appeared in the two small
cornua (minora) of the hyoid, and the ossification is con-
tinuous across the lingual cartilage. The median basal
Cartilage has two styliform bones, one in front of and the
other behind the " thyrohyals" (=lst branchials). The only
ossification in the latter cartilages in the last stage was a
lower one on either side; there is now a second upper
bone, but the curved tip is cartilaginous, as are the apices
of all these osseous tracts.

567. The types suggested by a view of the chick's
skull at this stage are the Hemipodiine, the Pterocline,
and the Columbine outliers of the Gallinaceous group :
subsequent development removes the Fowl greatly from
its lower and more generalized relatives on one hand, and
from its higher and more specialized congeners on the
other. Every detail which has just been given will be
found pregnant with meaning by the student who has an
aflSnity for this subject. The coalescence of the various
bones in the basisphenoidal region, the extension of the
exoccipitals and the supraoccipitals into the ear-capsules,
and the appearance of the epiotics and mesethmoid, ^re
the chief events that have affected the brain-case. The

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Vil] the fowl: sixth stage. 249

prenasal cartilage is relatively much reduced. The pre-
maxillaries and dentaries respectively now form but one
bone ; and bony deposit has arisen in the stapes, the
articular region of the mandible, and the small re-
presentatives of the hyoid and branchial arches.

Sixth Stage: Young Fowls up to nine months old,

568. In a chick three weeks old there are two
features to be noted, (1) a distinct osseous centre in the
sphenotic process (§ 542), subsequently uniting with the
alisphenoid, and (2) the appearance of a small opisthotic
plate on the outer occipital plane, in a notch of the supero-
exterual part of the exoccipital, soon becoming united
with it.

569. In a chicken of two months old the structures
have increased in massiveness, and the roofing bones of
the skull are connected by delicately-toothed sutures;
only narrow synch ondrosial tracts are left in the cranium.
An inner view of the occipito-otic region shows the three
bones of the periotic capsule, as well as the alisphenoid,
still remaining distinct; nor has the basisphenoid yet
coalesced with the basioccipital or with the alisphenoid.

570. Above the optic foramen, wedged in between
the alisphenoid and the interorbital septum, a four-sided
bone has arisen in membrane on each side, and there is a
smaller pair in front of and above the larger, helping to
fill in the fenestras left unoccupied by the orbital plates of
the frontals. These are the anterior and posterior orbito-
sphenoids (o.s. Fig. 66). The anterior half of the inter-
orbital septum is ossified, the mesethmoid encroaching on
the anterior margin of the interorbital fenestra.

571. In chickens from three to four months old all
the splint bones of the skull, with the exception of the
early engrafted para?phenoidal elements, are separable by
maceration ; only the symmetrical premaxillaries and
dentaries have united at the mid line, the former very
early, the latter soon after hatching. Looking at the

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fikuU below and behind, externally, the large supra-
occipital still shows sutures with the exoccipitals, and they
with the basioccipital — the latter being still distinct from
the basisphenoid and its basitemporal plate. The posterior
basicranial fontanelle is indicated by a deep median longi-
tudinal chink in the posterior part of the basisphenoid.
The complex basisphenoid is distinct from the alisphenoids,
prootics, and basioccipital; but the small epiotic is uniting
with the prootic and supraoccipital, and the opisthotic
with the prootic and exoccipital. The sphenotic is con-
fluent with the alisphenoid. The composition of the occi-
pital condyle of three elements is still manifest.

572. In young fowls of the first winter, from seven to
nine months old, a large number of embryonic characters
persist (Fig. 66). The skull has become very thick and
solid, with copious coarse diploe. The cranium proper is

Fig. 66.

Fowl of first winter ; median longitudinal section of skull.

h.o. basioccipital perfectly continuous with basitemporal, basisphenoid,
and rostrum; p'. prootic; s.o. supraoccipital; p.f. pituitary fossa;
p. parietal; sq. squamosal; a.s. alisphenoid; o.s. orbitosphenoid ;
/. frontal ; m.e. mesethmoid ; v. vomer ; px. premaxillary ; d, deniary ;
sp: splenial ; 8. a, surangular; a. angular; ar. articular.

Cartilage is seen in the interorbital septum behind the mesethmoid ;
in the nasal septum in front of that bone, sending forward the small
prenasal spur ; and in the lower jaw slightly between the angular and

not half the length of the elongated skull, and is extremely
narrow where it ends above the interorbital septum. The
bones of the occipital ring, the periotic mass, and the basi-

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sphenoid are entirely confluent, as also are the anterior
parts of the premaxiilaries and dentariea. All the other
lines of suture are still traceable.

573. In the more advanced specimens a small patch
of bone can be seen between the orbitosphenoids in the
cartilage of the septum, between the interorbital fenestra
and the optic foramen; this is the first appearance of the
presphenoid. Cartilage remains beneath this bone, in
front of the interorbital fenestra, down to a vertical suture
which divides the anterior part of the basisphenoid from
the postero-inferior part of the mesethmoid: this connec-
tion is above the original rostrum, and is produced by
extension of ossification. The mesethmoid (m.e.) has ex-
panded superiorly into the aliethmoidal cartilage on either
side, forming a flat plate, which can be seen between the
frontals and nasals and behind the nasal processes of the
premaxiilaries (see Fig. 70), The mesethmoid ossifies
the posterior part of the aliethmoidal roof, and extends
behind into the spike mentioned in § 544 : below this is a
groove for the olfactory nerve, pavssing beneath the alar
expansion on either side, in the angle between it and the
vertical plate.

574. The rest of the nasal labyrinth is entirely un-
ossified. The hinder part of the roof is flattened above,
and its lateral edges grow inwards to form the upper tur-
binals {u.tb. Figs. 69, 71), which near the middle line are
again bent outwards, forming a buUate pouch opening
outwardly; its edge is confluent behind with the ant-
orbital wall. In this region the nasal septum is deficient by
reason of the craniofacial notch, except for a slight carti-
laginous ridge above, separating the nasal branches of the
trigeminal nerve (n.n.) as they pass forwards to reach the

575. No middle turbinal coil is developed on the
face of the antorbital plate below the upper turbinal. In
front of and below the latter, the inferior turbinal (i. tb.
Figs. 68, 71) forms a not very uniform scroll, coiled twice,
the blind face of the coil being inwards, and its long axis

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Figs. 67—71.

Fig. 67.

Fig. 70.



Fig. 71.

Views of nasal structures of Fowl of first winter.

Fig. 67. Transverse section through most expanded part of alinasal
region; (n.tft. Fig. 71).

Fig. 68. Transverse section between aliethmoidal and aliseptal
regions (through Fig. 71).

Fig. 69. Transverse section in aliethmoidal region, with inferior
turbinal cut away, in region of cranio-facial notch of septum (through
u,th. Fig. 71).

Fig. 70. Transverse section through most solid part of ethmoid,
showing antorbital plate (pp. Fig. 71).

Fig. 71. View of nasal labyrinth from within, the septum being
removed ; less magnified than the preceding figures.

8.n, nasal septum ; al.n. alinasal cartilage ;, alinasal turbinal ;
n.w. nasal wall; i. t6. inferior turbinal; upper turbinal; al.e. ali-
ethmoid cartilage ; a.or, (Fig. 10)^pp. (Fig. 71), antorbital plate; n,y* nasal

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gland ; 1, olfactory nerve ; n.n. nasal nerve (from trigeminal), appearing
as displaced from relation to the septum in Fig. 71 ; m.n. middle narial

Bones : n,px. nasal process of premaxillary ; d.px. dentary process of
same ; n. nasal (in Fig. 68 the lower fork of the nasal is cut through and
seen separate from the upper) ; /. frontal ; L lachrymal ; p.e. mesethmoid ;
eth. ethmoid; v. vomer; pa, palatine; r. rostrum; mx, maxillaiy; palatal plate of maxillary.

directed obliquely. It grows out of the nasal roof and
outer wall in front of the aliethmbidal region. Behind
and below this, the nasal wall becomes imperfect for a
space ; and the hinder termination of the inferior turbinal
is confluent with the lower outer angle of the antorbital

576. Another turbinal occupies the fore part of the
nasal cavity ; it is the alinasal turbinal (n. tb. Figs. 67, 71),
whose blind face is also inwards ; at most it only forms
half a coil. It arises from the outer edge of the nasal
roof, but outside it the alinasal cartilage {al. n. Fig. 67)
forms a complete lateral wall, continuous with the nasal
floor except where it is slit obliquely for the anterior
nasal opening. Posteriorly the alinasal turbinal, where it
hes below the anterior part of the inferior turbinal, grows
inwards, and joins a remarkable wing of cartilage con-
tinuous with the nasal septum, and pierced on either side
by the nasal nerve Tthis is a development from the original
bridge over this nerve, § 545). Anteriorly, the upper part
of the alinasal cartilage terminates as a small ear-like
process on either side, and between these the prenasal
cartilage,' which has not lessened since the time of hatch-
ing, lies in a groove of the premaxillary. The whole alinasal
tract lies in an ovoid space formed between the processes
of the premaxillary, and also partly in the notch between
the processes of the nasals. In the Fowl the aliseptal
region is very small in comparison with the aliethmoidal
behind and the alinasal in fron^ of it.

577. The bones of the palate have merely grown
larger without altering their relations ; those of the lower
jaw are beginning to coalesce, and the posterior as well as
the internal angular process is ossified by the articular.

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The Skull in Fowls several years old,

578. By the time the fowl is a year old most of the
cranial sutures are completely closed ; and then, year by
year, the skull becomes more and more dense, the peri-
osteal layers filling in all spaces in the bony walls except
those needed for the transmission of vessels and nerves.
Thus in an aged bird (Figs. 72, 73) the occipital, otic, pos-
terior and anterior sphenoidal, and ethmoidal regions have
become one continuous bone, with the old landmarks all
removed ; and scarcely a sign is left of the once highly
complex conditions of these parts. This state of things is
quite normal for a Bird ; and although carried to a very
high degree in the Fowl, yet in some birds the obliteration
of all signs of the once composite condition of the skull is
still more perfect.

Fig. 72.

9J :/

Fowl several years old ; side view of skull.

Bones : sq. squamosal ; p. parietal ; pf, sphenotic process ; /. frontal ;
me, mesethmoid ; I, lachrymal; px. prem axillary ; mx. maxillary;
V. vomer; j, jugal; q.j. quadrato-jugal ; between v, and J. pt^atine;
pt, pteyrgoid ; g. quadrate ; ar. articular; a, angular ; d, dentarj.

579. In the side view of the skull of an old male
(Fig. 72) it is seen that the face has gained considerably
upon the cranium as compared with the condition found
in the newly-hatched chick ; although the maxillary bones
(mx.) are relatively smaller than ever, being minute styles
with a very small maxillopalatine ingrowth.

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580. In an upper view the anterior part of the suture
between the frontals is visible : the nasa's have coalesced
behind with the frontals, but are distinct from the forked
nasal process of the premaxillary. Between these bones
(frontals, nasals, and premaxillary) a considerable penta-
gonal ^ract of the top of the ethmoid is apparent. The
following bones are permanently separate: lachrymals,
raaxillaries, jugals, quadrato-jugals, quadrates, pterygoids,
palatines, vomer.

581. The whole interorbital plate (m. e.) has become
one bone. In some old birds a trace of the interorbital
fenestra can be seen ; in others its outline is distinguish-
able, but it is filled up with periosteal bone. The ali-
sphenoidal fenestra is also occupied by bone, and the
alisphenoid has coalesced with the orbital plate of the
frontal. Above this, the two pairs of orbitosphenoids, the
presphenoid, and the anterior part of the orbital plates of
the frontals, have coalesced together; and the fenestra
which formerly existed on either side, between the edge
of the orbital plate and the interorbital septum, is oblite-
rated excepting for the antero-superior chink by which the
olfactory nerve runs forwards on either side of the top
of the mesethmoid.

582. In the cartilaginous isthmus above the cranio-
facial notch (§ 557) two small osseous centres have
appeared, one behind the other; and there is a small nodule
on either side between them. The rest of the nasal
labyrinth is unossified save that in some cases *a little
bony matter passes from the mesethmoid to the antorbita,l
plate, which otherwise hangs down from the ethmoidal

Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 21 of 31)