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The morphology of the skull online

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The ascending laminae of the palatines curve round the
posterior nares and ascend to the base of the nasal laby-
rinth in its hinder region. The pterygoids are quite smalL
The ossification of the lower jaw. is almost complete ; the
coronoid process is narrow and curved backwards ; the
condyle is still cartilaginous.

Sixih Stage: Kew-horn Pigs,

662. Since the last stage the head has almost doubled
in length, and the process of ossification has gone on very
rapidly : the form of the skull has become much more
specialized. The whole of the occipital and sphenoidal
cartilages are ossified. The spheno-occipital synchondrosis
is of small extent, and a scarcely thicker tract of cartilage
remains between the basi- and presphenoid.

663. The perpendicular ethmoid and nasal septum
are still unossified ; but the inferior turbinals are almost
completely, and the middle and upper turbinals (so-called
lateral masses of the ethmoid) are partially converted into
endosteal bone. The cribriform plate is soft, and so is the
snout ; but this latter is everywhere burrowed with vessels
prior to ossification. The external nostrils are here in-
ferior, the alinasal cartilage being continued forwards and

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downwards, and having coalesced with the recui^ved tra-
becular horns and their continuation backwards along the
lower edge of the septum. The external nostril is bounded
anteriorly and partially externally by the alinasal cartilage,
externally by a cartilage of the ** labial" category, distin-
guished as appendix alse nasi, and within and behind
by the trabecular comiu The prenasal cartilage is now

664. The bony palate is now most compact, and its
postero-extemal angles are continued by the pterygoids
and the external pterygoid plates. The pterygoid is fixed
posteriorly to the solid nut -like tympanic, indenting it
— a, temporary state of things only. The lower jaw is well
ossified, while the adjacent meckelian bar has degenerated
into a band of fibrous tissue.

665. The three periotic centres have completely
ossified the capsular cartilage, and coalesced with one
another. The exoccipital (Fig. 85, e.o,) and its parocci-
pital process (p.oc.) are apposed to the periotic bone
behind, the process being separated from the stylohyal bar
(ath.) by the facial nerve (7a), which lies in the styloid
canal, and gives off at its upper part its anterior branch
(the chorda tympani, 7a) running upwards and forwards
to curve over the tympanic membrane and enter the
Glaserian fissure.

The processus gracilis of the malleus (the con-
tinuation of the primary mandibular arch) is reduced to a
style ending in fibrous tissue (Fig. 86, pgr.) ; the manu-
brium (m6.) is flat and slightly arcuate ; the head (ml.)
articulates with the incus by a synovial joint, the minia-
ture of the tibio-astragalar joint in the same animal. The
head of the malleus sends inwards a rounded process
(i.p.m.), and the manubrium at its base has a posteiior
process to which is attached the tensor tympani muscle.

667. The hollow of the tegmen tympani (tty,^ has
the head of the incus in its hinder recess, which pos-

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teriorly has a round cup-like facet for the short cms of the
incus (; the head is also partially roofed hy the
ingrowing squamosal. The hooked long cms of the incus
{Led.) is tipped by a distinct orbicular ossification {p.o.)

Fig. 86.

Fig. 86. The Pig at birth ; outer view of auditory capsule, &c., the squa-
mosal and tympanic having been removed.

0.6. OS bullae; pr. promontory; f,ov. fenestra ovalis; st. stapes;
oob, OS orbiculare; pr,o. prootic ; t.ty, tegmen tympani; Let. long crus
of incus; h.8C. p.8C. semicircular canals; short crus of incus ;
sq. a small piece of the squamosal in the hinder part of the tympanum ;
e.o. exoccipital ; oe.c, occipital condyle ; ep, epiotic ; s.m.f. stylo-mastoid
foiamen; 7a, facial nerve; 7a\ chorda tympani branch; /.r. fenestra
rotunda; p.oc, end of paroccipital process; st.h, stylohyal; ch, cerato-
hyal; b.h. basihyal; tKh. 'Hhyrohyal."

Fig. 86. Auditory chain of bones.

ml. body of malleus ; mb. manubrium ; processus gracilis; i.p.m,
internal process ; i. incus ; 8.c.i. short crus ; l.e.i. long crus ; i,hy, inter-
hyal; o.o. os orbiculare; st.m, stapedius muscle; st, stapes.

applied to the cup on the summit of the arch of the
stapes. The base of the latter fits into the fenestra ovalis.

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1 Till.]


t The little nucleus of cartilage which was early formed ia
I the interhyal ligament (i-hy.) is now attached by its base
I below the head of the stapes, while its pointed distal end
\ is buried in the fibres of the stapedial muscle {stm),

668. In the head of the stylohyal cartilage, confluent
%elow and behind the preceding parts with the auditory
capsule, a bony centre has appeared, forming the tympano-
hyal. The middle of the bar, outside the skull, ossifies as
the stylohyal. The distal end of the bar is tied by ligament
to the small hyoid comu (c.h.) ; the larger comua {thJi,),
belonging to the first branchial arch, are ossified.

Seventh Stctge: The Skull of a Pig six m(mths old.

669. In this stage the greater number of the sutural
landmarks, largely obliterated in the adult, are still in
existence. The long angular skull is an irregular pyramid,
with two equal and two unequal sides and an oblique base.
A complete contrast in outward form to the human skull,
that of the Pig is the straightest of all the types; it is
very strongly built, but its bone-tissue is inferior in
density to that of the Sheep, being iutermediate in this
respect between the bone of a Ruminant and that of a
detacean. The flat top of the skull, with its orbits flush
with the surface, indicates the semi-aquatic habits of its
owner ; and the depth and squareness of the base of the
pyramid is correlative with the high neck and strong

670. The long straight nasals overlap the snout in
front, and are articulated by suture along their outer
margin with the upper edge of the long premaxillaries,
and for a less extent with the maxillaries j they terminate
in a transverse line of suture with the frontals. The
latter together form a somewhat pentagonal plate, divided
along the middle line by the sagittal suture. The anterior
third is deeply grooved, the grooves issuing from the

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298 Morphology of TfiE skuLl. [cha^.

supraorbital foramina; the posterior half of the outer
margin of the bones bears the thick and somewhat promi-
nent supraorbital ridge. The frontals have a large orbital
plate, bounded behind and above by the short postorbital
process, and lower down and within by the orbitosphenoid.
The upper surface of the parietals is narrow, and divided
by the continuation of the sagittal suture ; laterally they
are much compressed, forming the inner wall of the large
temporal fossa ; behind, the upper part of the high supra-
occipital abuts against them.

671. ' The premaxillaries hav6 a large facial and a
lesser palatal region, the palatal spurs being slender and
compressed, lying together in the middle line, with an
anterior palatine foramen on either side. The huge
maxillary forms most of the side of the face and the
anterior root of the zygoma ; and the part containing the
hindermost tooth-socket is bound to the external -ptery*
goid plate and the descending part of the palatine. The
horizontal plates of the maxillaries form three-fourths of
the grooved and ribbed hard palate. The palatine bones
complete the palate, being much longer at the middle line
than externally. Altogether, the median suture of the
hard palate is two-thirds of the length of the skull.

672. The inner ascending plate of the palatine articu-
lates with the vomer, and sends forward a long scoop-like
process beneath the lateral ethmoidal masses. Externally
and behind they form a thick boss, which articulates with
the maxillary and the external pterygoid process on the
outside, and with the proper pterygoid on the inside. The
latter bone has a very thin ascending part, and an upper
squamous plate underlying the presphenoid — a separate
piece (mesopterygoid) in some examples.

673. The thin dentate posterior end of the vomer
extends to the same transverse line as the mesopterygoid
plate ; in front it reaches almost to the fore end of the
premaxillaries. Its lower edge lies, behind, on the up-

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turned edge of the palatal plate of the palatine bones, and
anteriorly to this, on the long harmony-suture of the
palatal plates of the maxillaries. Its upper edge is grooved
by the nasal septum. The complex nasal labyrinth is
well ossified, but the septum, between the inferior turbi-
nals, is still soft. The common trabecular and alinasal
cartilage projects forward beyond the bony structures of
the skull, and in the adult becomes ossified endosteally as
the median snout-bone.

674. The lachrymal is nearly equally developed with-
in and without the orbit ; it articulates with the frontal
above and behind, with the maxillary in front, and with
the jugal below. It has both an upper and a lower canal,
immediately in front of the rim of the orbit. The jugal
or malar bounds the lower and anterior portion of the
orbit : it is massive in front, while its hinder half shelves
away under the zygomatic process of the squamosal. About
an inch of space intervenes between the highest part of
the jugal and the postorbital spur of the frontal.

675. The squamosal by its squamous lamina formii
the lower posterior boundary of the side of the skull,
overlapping the hinder edge of the frontal and the lower
part of the parietal. It is turned outwards at its middle
region below, to form the transversely* extended glenoid
cavity for the mandibular condyle. The zygomatic pro-
cess passes forwards from the outer side of this articular
surface to rest upon the jugal. The line of the zygoma is
continued backwards and upwards by an acute ridge rising
towards the upper part of the supraoccipital, bounding the
deep temporal fossa. The squamosal also coalesces with
the upturned mouth of the external auditory meatus, and
thence gives two processes, the post- tympanic, fixed on the
side of the tympanic bone, and in front of this, a post-
glenoid process. In front of the glenoid facet the squa-
mosal is strongly sutured to the alisphenoid and to its
external pterygoid plate.

676. The main part of the tympanic bone is somewhat

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like a large filbert, ridged below ; it principally consists of
a mass of square-chambered diploe. It has a small cavity,
and across its mouth the tympanic membrane is stretched ;
from its outer angle behind, a tubular meatus has grown
upwards and backwards. There is a large foramen lacerum
for nerves and vessels along the inner and hinder edge of
the tympanic bone, between it and the basisphenoid and
basioccipital. Part of the periotic mass is visible through
this foramen ; it is very largely unanchylosed to the sur-
rounding parts. The tympanic abuts on the alisphenoid
in front, the squamosal externally, and the exoccipital
behind. The post-tympanic spur of the squamosal on the
outside, the tympanic bone on the inside, and the paroc-
cipital process behind, encircle a canal through which the
styloid cartilage and the facial nerve descend, and in which
the tympanohyal bone is impacted.

677. The great occipital plane is scooped above,
beneath a strong transverse backwardly-tumed ridge.
The supraoccipital sends a wedge forwards between the
parietals, expands laterally above, and descends so as to
occupy a small part of the margin of the foramen magnum.
The exoccipitals spread widely outwards, meeting the
hinder edge of the squamosals and their post-tympanic
processes, and lying behind the epiotic or " mastoid "
portions of the periotic bones. From this region the
exoccipitals run downwards to form the paroccipital pro-
cesses ; while their middle region projects and constitutes
the diverging semioval occipital condyles. Within the
paroccipital process, on the base of the skull, is a con-
siderable foramen for the hypoglossal nerve.

678. The basioccipital is a pentagonal bone joining
the exoccipitals by suture, and separated from the basi-
sphenoid by a narrow synchondrosis. The basisphenoid,
half the length of the basioccipital, is continuous with the
ascending alisphenoid s. A narrow cartilaginous tract
separates the basi- from the presphenoid, which is hidden
below by the vomer. The large orbitosphenoids appear

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in the posterior part of the orbit around and above the
optic foiumen. No change of importance has taken place
in the auditory ossicles : the preceding description (p. 295)
is sufficient.

679. The mandibular rami are quite distinct from
each other. They are large deep bones, with a small
notch between the coronoid and articular regions, the
coronoid process scarcely rising higher than the condyle.

680. In the three preceding stages the enlargement of
the face as compared with the brain-case, and the develop-
ment of the tracts which are specially strengthened for
muscular attachment, have given the skull approximately
its adult conformation. We may note that the ossifications
of the auditory capsule were found in the fifth stage, as
well as the small os bullae ; the turbinals, the os orbiculare,
the tympano- and stylohyals. and the hyoid cornua in the
sixth ; while it is only in the latest stage that the mes-
ethmoid and cribriform plates are ossified, and the tubular
part forming the meatus is added to the tympanic. Further
description of the adult skull of the Pig may be found in
Prof. Flower's Osteology of the Mammalia,


68L The Mammalian skull presents great uniformity in its broad
mori hological features, and the extraordinary varieties of external
coutoiu* which are found depend mainly upon the relative size and
prominence of bones, their distinctness or anchylosis, or their over-
lapping one another. Remarkable cases of asymmetrical skulls occur
among the Cetacea. The most important forms of the Mammalian
skull are admirably elucidated in Profl Flower's Osteology of the
Mammalia^ in Pro£ Huxley's Lectures on Comparative Anatomy ^
and his Anatomy qf Vertdrrated Animals, We are yet in great
ignorance of the history of the skull in most Mammdian groups;
conseqnently only a few facts of importance in general morphology
can be noticed here.

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682. Reptilian affinities in the skulls of Echidna and Omitho*
rhynchtM are found in the absence of an ascending ramus in the
mandible, in the slight bending of the cochlea, in the stapes being im-
perforate and columelliform, in the malleus being very large and the
incus small. In the Marsupials many of the cranial sutures persist
throughout life; the squamosal, the periotic mass, and the tympanic
remain separate, not uniting to form a " temporal" bone. Very fre-
quently the jugal extends far backwards and furnishes part of the
articular surface for the mandible. The internal carotid arteries
pierce the basisphenold and enter the bottom of the pituitary fossa,
as in Birds, instead of coming in at the sides of the basisphenoid,
There is an anterior tympanic recess comparable to that of Birds
(called alisphenoid bulla). The palatal plates of the palatines have
large deficiencies in Marsupials, and in the Hedgehog. Palatal plates
of the pterygoid exist in Myrmecopha^ga, in Manis, and many Ce-
tacm. The pterygoid is hollowed by cells in Cetacea. In Manis
several bones of the skull have air-passages, and by this means the
two tympanic cavities are in communication with one another.

683. In Ruminants the external pterygoid process comes dis*
tinctly from the basisphenoid. The whole sphenoid mass is developed
from a pair of alisphenoid and a pair of orbitosphenoid centres; the
basi- and presphenoid centres do not appear. The tympanohyal has
two centres in the Sheep. In Rhinoceros the squamosal sends down
a large postglenoid process, which unites with the post-tympanic
process of the same bone to form a sort of external auditory meatus,
the tympanic bone not possessing a tubular part. The *^ mastoid"
portion of the periotic bone is hidden by the combined post-tympanic
process of the squamosal and paroccipital process of the exoc-
cipital. The horns of Ungidata are well known and need not be
described here. In Sirenia the frontals are prolonged into broad
supraorbital processes, and the nasals are abortive. The anchylosed
tympano-periotic ma^s can be readily separated from the rest of th^

684. In Cetacea the supraoccipital and interparietal separate
the small parietals and unite with the frontals, each of which has a
great supraorbital plate. The very large maxillary extends far
backwards and outwards, often largely overlapping the frontal, as
well as far forwards, bounding nearly the whole ot the gape. The
premaxillaries have very long nasal processes extending to the an-

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tenor nostrils, which are superior in position. The nasal passages
are nearly vertical, the nasals and turbinals small or rudimentary.
The periotic mass is but loosely connected with the squamosal and
tympanic, and is reiidily isolated. The lower jaw has no ascending
ramus, and the condyle is posterior. The hyoid is broad and well
ossified, with two pairs of bony comua. In the Carnicora there is
a remarkable tympanic bulla partly due to the periotic cartilage
and partly to an extension of the tympanic bone. The periotic
portion is traversed by the internal carotid artery. For the struc-
ture and variations of the bulla see Prof. Flower's Osteology and his
important paper in Proc, Zool, Soc, 1869.

685. In Rodentia the presphenoid is mostly very distinct as a
median bone ; it is seldom so in the other Mammalian groups, the
median tract being ossified by the orbitosphenoids. There is fre-
quently a large distinct posterior ossification of the alisphenoid.
In the Rabbit the presphenoid is high and much compressed, so as
to form an iuterorbital septum; and the optic foramina run into
one, as in some Seals. The tympanic and periotic bones are anchy-
losed together but not with other bonnes. The pituitary fossa is
permanently uufloored by bone, so that there is a perforation in the
middle line in the adult skull. The premaxillary is very large, with
great palatal plates, and is much perforated. In the Guinea-pig {Cavia
aperea), the pterygoids do not reach the base of the cranium, but are
fixed on basipterygoid processes like those of Ostriches, which are
really homologous with the external pterygoid plates of the Pig.
From the sides of the basis cranii above and behind these basiptery*
gold processes a long continuous bony lingula passes backwards on
either side, extending to tlie front of the auditory bulla ; at the end
of each is a delicate sigmoid bone, separate at first, anchylosed
afterwards. The first answers to the pretympanic wing 'of the Bird,
the second to the basitemporal In old specimens the vomer is still
found in two separate moieties. These facts indicate the high anti-
quity of this type. The vomer appears on the palate in the Cat and
the Pangolin. In the Capybara and the Guinea-pig the alveolar
border of the maxillary is prolonged far backwards beneath the
orbit so as to unite with the squamosal at a level with the anterior
border of the glenoid fossa. In some Rodents, as Dipus^ Chinchilla^
there is in addition to the tympanic bulla another expansion above
the tympanic cavity, forming a rounded prominence on the postero«

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external angle of the skulL In the Beaver it forms a definite angular

686. The Hedgehog has large pretympanic wings whose posterior
mai-gin nearly reaches the foramen magnum. In Moles and Shrews
much of the skull is primarily cartilaginous: a large periotic cre&t
projects from the upper edge of the ear-capsule, distinct from the
supraoccipital, and becomes ossified, occupyiug a large part of the
cranial wall The squamosal is partly outside it, also appearing in
front and below.

The Human Skull.

687. The circumstances which contribute most to modify the
form of the human skull and the condition of its component bones,
as compared with that of animals, are — 1st, the proportionally large
size of the brain and the corresponding expansion of the cranial
bones which enclose it; 2nd, the smaller development of the face as
a whole, and especially of the jaws, which brings the facial bones
almost entirely under the fore part of the brain-case, instead of in
front of it, as occurs in all animals, with the partial exception of the
anthropoid apes ; and 3rd, the adaptation of the human skeleton to
the erect posture, which, as regards the head, is attended with the
sudden bend of the basicranial axis at a considerable angle upun the
line of the erect vertebral column ; and along with this the advance
of the occipito-vertebral articulation to such an extent as to make
the head nearly balanced on the upper extremity of the spine. The
downward opening of the nostrils, the forward aspect of the orbits
and eyes, the nearly vertical forehead, and more or less oval-shaped
face, are accompaniments of these human peculiarities in the form of
the head, which, together with those already mentioned, strongly
contrast with the smaller cranium and its strong crests of bone, tne
larger projecting face and jaws, and the other characteristic features
of the skull in most animals^.

688. A general acquaintance with the topography of the human
skull will be assumed, in order that the references to its structure

1 This paragraph is quoted from Quain and Sharpey*s Anatcmj^ 8th
edition, Vol. i. p. 73 ; this work also contains much of the inlormation
^ven below, but diif erently arranged. See also Huxley and Flower in
the works before mentioned.

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and development may be abbreviated. There is on the whole a more
complete consolidation or anchylosis of the osseous elements than in
most other mammals. Features of contrast between man and many
lower forms are the imion of the frontals and the moieties of the
mandible; of the premaxillary with the maxillary; the distinctness
of the presphenoid; the fusion of the interparietal with the supra-
occipital; the large proportionate size of the squamosal and its
union with the periotic mass; the comparatively small size of the
tympanic, and the absence of an auditory bulLi; the occurrence of
fewer and less extensive elements in the hyold arch.

689. BriejSy catalogued, the following are the elements distinctly
developed in the human skull, in comparison with the bones as named
in works on human anatomy :

Occipital =ba8ioccipital, exoccipitals, supraoccipital, interparietal.

Sphenoid = basisphenoid (including sella turcica), alisphenoids
with. external pterygoid plates, pterygoids, presphenoid, orbitosphe-

Ethmoid = mesethmoid, ectethmoids with proper superior and
inferior turbinals. Inferior turbinals or maxillo-tm*binals ; vomer;
sphenoidal turbinal.

Farietals, frontals, nasals, lachrymals; premaxillaries, maxillaries,
palatines; jugals (malars).

Temporal, composed of several principal parts : — squamosal (in-
cluding zygomatic process), tympanic (iucluding tympanic ring, ex-
ternal auditory meatus, and postglenoid process), and petromastoid
(including prootic, epiotic, opisthotic) ; tympanohyal, stylohyal.

Malleus, incus, os orbiculare, stapes.

Inferior maxillary =dentary, coronary, splenial, mento-meckelian.

Hyoid = basibranchial (body), ceratohyals (lesSer cornua), first
branchials (greater cornua).

690. The occipital bone is almost horizontally placed, and the
condyles are not far from the middle of the floor of the cranium. The
supraoccipital originates by four centres, in pairs above and below :
these speedily unite, but four fissures long remam between them.
The upper pair, developed in membrane, represent the interparietal
There are in early life anterior and posterior fontanelles in the
middle line, at the anterior and posterior angles of the parietals ;
there is also a fontanelle at the postero-mferior angle of each parietal.

Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 25 of 31)