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The morphology of the skull online

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and the ear-sac is a large shallow fossa for the Gasserian
ganglion, and the largest division of the trigeminal nerve

Fig. 79.

Embryo Fig, an inch and a third long ; median longitudinal section of
head, with nasal septnm removed.

aLth. alinasal tnrbinal; i.tb. inferior torbinal; nasal torbinal
m,tb. middle turbinal; u.tb. upper turbinal ; cr.p. cribriform (ethmoidal)
plate (the line should be prolonged to a point over the turbinal folds) ;
p.8. presphenoid region; 0.8. orbitosphenoid ; al.s, alisphenoid; py,
pituitary fossa; posterior clinoid ridge; au. ear-capsule; a.8.c. an-
terior semicircular canal; p.8.c. posterior semicircular canal; «.o. supra-
occipital cartilage; 1.8. position of the lateral sinus in dura mater:
f.m. foramen magnum ; o.c. occipital condyle ; b.o. basilar plate ; 2, optic
foramen ; 5, foramen ovale ; 8, foramen for glossopharyngeal and vagus
nerves; 9, foramen for hypoglossal; mk. meckelian cartilage; above it,
the tongue; chy. ceratohyal.

(the inferior maxillary) passes out between these two.
cartilaginous regions (5). The superior maxillary division
of the trigeminal passes out between the orbito- and ali-
sphenoids. The orbitosphenoids (0,8,) are far the larger,
spreading from .their presphenoidal base into a sickle-
shaped lamina extending from the fore part of the cranium
almost to the auditory masses. The optic nerve (2) passes
out by a foramen in the base of the orbitosphenoid.
The thin frontals have arisen in the stroma outside and
above the outer edge of the orbitosphenoids.

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635. The nasal cavities extend backwards beneath
the cranial cavity in its ethmoidal portion, and on each
side of the median ethmoidal plate the nasal roof is soft,
and the olfactory lobes of the brain lie upon it. The
nerve filaments perforate the roof to reach the turbinal
growths of the nasal sacs ; and very soon cartilage appears
between the groups of nerve fibres, and gradually becomes
perfected into a cribriform plate (cr, p,).

636. The nasal cavities already occupy more than
half the length of the head. Rather more than half their
ext«[it belongs to the snout, which is a rostrum-like pir^
jection from th« l^boas ^ead. The maaai aqptam is a
complete vertical lamina of cartilage, of considerable and
almost uniform height, being most elevated at the junc-
tion of the snout with the cranial box. The septum bears
no conspicuous projections on its lateral surfaces; but
superiorly it is continuous with the upper and outer carti-
laginous nasal walls, which are distinguishable from before
backwards into alinasal, aliseptal, and aliethmoidal regions*
Anteriorly the septum is broader at the base, owing to its
continuity with the primary trabecular cornua; but a
little posteriorly these cornua have become recurrent car-
tilages distinct from the septum ; and they lie for some
distance close to its base, diminishing to small styliform
rods. The greater portion of the septum is floored by
condensed tissue very like nascent cartilage, separating it
from the roof of the mouth, or from the palatal plates
where they are uniting to form the true palate. This
tissue early ossifies as the vomer.

637. The outer nasal wall becomes more complete
and more complex in passing from before backwards;
each of the main turbinal outgrowths from the upper and
outer walls contains a small cartilaginous. blade continuous
with the capsular wall. The nasal sacs are widest where
they underlie the fore part of the cranial cavity. They
diminish in size as they approach the presphenoidal region,
under which they die out : this is the part of the nasal
cavity known as the sphenoidal sinus. The premaxillariea

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vhlJ the pig: third stage. 283

have appeared in the roof of the mouth, beneath each
nasal mass, short of its extreme anterior extremity,

638. The trihedral mass of stroma forming the outer
part of the palate roof and the uppw lip, sends inwards in
its middle region a thin tongue of tissue to meet its fellow
and form the secondary palatal roof. This meeting takes
place at present only for a short distance ; elsewhere the
palate is still cleft. The palatopterygoid rod lies just at
the junction of this inner palatal region with the outer
stroma ; it is kidney-shaped in transverse section. Beneath
the hinder part of the nasal capsule this imperfect carti-
lage is ossifying to form the palatine ; further back a small
rudiment of the pterygoid appears (the future internal
pterygoid plate), adjacent to the downgrowth from the
alisphenoid which becomes the external pterygoid plate.
The maxillary bones first arise beneath the nasal sacs
under the anterior cranial region, in the palatal roof. It
is evident that it is the palatal part of the maxillary which
is the first formed ; outside and behind it, a row of tooth-
pulps is developing, close to the edge of the lip, corre-
sponding to a similar set in the lower jaw.

639. The floor of the mouth is occupied by the pro-
minent tongue, and below its level on either side is the
stroma in which the meckelian cartilage is embedded.
This primary rod is flanked externally and partly em-
braced by a dentary stroma, which ossifies rapidly, be-
coming however distinctly cartilaginous in the interim.
The dentary bone is vertically elevated behind ; a row of
tooth-pulps lies above it in the greater part of its extent.
The meckelian cartilages have fused together anteriorly.

640. The arrangement of the parts of the ear-capsule
is as follows. The coils of the cochlea (cZ. Fig. 80) are
fully developed, and are entirely anterior, and near the
middle line and base of the skull. The facial nerve
passes into the capsule above and externally to the cochlea,
and curves backwards and outwards to reach the roof of
the tympanic cavity. The anterior semicircular canal

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arches high up on the antero-external part of the auditory
mass : the horizontal canal is almost directly below it, and

Fig. 80.

Embryo Pij?, an inch and a third long; posterior view of a section
through the basal region of the skull. The section being not per-
fectly transverse shows a different condition on each side.

The left-hand view shows parts in front of those displayed on the
right ; the manubrium of the malleus mb. is cut through at its thickest
(posterior) part, and the main part or shoulder ml. is seen, at the part
which articulates with the incus ; within mb. -is the tympanic cavity t.c.
On the right side the incus i. is seen, covering the malleus, and having
its short eras cut away; its orbicular head o.ob. is applied to the stapes sU
The stylohyal cartilage st.h. is seen in oblique section as it passes down-
wards and forwards. The supraoccipital cartilage s.o. is seen as if discon-
tinuous with the auditory tract on the left side, in consequence of the
section passing through the notch in the front end of the cartilage.
h.o. basilar cartilage, narrow between the ear- capsules ; a little of its
upper surface is seen, as well as the section enclosing the notochord nc. ;
ba.a. basUar artery ; j.v. jugular vein, with a coiled branch ; Let. long
crus of incus ; aq.v. aqueductus vestibuli ; cl. cochlea. Under the basilar
cartilage the throat is partially represented.

the main part of the labyrinth is internal, behind the
cochlea. The posterior canal, convex backwards, touches
the horizontal canal externally, and ends internally in the
labyrinth. The middle and hinder part of the external
edge of the ear-mass, containing part of the posterior
canal, projects as a roof or tegmen tympani (pterotic ridge)
over the tympanic cavity ; so that the fenestra in which
the stapes lies places the labyrinth in direct contiguity

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vni.] THE pig: third stage, 285

with the tympanic cavity. The short or external cms of
the incus articulates with the tegmen,

641. The tympano-eustachian canal has increased in
length and become more de£nite; it is completely oc-
cluded by the tympanic membrane, in which the handle of
the malleus (Figs. 80, 81, mb,) is embedded. The latter
piece has become flattened, but is not segmented from the
remainder of the mandibular arch. The incus (i.) has

Fig. 81.

Embryo Pig, an inch and a third long ; side view of mandibular and
hyoid arches. The main hyoid arch is seen as displaced backwards
after segmentation from the incus.

tg, tongue ; mk, meckelian cartilage ; ml, body of malleus ; wh, manu-
brium or handle of thia malleus ; t.ty. tegmen tympani ; i. incus ; st, stapes ;
i.hy. interhyal ligament; it.h, stylohyal cartilage; h.K hypohyal;
h,K basibranchial; th.K rudiment of first branchial arch; 7a, facial nerve.

similarly progressed, applying the orbicular head {o.oh.) of
its long crus to the stapes (Figs. 80, 81, st) The latter
has begun to assume its distinctive shape, the two tubercles
which it bore at first being now connected by a bridge of
cartilage in the fenestra ovalis. The main hyoid rod
(st.h) is attached by ligament to the orbicular end of the
incus, and is in contact with the auditory capsule behind
the stapes. In this interhyal ligament {i.hy,) a small
cartilage has arisen. The facial nerve (7a.) passes (above
the tympanic membrane) to get behind the hyoid arch,
parallel with which it proceeds to its destination.

642. The main hyoid bar passes directly downwards
from the auditory region for some distance, and then

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curves forwards and inwards, being connected with the
small hypohyal segment (A.A.). The latter is articulated
to the median azygous rudiment of the next arch (basi-
branchial), and to its lateral or branchial cartilages (so-
called basi- and thyrohyal). ,

" The proper territories of each investing bone in the Pig
evidently only want time that they might all become true
cartilage ; ossification sets in too soon for the formation of the
intercellular substance, but each tract, before ossification, is a
true morphological element or organ."

{FhU. Trans, 1874, p. 306.)

643. We have now considered the structure of the
skull in an embryo Pig only an inch and a third long, and
have found it completely constituted as to cartilage, with
all the normal elements continuous, viz. basilar plate,
trabeculae, occipital ring, auditory capsules, ali- and orbito-
sphenoids, nasal septum, walls, and turbinals. The princi-
pal arches have almost perfected their permanent rela-
tions. The predominance of the orbitosphenoid cartilages,
the large size of the nasal capsules, their extension back-
wards beneath the brain-case, and the formation of the
cribriform plate are to be noticed. The true cartilage-
bones have not yet appeared ; but there are present the
vomer, frontals, premaxillaries, maxillaries, palatines, ptery-
goids, and dentaries.

Fourth Stage : Emlryo two and a half inches long.

644. Development has proceeded very rapidly; the
snout is proportionately longer, and the forehead has
become sloping instead of vertical. The well-marked
granular territories that at first invested the primordial
skull are now largely ossified, and these ossifications are
massive in relation to so small a skull. The cartilaginous
parts also are undergoing endostosis at many points.

645. The cartilaginous cranium has not changed much
in its main outlines : the floor has thickened considerably.

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VIII.] THE pig: fourth stage. 287

The cribriform ethmoidal plate has now four slender bars
passing between the olfactory filaments, and a common
outer band which does not extend backwards to the orbito-
sphenoidal cartilage. The latter has grown forwards to
unite with the aliethmoid cartilage, and backwards to
overlap the antero-external angle of the auditory cartilage,
quite overshadowing the alisphenoid. But the external
pterygoid plate (Fig. 82,,) arising from both the
basisphenoidal and the alisphenoidal regions, is well de-
veloped, and articulates with the palatopterygoid bar.

Fig. 82.


Embryo Pig, 2 J inches long; under view of skull with lower jaw removed.

f.m, foramen magnum ; occ, occipital condyle ; au. auditory capsule ;
m.t. tympanic membrane; external pterygoid process ;j>.s. presphenoid
region ; c. tr. trabecular comua.

Bones: b.o. basioccipital ; e.o, exoccipital; «.o. supraoccipital ; ty. tym-
panic ; b.8, basisphenoid ; sq. squamosal; pg. pterygoid; pa, palatine;
V. vomer ; d.px. dentary plate of premaxillary ; p.px, palatal process of
same ; nix. maxillary ; j, jugaL

646. The cartilage bones are principally endosteal at
present, although they rapidly gain the surface and afifect

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the perichondrium. There is a thick basioccipital (b,o ) in
the basilar plate, seen both above and below ; it is shaped
like a spear-head, and is fast obliterating the notochord.
In the exoccipital region there is a large growth of bone
(e.o.) extending inwards towards the upper edge of the
foramen magnum, and outwards and downwards in the
external or paroccipital process. Besides this, a distinct
ossification arises in the massive condyle, which coalesceis
with the exoccipital before long. The supraoccipitalis.o)
is double at first, but the two patches run into one another
in a day or two.

647. The cartilage in the pituitary floor is ossifying
to form the basisphenoid {b,8.) ; a little later, a distinct
centre arises on either side for the alisphenoid. There is
no presphenoid, but a small orbitosphenoid ossification is
found outside and behind each optic foramen. The
auditory capsule is unossified at present.

648. The hinder nasal region has become much
broader than the anterior. The upper and middle turbinal
folds with their included cartilaginous laminae occupy the
back part of the nasal cavity ; and the inferior turbinal
is further forward. Anteriorly the recurrent trabecular
cornua {clr,) still persist, on either side of the base
of the septum.

649. The vomer (Figs. 82, 83, v.) embraces the base
of the septum along the hinder two-thirds of the nasal
region. The premaxillaries are found on the under surface
of the fore part of the snout, not reaching to its extremity.
Each has an expanded dentary portion (d.px.) grooved by
tooth-sacs, and a small medio-palatal spur (p.px,) ex-
tending towards the vomer. The end of the snout around
and above the external nostrils is unclothed by bone : but
behind this region the premaxillary of each side sends up
a facial lamina to meet the outer edge of the nasal, also
coming in contact behind with the facial plate of the
maxillaiy. The nasals^ frontals, and parietals form a

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regular double series of thick bones. The p€urietals do not
extend so far back as to the supraoccipital ; they occupy
the main part of the vertex of the skull. The frontals
are the largest pair. They stretch considerably over the
face, touching the lachrymal and maxillary bones ; they
arch over the orbit, and send inwards from the eave an
orbital plate which is in the side wall of the skull, and
reaches the orbitosphenoid cartilage. The anterior and
posterior median fontanelles (ifronto-parietal and parieto-
occipital) are still widely open.

650. The maxillary {mx.) has its facial and palatal
plates largely developed. The facial lamina extends from
the premaxillary along the lower part of the face to under-
lap the jiigcd. Above, the maxillary is in contact with the
frontal and lachrymal, and does not form any part of the
margin of the orbit. The lachrymal is a small bone en-
closing the upper end of the orbitonasal (lachrymal) canal,
and occupying some space on the face between the frontal
and jugal bones. The latter (j.) bounds the lower margin
of the orbit, and passes backwards underneath the zygo-
matic process of the squamosal, nearly to the ear-sac.

651. The squamosal (sq.) is a large triradiate mem-
brane bone, with a squamous part lying on the infero-
lateral wall of the cranium, between and below the frontal
and parietal; it has also a long sickle-shaped process
directed backwardly by the side of the ear-capsule above
and outside the tympanum ; and a zygomatic process
growing outwards and forwards to the back part and
external edge of the orbit, resting on the jugal. At the
point of junction of these three parts the squamosal has
an orbicular concAve surface, external to the tympanic
membrane; with this surface the osseous mandible ar-

652. Returning to the palate, the greater portion of
it is occupied by the large oblong palatal plates of the
maxillaries, which do not meet in the middle line but
leave the vomer exposed between them. The maxillaries

B. M. 19

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are grooved by vessels in the middle of their palatal plates,
whilst the dentary portions are hollow and shell-like,
containing large growing tooth-germs. The palatines ( pa.),
pointed in front, rod- like behind, extend outwards and
backwards on the palate from the maxillaries to the
external pterygoid cartilaginous plate. Their ascending
lamina reaches to the hind part of the nasal septum. In-
ternally to the hinder portion of each palatine, is a small
osseous nodule, the pterygoid {pg)»

Fig. 83.

Embryo Pig, 2 J inches long; vertical section of head, showing structures
between and beneath the orbits.

p.g. mesthmoid plate; middle turbinal coil; «. eyeball;
m.n. nasal passage above secondary palate; mk. meckelian cartilage;
h.hy. basibranchi^ (body of hyoid) \c.hy. ceratohyal; th,K first branchial
aich ; tg, tongue.

Bones: v. vomer; pa. palatine; cr, coronoid process; ar, articular
tract ; d. dentary ; J. jugal.

653. The proper mandible is now an extensive ossifi*
cation of considerable vertical extent, ensheathing its own
cartilage, sending up a backwardly curved coronoid procesfj
behind (Fig. 83, cr.), and having a cartilaginous condyle
beneath the curve of the coronoid, articulating with
the squamosal. There is a protuberant angular region of
cartilage below the condyle. The osseous symphysis is
not formed at present : but the symphysis of the meckelian
rod is very extensive. This cartilage lies within the bony
mandible unchanged ; the malleus and its manubrium
have become ossified, but are still perfectly continuous

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vnr.] THE PIG: fifth stage, 291

with the rest of the primary arch, just within the articular

654. The tympanum Is becoming rapidly perfected ;
the Eustachian tube opens into the back of the mouth
beneath the basisphenoid. The tympanic membrane is
partially enringed by a tympanic parostosis (Fig. 82, ty.)
deficient behind. The incus is ossifying : behind it the
stylohyal is continuous with the ear-capsule below the
horizontal semicircular canal. At its lower end the stylo-
hyal region passes into the hypohyal. The hypohyals,
basibranchial (6. Ay.), and the pair of branchials [th,h.) are
articulated together; and they support the root of the

655. This stage is an excellent one for comparison
vith the adult Fish, Amphibian, or Reptile, or the ripe
chick of the common Fowl. It also corresponds very
closely with an early stage of the skull of Balcena japonica,
figured by Eschricht, The non-segmentation of the car-
tilage of the mandibular arch, and the articulation of the
dentary with the squamosal, at once distinguish the mam-
malian type ; yet if the palate be compared with that of
the young Ostrich {Phil. Tram. 186B, Plate vii. Fig. 4),
the conformity is more remarkable than the difference.
The new bones which have appeared are the four occipi-
tals ; the ba^-, ali-, and orbitosphenoids ; the nasals and
parietals ; the lachrj'mals, jugals, squamosals, and tym-
panies ; the incus and the malleus.

Fifth Stage: Embryo six inches long from snout to

656. The general form of the head has not altered
much ; the snout is longer and its upper surface slopes
more gradually from the frontal region forwards. The
bones have become very dense ; on the roof of the skull
they are applied to each other edge to edge by sutures, in


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certain places even overlapping. The anterior fontanelle
(fronto-parietal) is still open, but is much lessened ; the
parietal and occipital bones form a good lambdoidal suture.
The sickle-shaped process of the squamosal which lies
alongside of the ear-capsule, has an increased surface,
projecting backwards and upwards to the supraoccipital
(Fig. 84, s,o,), and downwards in apposition with the
paroccipital process (p.oc.) of the exoccipital. The end of
the snout, around the small external nostrils, is unclothed
by bone. The nasals project forwards mesially. The
orbital plate of the frontal has developed very largely, so
as to form the whole bony roof of the orbit. The upper
and lateral bones of the skull require but little relative
change, with increase of size, to bring them to their adult

657. The previously existing cartilage-bones have in-
creased in size. The bones forming the occipital ring are
not yet in contact with one another. The exoccipitals
(Fig. 84, e.o.) ossify the condyles and the paroccipital
processes. The supraoccipital (5.0.) is a large concave
shell ot bone, separated by a considerable tract of cartilage
from the exoccipitals.

658. The basisphenoid is a thick bone, extending
partially into the posterior clinoid ridge, and continuous
with the all sphenoids, which are of considerable size, and
extend downwards into the external pterygoid plates.
The prespheuoid cartilage is well ossified, and continuous
with the orbitosphenoids, which have ossified the inner
half of the corresponding cartilages. The latter have not
grown with the growth of the skull, and. con.sequently
appear relatively contracted ; they are separated ifrom the
aliethmoid cartilage anteriorly, and are at some distance
from the auditory capsule behind. The remainder of the
facial axis and nasal septum is one sheet of solid cartilage,
perfectly continuous on either side with the nasal laby-
rinth and its now highly complex turbinal growths and
cribriform plate*

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vin.] . THE pig: fifth stage. 293

659. The proper periotic bones ate now distinct.
The prootic (pr,o.) is an endosteal patch surrounding the
meatus internus, lying under the fore part of the cochlea
and extending supero-posteriorly to the junction of the
anterior and posterior canals : externally it is seen above
and in front of the fenestra ovalis (f.ov.). The opisthotic

Fig. 84.

Embryo Pig, six inches long; outer view of occipital and auditory regions.
t.ty, tegmen tympani;/.ov. fenestra ovalis; a.8.c. anterior, h.8.c. hori-
zontal, p.«.c. posterior semicircular canals ;/.r. fenestra rotunda, the line
should be carried upwards to the angle behind f.ov. ; 7a, facial nerve ;
p,oc. paroccipital process ; o.c. occipital condyle ; tLh. stylohyal cartilage.

Bones, in some cases not well indicated: e.o, exoccipital; i,o. supra-
occipital ; pr,o, prootic ; op. opisthotic ; ep. epiotic.

(pp.) is on the under surface of the cap?5ule behind, and
covers the most bulbous part of the cochlea below : one of
its processes lies between the fenestra ovalis and the
fenestra rotunda (/.r.) close in front of the head of the
stylohyal cartilage. The epiotic centre {ep.) is at present
but a small scute ahove tne head of the latter cartilage
and below the hinder end of the tegmen tympani or
pterotic ridge {t.ty.).

G60. The structures of the middle ear have acquired
almost their full development. The stapes is not yet
ossified. The tympanic bone is still an imperfect ring,
but is thicker and broader, although no meatus extemus


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is as yet formed. There is an additional small ossicle
clinging to the inner edge of the tympanic : it arises in
the connective tissue forming the floor of the tympanum ;
it is a small 08 hullcB,

661. The palate has not become much broader : it is
now regularly oblong. The maxillaries meet the pre-
maxillaries, leaving the incisive foramen near the middle
line on each side. The palatal plates of the maxillaries
meet all along the middle line, hiding the vomer. The
palatal plates of the palatine bones continue the general
surface of the palate, being triangular with the bases ad-
jacent and the apices directed outwards and backwards.

Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 24 of 31)