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The morphology of the skull online

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the antero-inferior extremity of the head ; the eyeballs (e)
are above and behind them in the angle between the first
and second cerebral vesicles; and the pyriform ear-sacs {au),
of approximately the same size as the other sense capsules,
are below the hinder half of the third vesicle. The skin
over them, as well as over the adjacent cerebral vesicle, is
very thin as yet.

604. The side wall of the mouth and throat presents a
continuous skin, interrupted below by four successive vertical
clefts. The maxillopalatine process {pa. mx.) is perfectly
continuous with the wall of the mouth and throat above,
and projects downwards and forwards in the 3ubocular

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region, a cleft remaining between it and the eyeball. The
hinder visceral clefts are overlapped by the palmate rudi-
ment of the forwardly-directed fore limb.

605. In a median longitudinal and vertical section
the brain vesicles are seen to be hollow, having only a
film of soft neural substance lining the membranous cra-
nium. Tlie mesocephalic flexure is even more intense
than in other types. The notochord, underlying the hinder
vesicle, bends suddenly upwards so as to be situated in
front of it. It does not extend so far as the summit of
the third vesicle or the base of the second, but ends in a
free blunted point, just opposite to that superior and
posterior infolding of the membranous cranium which
partially severs the second and third vesicles. Above the
apex of the notochord is a mass of gelatinous stroma (the
" middle trabecula ") forming the hilus of the long kidney-
shaped second vesicle. In front of the notochord at its
upward bend is the pituitary body, and immediately an-
terior to that, the small first cerebral vesicle giving off its
large hemispheres. The internal carotid artery is distinct
on either side of the pituitary body,

606. Parachordal hyaline cartilages (Fig. 77, pa, ch)
already exist, being rather below the level of the noto-
chord (nc.) on either side ; they follow its curve, stopping
somewhat short of its apex. The ear-capsules lie external
to the parachordal cartilages, with walls commencing to
chondrify. They are tuberous bodies, having a nearly
straight inner margin, and a sublobular outer edge, and are
broadest behind. The aqueductus vestibuli is left as an
opening in the cartilage on the upper and inner edge:
the facial nerve enters the capsule a little behind this,
its passage being the aqueductus Fallopii. The semi-
circular canals are but just beginning to bud out from the
postero-superior region of the ear-cavity, and the cochlea
is indicated by a styliform anterior projection of the same
cavity. There is an external deficiency in the capsule,
filled with a plug of gelatinous stroma. The glossopharyn-

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geal (8a), vagus (8i), and hypoglossal (9) nerves pass
through soft stroma in the posterior angle between the
auditory sac and the parachordal cartilage. The trigeminal
is distinguishable in front of the ear-sac.

Fig. 77.

Embryo Pig, two-thirds of an inch long; elements of the skull seen
somewhat diagrammatically from below. parachordal cartilage ; nc, notochord; au. auditory capsule ;
py, pituitary body; tr. trabeculce;, trabecular oomu; |)w. prenasal
cartilage; e.n. external nasal opening; ol. nasal capsule;, palato-
pterygoid tract enclosed in the maxillopalatine process ; wiw. mandibular
arch; hy. hyoid arch; th.h^ first branchial arch; 7a, facial nerve; 8«,
glossopharyngeal; 86, vagus; 9, hypoglossaL

607. The trabeculae (tr,) are well marked out in the
fore part of the cranial floor, lying beneath the olfactory
sacs and the first cerebral vesicle, and extending upwards
and backwards to embrace the pituitary body. They are
shaped like callipers, and their pointed (notochordal)
apices are some distance apart. They reach nearly as
far back as the fore end of the parachordals, but are con-
siderably below them in position. The trabecular curve is
not moulded on the sides of the pituitary body, but is
very distinct from it; and the intertrabecular space is
much more extensive than that body. In their fore part
they very early coalesce and thicken: the commissure

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gradually extends backwards, until the uncoalesced part
merely corresponds to the pituitary body. The fore end
of each trabecula does not take part in the fusion, but
becomes first clubbed and then bent somewhat outwards
and inferiorly, forming the trabecular corn)ia (ctr.); and
these cause the skin to project on either side in the palate,
mesiad of the internal nostrils. Between these promi-
nences of the comua a median backwardly directed process
soon arises, which includes an azygous growth from the
trabecular commissure, the prenaial or rostral process.
(Figs. 77, 78,^/1.)

608, The trabeculse, as well as the other skeletal ele-
ments, are not yet truly cartilaginous, although unmistakeable;
they are embedded in a gelatinous tissue rich with young cells,
whose protoplasmic substance takes up carmine very freely.
The differentiation of the rods is at present a matter of degree,
that part of the blastema which will become hyaline cartilage
being the most compact and crowded with yoimg cells ; next to
this the nascent perichondrium ; and the most gelatinous paro
outside is the rudimentary condition of the areolar connective

609. The nasal sacs are intimately related to the
front part of the trabeculae ; and are already complicated.
There is a squarish median intemasal region, of consider-
able thickness, beneath the first vesicle, and in front of
the at present down-turned trabecular extremity. The
inner walls of the nasal cavities are simple, but the outer
walls exhibit various processes projecting inwards. There
is a lower bilobate process, the anterior lobe of which be-
comes the alinasal turbinal (see Fig. 79, p. 281), and the
hinder lobe the inferior turbinal. Another swelling below
the fore part of the nasal roof is the rudiment of the nasal
turbinal ; while a mass in the hinder angle beneath the
olfactory cms gives rise to the upper and middle turbinals
and the true olfactory region. By reason of these processes
the nasal meatus is already tortuous, first ascending, pro-
ceeding backwards, and then descending; but the external
and internal nares are on the same level, partitioned from

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one another by the forward part of the maxillopalatine
process and the tissue containing the trabecular cornu.

610, The intemasal mass contains a pair of cartilagi-
nous septal laminae, each being continuous below with the
corresponding trabecular moiety at its inner and upper
edge, very near the middle line, and forming a concave
inner wall to each nasal sac. The arch extends considerably
into the upper nasal boundary ; it may be called aliseptal

611. It has already been stated (§ 602) that the nasal
sacs are beneath the cerebral hemispheres. A transverse
vertical section through the head at the region of the
internal nares cuts through the olfactory crura partially

Fig. 78.

Embryo Pig, two-thirds of an inch long; palatal view, the lower jaw and
lower side of throat having been removed,
p. prominence enclosing prenasal cartilage; that covering the
trabecular cornu; oh nasal sac; e,n, external nostril; i.n. primary int>
ternal nostril; pa,mx. maxillopalatine process; mk. section of meokelian
cartilage; hy. section of hyoid arch; th.h, section of branchial arch, ap-
plied to lower extremity of hyoid ; py, tissue beneath the pituitary fossa.

separated by a median ethmoidal partition of mesoblastic
tissue ; the trabeculae lying as rounded rods somewhat
below this partition; and the irregularly shaped nasal

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sacs beneath the cranial cavity and external to the trabe-
culae. The thick maxillopalatine process forms the outer
and lower margin of the section (see Fig. 78), enclosing, in
that part which is close to the internal nostril, the small
rudiment of the palatopterygoid bar. In the upper and
outer tract of the maxillopalatine process the cleft between
it and the upper facial region (§ 604) is cut through;
it passes downwards into the posterior portion of the
nasal passage, and upwards to the base of the fore part of
the orbital region (the future "inner canthus"); it be-
comes the lachrymal passage.

612. The palatopterygoid bars (ppg, Fig. 77) are less
definitely developed at present than the other skeletal
elements; they are small sigmoid granular rods, in the
inner edge of the maxillopalatine processes, which extend
inwards to form a considerable part of the palatal roof.
They are continued forwards beneath the trabecular region,
approximating as they advance ; but they do not meet, so
that there is a primary cleft palate or median palatal
hollow. The posterior ends of the palatopterygoid bars
diverge outwards, instead of turning inwards like the
postoral arches.

613. The mandibular bars {mn. Fig. 77) are much
stouter and more perfect than the palatopterygoids ; they
occupy most of the lower jaw, but do not meet in the
middle line. Each bar is sigmoid, and strongly inhooked
proximally, where it extends towards the auditory sac. It
comes into close relation with the first viscaral or mandi-
bulo-hyoid cleft, which at its upper part is of considerable
extent owing to the thickness of the throat-wall. The
cleft becomes constricted at the point where the mandi-
bular bar is in contact with its anterior wall ; this is the
position of the future tympanic membrane.

614. The hyoid arch (Fig. 77, hy.) is much like the
mandibular ; but it is flatter, and the right and left bars
approach one another more closely below, at an obtuse
angle ; its apex also is more sharply inturned, making the

B. M. 18

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shoulder stand out like the tubercle of a rib. The apex
of the bar grows into relation with the mandibulo-hyoid
cleft behind, in a similar manner to the bar in front. The
main part of the facial nerve (7a) curves backwards round
the fore part of the auditory capsule, and behind the angle
of the hyoid bar. At some distance behind the hyoid, in
the lower part of the third visceral arch {th.h)y is a much
smaller cartilage on each side, attached to the fore part of
the sides of the larynx. This is homologous with the
branchial rods of lower vertebrates.

615. The primordial skeleton of this most highly-
specialised Mammal is as simple as that of the lowest Fish
we have examined; the head, with its intense mesocephalic
flexure, compares very well in shape with that of the
Skate or the Frog. The elements are more distinct from
one another than in the Fowl. There is everything to
show the persistency of morphological impress; the vis-
ceral clefts which appear, all of them transitory except
one, are more in number than the arches which contribute
anything to the adult skeleton. The embryo of the air-
breathing Pig gives evidence of its relationship with forms
breathing through water. The prenasal nodule and the
little cornua are further manifestations of unity with other
types, cunningly adapted and hidden in subsequent his-

616. But the Pig even at this stage shows evidence
of its high grade. The notable posterior clinoid ridge is
prepared for by the high upward curving of the para-
chordals, above the trabecular level. The early intemasal
fusion of the trabeculse and the development of the rudi-
ments of turbinal folds while as yet the sense capsules are
sub-equal in size, are indications of the future predomi-
nance of the olfactory organs. A distinctive palatopterygoid
tract exists in the maxillopalatine process of the mandibular
arch, but the palatopterygoid itself shows no appearance
of being a process from the mandibular cartilage. The
principal arches of the skull at their proximal extremity

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yin.] THE PIG : second stage. 275

already begin to be specialised ; the first visceral cleft is
becoming tortuous, and the apex of the mandibular arch
(future manubrium of the malleus), which is plainly com-
parable to the apex of the otic process of the quadrate iu
Birds, grows inwards into the cleft in a way not occurring
in the Fowl, so as to become involved in the tympanic

617. Here then in the last of our types we find the
same parachordal and trabecular elements in the cranial
floor and the same relations of the notochord as in pre-
vious cases ; with two principal arches, mandibular and
hyoid, and smaller rudiments, the palatopterygoid and first
branchial. All these parts are quite distinct at first. But
there are already two openings to each nasal cavity, and
these are approximately on the same level ; the "posterior
nares " are in the anterior part of the oral cavity, as in
Dipnoi and Amphibia. The roof of the mouth, or primary
palate, is the floor of the cranium. A free growth of car-
tilage, binding the various bars together, would produce a
very good parallel to the cartilaginous skull of Lepidosiren.

Second Stage : Embryos one inch long,

618. In this stage chondrification has fairly set in,
although the cells of the hyaline cartilage are still close to-
gether. Ossification has also commenced in fibrous tracts
related to the month. Differentiation of parts has gone
on very rapidly in the olfactory and auditory regions,
whilst the embryo has merely become longer by one half.
The mesocephalic flexure is partially lost, but the apex of
the notochord and the direction of the parachordals still
make a considerable angle with the trabecul«e.

619. The parachordals have grown round the noto-
chord to a large extent, but do not unite in front of it.
They are narrow in the interauditory region, and much
broader behind where they coalesce with the inner and
hinder edges of the periotic masses ; they extend backT


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wards considerably behind the latter, ending pointedly on
either side. The occipital ring of cartilage is developing.

620. The trabeculse have coalesced in their whole
extent except around the pituitairy space, which is itself
becoming floored with cartilage; and this cartilage unites
with the parachordal below the level of the clinoid ridge.
The cranial floor is being extended laterally by trabecular
outgrowths, after the manner in which the occipital ring
is being formed; thus alisphenoidal and orbitosphenoidal
laminse are arising.

621. The alinasal cartilage has become completely
chondrified up to the fore end of the nasal region, and the
primary moieties are more or less confluent with the
trabeculae and their cornua into a single cartilaginous
septum. This is thick and low in front, but much thinner
and more elevated behind, in the mesethmoid region. The
outer nasal wall is fully chondrified, and sends cartilaginous
processes into the various turbinal folds. There is no trace
of separate trabeculse in the base of the mesethmoid ;
that lamina merely gradually thickens towards its base,
and just outside the mucous membrane covering it below
is the internal nostril on either side. The base of the
mesethmoid tract is underlaid by an elongated granular
mass which is undergoing endostosis to form the azygous
vomer. A dense premaxUlary stroma is found under the
trabecular cornua in front, ready to ossify; while far on
either side of the ethmoidal region there is a faint beginning
of the maodllary, above and quite distinct from a rudi-
mentary tooth-pulp. The compressed granular palato-
pterygoid rod is considerably mesiad of this tooth-pulp,
bounding the median cleft of the palate.

622. In the ear-sac the semicircular canals and the
coils of the cochlea have now become diflferentiated. The
median lobular protuberance of the outer wall of the
capsule seen in the first stage is separate to a great extent
from the rest of the auditory wall, but remains as a plug,

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thus forming the stapes, lying in the fenestra ovalis. It
has two papular elevations on its external surface, and is
covered externally by tissue commencing to chondrify.

623. By the thickening of the side-wall of the throat
the first visceral cleft is so much lengthened that it may now
be called tympano-eustachian canal ; its Eustachian part
is directed forwards into the mouth ; its tympanic portion
is becoming closed by the formation of the tympanic
membrane as the upper end of the mandibular arch grows
inwards towards the ear-capsule. An external tube out-
side the tympanic chamber is being formed by the growth
of an opercular skin-fold round the primary orifice of the
cleft; cartilage soon arises in this fold. The external
edge of the periotic mass is produced as a pterotic ridge
or tegmen tympani, covering and partially surrounding the
upper ends of both the mandibular and the hyoid arches.

624. The mandibular bar becomes more and more
incurved at its proximal extremity, so that it is very
hook-shaped (see Figs. 80, 81, representing a later stage).
It has a small bulbous end, and a thickened portion at
the bend or shoulder, which articulates with the upper
part of the next bar. The incurved apex becomes the
manubrium, or handle of the malleus, and the remainder
is the meckelian cartilage. It is the ingrowth of the
manubrium into and over the tympanic canal that, carry-
ing the skin with it, gives rise to the tympanic membrane.
The head of the malleus further grows out into a boss at
its junction with the main bar, for the attachment of the
tensor tympani muscle.

625. The proximal region of the hyoid bar is more
diflferentiated than the preceding arch, and also undergoes
distinct segmentation. The incurved and backwardly-
directed hook has been cut off from the shaft, and
becomes shaped into the incus with its processes; while
the remaining bar shifts its position backwards along the
incus ; its proximal end becomes somewhat two-headed, to
articulate extearnally with the tegmen tympani, and to



unite internally with the ear-sac close in front of the exit
of the facial nerve.

626. It was the tubercular part or shoulder of the un-
segmented arch which came into contact with the man-
dibular arch growing backwards and forming the tympanic
membrane : while the apex or head was bent inwards and
backwards towards the ear-sac. Consequently, when seg-
mentation takes place, the tubercular part of the hyoid is
left as the front and outer end of the incus, articulating
with the head of the malleus (Fig. 81). A small boss arises
on the outer edge of the incus, to form the short crus; the
part of the incus growing towards the ear-sac remains as
the long crus, and its small rounded head is the orbicular
process, coming into close contact with the stapes.

627. The distal part of the mandibular arch extends
forwards in the lower jaw ; and half encircling it on the
puter side is a mass of granular tissue or nascent cartilage,
reniform in section, in the axis of which the dentary bone
is being formed. The remainder of the hyoid arch (which
may be called stylo- and ceratohyal) extends downwards
to the fore part of the larjrnx, and is fastened to a median
cartilage, which is the first basibranchial (basihyal of
human anatomy) ; this lies between a pair of small carti-
lages which represent the first branchial arch.

628. In this minute embryo some of the features
most characteristic of the adult as belonging to the mam-
malian group are already distinguishable. Chondrification
has advanced greatly, and is rapidly completing its work.
The notochord is still visible, but the basilar plate is well
formed, though incompletely united as yet with the
auditory capsules; and the exoccipital region is arising.
The trabeculae have fused together almost completely,
but are not yet united to the basilar plate ; lateral carti-
lages, however, are growing in the ali- and orbitosphe-
noidal regions. The cochlea and semicircular canals are
well formed. The nasal labyrinth has increased in size
and completeness ; and a median septum is found instead

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yin.] THE PIG : THIRD STAGE. 279

of two distinct inner nasal walls. The trabecular comua
appear as recurrent cartilages.

629. Thepalate is still distinctivdy cleft; there are rudi-
ments of the vomer and the maxiilaries. The palatoptery-
goid bar does not completely chondrify, but has already
begun to be ossified. As a contrast to this, the stroma
of the dentary (vary distinct from the meckelian cartilage)
chondrifies very largely before it ossifies. The outer ear
is alec becoming cartilaginous, while the proximal parts
of the two principal arches are rapidly assuming their
adult relations. The mandibular apex becomes shaped
into the parts of the malleus, without any segmentation
taking place ; while the hyoid, becoming bent so as to
articulate with the malleus, gives rise to the incus, which
is also in relation to the commencing stapes formed out of
the tissue of the ear-capsule. The incus being segmented
from it, the main hyoid arch is withdrawn backwards,
approaching the stapes and becoming connected with it
and the ear-capsule by an interhyal ligament.

Third Stage : Embryos an inch and a third long,

630. There is now continuous cartilage occupying
the whole median basicranial and facial regions; the
lateral cranial walls are partially chondrified, the occipital
rin^ being complete. The parachordals have united in a
basilar plate, entirely enclosing the diminished notochord
(Fig. 80). The anteri4)r end of the basilar plate forms
the hinder boundary of the pituitary space (posterior
clinoid wall, p. cl. Fig. 79) ; but does not rise so high
relatively, nor form so great an upward curve as did the
parachordals in the first stage. No remnant of the noto-
chord is found in this posterior clinoid wall — a sharp
contrast to the first stage, when the notochord projected
beyond the parachordals,

631. The basilar plate {b. o.) is narrow between the
ear-capsules, which are continuous with it; internally

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and especially posteriorly they are on the same level
with it, but externally they are elevated so as to con-
stitute the lower half of the lateral cranial wall. Behind
tlie auditory masses the basilar plate widens, and curves
gradually upwards on both sides to form the occipital
arch. The supraoccipital region {s.o.) rests on and is
continuous with the hinder half of the ear-capsules. The
foramen magnum (/. m.) is large and transversely ellip-
tical, with a supero-median notch : the crescentic condyles
are external to the foramen, extending towards the
middle line below.

632. The foramen for the vagus and glossopharyngeal
nerves (Fig. 79, 8) is immediately behind the ear-mass
internally ; the hypoglossal foramen is somewhat behind
this. The auditory cartilage, protruding into the cranial
cavity, presents several depressions and openings. There
is a depression beneath the arch of the anterior canal
to receive the cerebellar process (flocculus) ; antero-
inferiorly the facial and auditory nerves perforate the
cartilage, close together, but separated by a distinct bridge.
The condition of the auditory cavity will be described

633. In front of the basilar plate, the floor of the
cranial cavity presents a deep pituitary cup'(joy.), floored
with cartilage ; a slightly elevated anterior clinoid wall,
with a depression in front of it on which the optic
chiasma lies (presphenoidal region, p. s.) : and a more
elevated ethmoidal region, the lev^l of which is approxi-
mately the same as that of the basilar plate. The
ethmoidal cartilage projects slightly upwards at the
extreme fore part of the brain case, forming a rudimentary
anterior walL

634. The lateral portions of the cranial floor and
the lowest regions of its side walls contain cartilaginous
orbitosphenoidal and alisphenoidal growths, continuous
with the median floor. The alisphenoidal cartilage (oZ. s.)
is short and thick, and is attached to the anterior and

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posterior clinoid tracts. Its most noteworthy part ex-
tends downwards to the palatopterygoid bar, to form the
external pterygoid cartilage. Between the alisphenoid

Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 23 of 31)