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also dense, but instead of being welded together, they are



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204 MORPHOLOGY OF THE SfeULL. [CHAP.

for the most part loosely connected with one another,
as well as with the brain-case, by elastic ligaments
constituting the most mobile facial apparatus to be found
among Vertebrates. The axial parts are condensed into
a comparatively small space, while the jaws even in their
most contracted state are widely extended on either
side of and behind the rest of the skull. The brain rests
upon the almost flat cranial floor, without being elevated
upon an interorbital septum. The hyoid apparatus is as
insignificant as in any vertebrate type, while branchial
arches are non-existent

478. The cranial box is of tolerably even breadth,
for the ear-masses do not project laterally as in other
types, but are rather elongated. The occipital ring is
much flattened, and is continuous to a great extent with
the auditory capsule. The basioccipital is a broadly
heart-shaped plate (6.0. Fig. 52), its rounded posterior
apex constituting the median transversely-oval occipital
condyle. A considerable part of each exoccipital (e.o.)
appears in the cranial floor, and an equal portion in the
cranial roof; and the postero-superior edge of each over-
laps the corresponding part of the atlas. The exoccipitals
meet above (Fig. 53), and are separated by the condyle
below : each has coalesced with the contiguous opisthotic.
The supraoccipital is now apparently larger than before,
for it has annexed to itself the two epiotics.

479. The largest of the periotic bones, the prootic
(Fig. 51, pr.o.), is apparently quite distinct; but it has
coalesced by its outer face in front with the little four-
sided perforate alisphenoid. Internally the prootic rests
equally upon the antero-external edge of the basioccipital
and the outer edge of the basisphenoid. Anteriorly it is
in contact with the thick hinder edge of the lateral plate
of the parietal, with its intumed thick vertical crest
Superiorly at its fore part it adjoins the postero-lateral
edge of the parietal, and the epiotic behind. Here, as
in Reptiles generally, a Y-shaped suture is persistent
between the three periotic bones, which never unite with



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VI.] THE ADULT SNAKE. 205

each other, but always with some adjacent bone. fWhen
there is no alisphenoid, the prootic remains distinct from
any other element.) The nerve foramina do not need
further description beyond that given in the last stage.

480. The large irregularly four-sided basisphenoid
has coalesced with the parasphenoid, to form a very
curiously-shaped bone (Fig. 52). There is a posterior
chnoid wall arching over the hinder part of the pituitary
body. The latter rests upon a bony floor formed by the
backward growth of the parasphenoid and by a concomitant
growth of bone (basisphenoidal) from the whole margin
of the intertrabecular space. Thus a sort of subcranial
hollow is formed, which besides being partially roofed by
the posterior clinoid wall, is covered at either side by
a shelf passing inwards from the base of the parietal, on
which the brain very largely rests. The subcranial hollow
contains the pituitary body, a quantity of fibrous tissue,
and the internal carotid arteries, which pass into it laterally
beneath the parietal shelf, having previously perforated the
basisphenoid.

481. The basisphenoidal ossification proceeds forwards
on either side of the intertrabecular space to a point just
under the very large optic foramen. Anteriorly to this point
the trabeculae are entirely unossified, and lie in a pair of
almost closed channels, on either side of the parasphenoid.
The latter has a rather broad base and a crested summit
which is wedged in between the frontal bones. Anteriorly
to the brain-case the parasphenoid becomes compressed
and knife-like, wedging in between the hinder ends of the
vomers.

482. The two parietals have completely anchylosed
into one, with no crest along the line of union. The bone
is somewhat pentagonal above, partly overlapping the
supraoccipital behind, and the epiotics laterally. Just in
front of its projecting postorbital angle the parietal is
overlaid by a small oblong but slightly curved postorbital
membrane bone. The front edge of the parietal recedes
at the mid-line, and is slightly overlapped by the frontals.



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206 MORPlBrOLOGY or THE SKULL, [CHAP,

483. The side-wall of the cranial cavity is provided
by three bones, prootic, parietal, and frontal, the larger

Fig, 51.



Adult Snake ; side view of skull^ with jaws removed.
' tr, trabecula ; aM, anterior nostril ; II. optic foramen.

Bones: ho, basioccipital ; e.o, exoccipital; 8,o, supraoccipital ; pr.o.
prootic; &.«. basisphenoid; a2.». aUsphenoid ; pa. parietal; pa', lateral
plate of parietal; /r. frontal; fr', lateral plate of frontal; o.8. orbito-
sphenoid; pa,8, parasphenoid ; vo. vomer ; rui, nasal ; prnx, premaxillary ;
L septo-maxillary ; a.<yr, antorbital or prefrontal; p.or, postorbital or
post^ontal ; coL columella,

third being aflForded by the parietal (Fig. 51). The
alisphenoid does not appear in the cranial wall at all,
being thrust out by the parietal and prootic. The inner
face of the parietal wall is deeply scooped for lodging the
optic lobes ; and the external face is correspondingly
convex. The hinder edge of the hollow is very thick and
almost vertical, projecting inwards as far as the prootic
does ; from the lower edge of the bone on either side the
shelf over the subcranial cavity proceeds inwards.

484. The optic foramen is much larger than the optic
nerve which passes through its centre ; its boundaries are,
behind, the antero-inferior part of the parietal wall; in
front and below, the pafasphenoid ; above, a fan-shaped
fenestra between the anterior edge of the parietal and the
posterior edge of the frontal wall. The anterior part or
handle of this space is occupied by a very small trilobate
bone, the orbitosphenoid.

485. The frontals are very solid bones, not anchylosed
to one another, but coapplied so as to form a single small
tubular cavity. The inferior meeting-place is in the cranial



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VI.] THE ADULT SNAKE. 207

floor abovie the parasphenoid ; supero-extemally they
expand to form a supraorbital ridge over each orbit. In the
extreme anterior part of the cranium a median wall grows
downwards from each frontal and separates the fore part of
the cerebral hemispheres, which are somewhat divergent
in front; but even these partitions, though in contact, are
unanchylosed to one another. The frontal floor is here
more expanded than behind; while at the same time the
outer wall is deficient infero-laterally, the space being
filled in by fibrous tissue, supported by an ingrowth of
the prefrontal. This membrane-bone (a. or.) lies at the
side of the fore end of the frontal, and bounds the orbit
anteriorly; its upper angle wedging in between the frontal
and the nasal.

486. The trabeculse have united underneath the fore
part of the frontals, and become compressed into a vertical
ethmoidal plate passing on into the nasal septum. The
hinder part of the nasal capsules are also wedged in beneath
the fore part of the frontals. The nasal septum is. highest
behind, but nowhere very high, for the nasal roofs con-
tinuous with it diverge very gently at first and are highly
arched. The nasal bones {na.) lie like shells upon the
cartilages, dipping down between the two almost to the
septum. Externally they narrow as they descend towards
the maxillaries: in front of these they are concave for the
passage of the external nostrils. Anteriorly to the nasals
the walls of the capsules dip gradually and are con-
fluent, the trabecular comu forming a bulging front-wall,
resting on the premaxillary, and scooped by the external
nostril. A very small spur of cartilage (prenasal) grows
backwards between and underneath the extreme fore end
of the capsules,

487. In the fore part of the nasal floor, on either side
of the vomers, is a small tongue of cartilage, which may be
connected on one side with a trabecular cornu, and which
passes more directly outwards behind towards the outer
process of the vomer. At the angle where the outward
divergence takes place is another small piece of cartilage,



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208 MORPHOLOGY OF THE SKULL. [CHAP.

which converges a little towards its fellow of the opposite
side, and lies in a rounded notch of the vomer. These are
two upper labial cartilages.

Fig. 52.



Adult Snake ; under view of sknll.

ox, occipital condyle ; n.g, nasal gland.

Bones : 6.0. basioccipital ; e.o, exoocipital ; pr.o, prootic ; b,s, basi-
sphenoid ; pa,8. parasphenoid 5 vo. vomer ; pmx. premaxillary ; mx.
■maxillary ; pL palatine ; pt. pterygoid ; tr, transpalatme ; qu. quadrate ;
st.hy. small stylohyal on quadrate; art. articular; ang. angular: s.ang,
Borangular ; cor, coronoid ; 8pl, splenial ; d, dentary.

488. The greater part of each nasal floor is occupied
by the bone called septo-maxillary, which is vertically
extended along the low nasal septum, and after under-
lying the cavity extends upwards for some distance in the
outer nasal wall towards the nasal. The transverse plate of
the septo-maxillary which is a floor to the nasal cavity is a
roof to the large reniform nasal gland, which itself is bedded
in a hollow of the vomer. The latter bone (vo,) has a
longitudinal plate applied to its fellow beneath the nasal
septum, and a transverse wing which is cupp<Bd to receive
the nasal gland, and notched antero-laterally to give exit



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VI.] THE ADULT SNAKE. 209

to its duct. The inner (posterior) labial applies itself
closely to this notch and extends inwards in the hilus of
the gland, expanding upon its duct.

489. The azygous premaxillary (pmx,) is a small
triradiate edentulous bone, with a short median nasal
process above, and a longer double palatine process below.
On either side it reaches the maxillary, which is a rather
long slightly-curved dentigerous rod, thicker in front and
flatter behind. It is attached by fibrous tissue at its
anterior end to the premaxillary, at its anterior third (inter-
nally) to the descending edge of the prefrontal, and behind
passes under and is supported by the broad transpalatine.
It occupies less than half of the gape, the rest of which
in the upper jaw is margined merely by fibrous tissue,
there being neither jugal nor quadratojugal.

490. The palatine (pi) is a small dentigerous rod of
bone not half as long or wide as the pterygoid : in front of
the middle it sends inwards a email plate beneath the
posterior nasal passage. Its anterior third lies beneath
the prefrontal: behind this it diverges gently outwards,
not lying at all under the brain-case, and is attached to
the outer side of the fore end of the pterygoid. The latter
(pL) is a large somewhat falcate bar, thick in the middle,
and nearly as long as the distance from occiput to pre-
maxillary; it reaches from the palatine behind the occiput
to an extent nearly equivalent to half the length of the
skull; it obliquely crosses the inner face of the suspensorium,
and extends even behind the quadrate condyle to the most
posterior part of the angle of the mandible. The trans-
palatine (tr,) is a curious hatchet-shaped bone, passing
obliquely from the posterior end of the maxillary to rest
upon the broadest part of the pterygoid. Its antero-
extemal process touches the palatine and is attached to it.

491. The squamosal (sq.. Fig. 53), the uppermost bone
of the suspensorium, is almost horizontally placed, passing
also a little downwards ; it is oblong, rounded in front, and
wedge-shaped behind, where its downward face is covered

B. M. 14



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210 MORPHOLOGY OF THE SKULL. [CHAP.

with articular cartilage. It lies on and is attached by
ligament to the anterior part of the prootic, and is slightly
in contact with the temporal region of the parietal. The
quadrate (qu,) is obliquely extended above, with a carti-
laginous face lying over the bevelled hinder and external

Fig. 53.



Adult Snake : skull seen from above.

Bones: s.o. supraoccipital ; e.o. exoccipital ; pr.o. prootic ; pa. parietal ;
fr. frontal ; na, nasal ; pmx. premaxillary ; a.or. antorbital or prefrontal ;
^.or. postorbital or postfrontal; pi. palatine ; pt. pterygoid; tr, transpala-
tine ; mx, maxillary ; sq. squamosal ; qu. quadrate ; at.hy, minute
stylohyal on quadrate; art, articular; 8,ang, surangular; ang, angular;
d, dentary.

edge of the squamosal: there is a synovial joint between
the two bones. The quadrate becomes more slender and
thicker below, and bears a rounded condyle for a perfect
hinge joint. Its direction is outwards and backwards at
an angle of abgut 45® with the middle line.

492. The lower jaw reaches as far as to the front of
the maxillary bone, and is gently arcuate : its dentigerous



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VI.] THE ADULT SNAKE. 211

dentary (d.) is just half the length of the ramus. The
bones are on the whole unchanged from the last stage.
The broad junction of the coronoid and splenial is at two-
fifths the length of the jaw from its anterior extremity.
The angular (ang.) and surangular (s. ang) are of great
length, and are in some degree consolidated with the
articular {art)\ the meckelian cartilage is almost entirely

absorbed.

*

493. The columella with its stapedial plate lies under
the horizontal canal, in a recess which leads to the fenestrse
ovalis and rotunda ; it is relatively much diminished, is
entirely bony, and the sigmoid mediostapedial part is
now no longer than the long axis of the stylohyal. This
latter is a little scale-like bone, partly anchylosed with
the posterior edge and inner face of the quadrate a little
above its middle. By the growth of the suspensorium it is
cajried away from the columella to a distance of four
times its own length. The small supratemporal has com-
pletely coalesced with the squamosal. A small pair of
cartilages is sometimes found at the sides of the larynx,
which may represent the distal part of an arch (Huxley).

494. Summary. In this type the non-coalescence of
the trabeculse in their postnasal tract, combined with their
late persistence, is a feature of striking interest. Further-
more, we note a marked diminution in the relative pro-
portion of the cartilaginous structures to the rest of the
skulL The cranium is only posteriorly roofed by cartilage ;
the anterior part of its lateral wall also acquires very little
cartilage, and the orbito- and alisphenoids are very feeble,
and contribute but slightly to the cranial investment.
The occipital and auditory bones are very complete,
showing much more likeness to Osseous Fishes than to
Amphibians ; the basisphenoid is ossified by two centres,
and then coalesces with the large parasphenoid which by
its median upward growth separates the trabeculae. But the
fore part of the cranium has yet more notable features, in
the first appearance of the frontals and parietals being low

14—2

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212 MORPHOLOGY OF THE SKULL. [CHAP.

down laterally, in the perfect coalescence of the parietals
superiorly, and the formation of inferior plates, above
the trabeculae and supporting the brain.

495. The palate is of a distinctive " cleft " type, and
the palatal bones present an extraordinary elongation and
high specialisation of parts. A new bone, the transpalatine,
is added, extending between the posterior end of the
maxillary and the middle of the pterygoid. The latter,
as in forms previously described, is connected with the
quadrate, but it has a perfection of structure not before
manifested. The premaxillary is azygous ; the septo-
maxillaries are very perfect and interesting ; " labial "
cartilages are again found in relation to the posterior
nares. The suspensorium of the mandible is notable
(1) as giving rise to but one bone, the quadrate, which
ossifies it entirely; (2) for altering its direction during
development from forwards to backwards; (3) in being
quite dissociated from contact with the cranial wall, and
loosely united with the long squamosal, both bones being
backwardly directed. The lower jaw has a very full
number of distinct ossific centres. The side wall of the
skull possesses membranous antorbital (prefrontal) and
postorbital (postfrontal) bones. The small representative
of the hyoid arch is divided into two parts, of which one
persists as the simple columella, separated, by some distance
from the minute stylohyal piece.

496. There are but few points of special community
between this skull and the types examined in the
Amphibia, while there are many features which remind us
of the Osseous Fish and the Bird. While the specialisation
of membrane-bones and diminution of cartilage and
cartilage-bones is far beyond what we have seen in the
Salmon, yet in the occipital and otic bones, the para-
sphenoid, and the palato-mandibular structures there are
many resemblances between these two forms. But there
is lost almost entirely the conformation of parts necessi-
tated by the function of breathing in water; and the
remnants of the postmandibular arches very early after



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VI.] THE SKULL OF THE TUBTLE. 213

their appearance take up the relations to which they are
adapted in the adult. In many respects we are on the
high plane of vertebrate development; the membrane
booes are co-applied and united with the cartilage-bones
to form a strong defence for the brain ; the bones of the
palate have a form and relations which need but little
modification to present the Avian type; the suspensorium
is one definite bone, the quadrate. From the consideration
of this skull alone, the association of Amphibians and
Reptiles in a group distinguished on one hand from Fishes,
and from Birds on the other, becomes a contradiction to
anatomical truth.



APPENDIX ON THE SKULLS OF REPTILES.

497. The skulls of venomous snakes vary in thoir palatal
structures in a manner which finds its extreme expression in
Crotaliis (the Rattlesnake). Here the shortening and specialisation
of the maxillary, bearing the poison-fang, combined with great
mobility of the other palatal bones, are the distinctive characters.
The premaxillaries are small; and the maxillaries are capable of
being moved from their normal position so as to erect the poison-
fangs, by the same muscular action which opens the mouths In
Typhlops the palatines are slender, and transversely placed, behind
the posterior nares. The pterygoids lie beneath the cranial floor,
and do not extend to the quadrates. In Tortrix the quadrate is
short, and directly connected with the cranium, the squamosal
being insignificant.

The Skull qf the Turtle.

498. In the Turtle {Chelone) at the time of hatching', the
chondrocranium is much more developed than in the Snake. The
occipital roof is extensive, reaching beneath the hinder fourth of

^ ^ See Hnxley, Anatomy of Vertebrates, p. 239.

s This description will explain most of the structure of the adult
skuU.



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214 MORPHOLOGY OF THE SKULL. [CHAP.

the parietal bone; yet the supracranial fontanelle is lai^. The
floor of the cavity presents cartilage almost perfectly continuous,
the posterior clinoid ridge being prominent ; but there is a posterior
basicranial fontanelle. The ossifications of the floor of the cranium
are a large basioccipital, and a basisphenoid arising by three centres,
two paired and lateral, behind the pituitary body, as in the Snake,
and one anterior and median, representing the rostrum of the same
animal. In front of this the presphenoidal cartilage is low at first,
and then becomes suddenly compressed vertically, forming an
interorbital septum beneath the optic neiTe. This cartilage attains
its greatest height as mesethmoid, and then is gently lowered where
it constitutes the nasal septum, finally terminating antero-infenorly
in the median prenasal cartilage.

499. From the whole of the upper edge of the interorbital
septum arises on either side a large semilunar orbitosphenoid
cartilage, which ends posteriorly just above the optic nerve. The
space behind this in the cranial wall is largely ossified by a descend-
ing plate of the parietal, there being no alisphenoid cartilage. The
nasal cartilages form very perfect roofs and side-walls to their
capsules, and, curving inwards laterally, give rise to rudimentary
turbinal plates. Posteriorly the nasal wall constitutes a steep
antorbital plate. In its hinder part the capsule is almost entirely
invested by the large ethmo- nasal bene. Anteriorly the lateral wall
is continuous in cartilage with the nasal floor, so that the capsules
form two perfect cylinders.

500. The exoccipitaJs are vertical and oblong, perforated by the
vagus and hypoglossal nerves. The supraoccipital region is large,
with a great crest or spine growing backwards ; it possesses one
considerable ectostosis. There are three auditory centres, but much
of the capsule remains cartilaginous. Anteriorly is the prootic;
supero-posteriorly the epiotic, passing forwards and inwards, and
uniting with the supraoccipital ; and postero-inferiorly the opisthotic,
remaining permanently distinct. Between these bones is a triradiate
tract of cartilage, the remains of which persist through life as a
suture. There are large parietals (with lateral cranial plates), and
frontals.

501. The quadrate has a large swollen otic region closely appliecT
to the auditory capsule, and an anterior (orbital) process growing



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VI.] THE SKULL OF LIZARDS. 215

forwards and inwards to the onter side of the descending plate of .
the parietal This is not ossified in the adult. The pterygoids
are long and broad, and unite with one another in the middle
line beneath the cranial floor. Each pterygoid has an angular
ascending process applied to the inner face of the quadrate, and
also abutting against the outer face of the parietal beneath the
orbital process of the quadrate ; it is ossified from membrane. In a
grooye on this osseous process is found a tract of cartilage, whose
apex nearly reaches the apex of the orbital process, forming a right
angle with it It becomes invested with a bony shafts and constitutes
a small epipterygoid ; it is subsequently developed into an oblong
plate of bone wedged in between the parietal and the ascending
process of the pterygoid, and has been mistaken for an alisphenoid.

502. The palatines are united with the pterygoids behind, and
with the median vomer in front and above. The latter bone is
expanded beneath, on a level with the small inferior palatal plates of
the palatines, which are here seen for the first time, tending to carry
backwards the orifice of the posterior nares in the palate. There is
no transpalatine, and no septo-maxiUary. The premaxillaries are
small; the maxillaries large, and jugals and quadrato-jugals con-
tinue the series to the quadrate. There is a postfrontal (membrane
bone) behind and above the orbit ; and' further back, a squamosal at
the sides of the auditory capsule and above the quadrate. The
temporal fossa is roofed over by the junction of the postfrontals and
squamosals internally with an external plate of the parietals, and
inferiorly with the jugals and quadrato-jugals. The dentaries are
anchylosed in the adult into one bone. The postoral arches present
much more development than in the Snake. There is a broad
basihyal with anterior (larger) ceratohyal comua and posterior
smaller (branchial or '' thyro-hyal ") processes. These comua are all
ossified. They are not directly connected with the axial parts; but
there is a long columella auris, ending above in the stapes.



The Skull of Lizards,

603. In a typical Lacertilian (as Yaranus, Iguana) the hinder
region of the skull is very completely ossified. Basi- and exoccipitals
enter into the composition of the single condyle. The basioccipital
is broad, the exoccipitals ascend to two-thirds of the height of the



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216 MORPHOLOGY OF THE SKULL. [CHAP.

foramen magnum, which is completed by the large supraoccipitaL
The auditory capsules, together with the exoccipitals, are extended
widely outwards; the opisthotic unites with the exoccipital; the
epiotic with the supraoccipital ; the prootic remains distinct

504. The basisphenoid is ossi6ed from a pair of ectosteal centres
behind the pituitary body; and sends out on either side a large basi-
pterygoid process to articulate with the inner aspect of the ptery-
goid. The basisphenoid extends forwards for some little distance



Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 18 of 31)