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The morphology of the skull online

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margins. Far forwwds there is a small nasal septum. The labial
tentacles are one-third shorter: but there are two new pairs of
labials around the inferior and two around the superior nares. The
quadrate condyle has not relatively receded. There are two apposed
frontals covering the supracranial fontanelle and bearing on then*
hinder surface a pair of small parietals. The parasphenoid (the
first bone to appear) is dagger-shaped; the cranial cartilage is
afiected by one continuous bone on either side, occupying the
exoccipitaL and prootic tracts.

420. In Dactylethras with large legs and diminishing tail one
of the most striking changes is the retraction of the quadrate condyle
for a distance equal to one-third the length of the skull, it being now
oppoi>ite the fore margin of the frontal. The precranial ethmoidal
cartilage is still of great width, as also is the suspensorium ; a much
narrower palatopterygoid pedicle has arisen, and the arcade is
disengaged from the trabecular comu. The tympanic annulos is
definitely segmented off, and has become semicircular. The stapes
is well chondrified, but there is no columella. The parasphenoid
has grown forwards to the subnasal region, and bears a small
vomer. The frontals have coalesced. Nasals, maxillaries, and pre-
maxillaries have appeared. The meckelian cartilage has grown veiy
much, with a thick proximal end, an ectosteal articular within, and
a small dentary in front. The mouth is no longer Siluroid, bnt
Batrachian. The hyoid arch is still very large and strongly marked,
but somevihat disengaged from its suspensorial articulation.

421. In the adult Dactylethra an enormous transformation has
taken place, for we find a narrow elongated cranial cavity, quadrate
condyles at the level of the periotic masses, and a very large
subocular fenestra separating the cranium from the palatopterygoid
bar. The part of the cranial walls and floor between the nasal and

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the auditory re^ons is nearly all solid bone, but this sphenoidal
bone scarcely extends into the ethmoidal region. The long superior
fontaneUe is covered by the large parieto-frontal. There is a
triradiate supraethmoid upon the cartilage in front of the cranium,
its median ray overlying the nasal septum. The nasal region, with
its comua, abbreviated first labials, and numerous pairs of valvular
secondary labials, presents many features of resembknce to the

422. The parasphenoid reaches nearly to the extremity of the
snout, the single vomer lying beneath it behind its termination.
There is no palatine and no jugal; and the hinder end of the
maxillary is far distant from the quadrate. The large pterygoid
behind underlies the suspensorium, extending inwards to touch
the short basitemporal wings of the parasphenoid. The suspensorium
is not quite directly attached to the cranial wall, but is confluent
with the auditory cartilage without dividing into very marked
processes. The elongated mandible presents much cartilage, and
has but two bones, the articular and dentary. There is a very large
auditory columella, in which the cartilage is not segmented, but
there are two shaft-bones of nearly equal length, the inter- and
mediostapediaL The extrastapedial is a large cartilaginous plate
applied over the outer face of the squamosal, quadrato-jugal, and
quadrate, and embedded in the tympanic membrane; around it
is the attenuated tympanic annulus^ whose upper deficiency is
supplied by two small tympanic bones. The lower hyoid tract, from
being massive, has become very slender ; it is not closely related
to the auditory capsule, but is suspended from it behind and below
the attachment of the suspensorium by a ligament of some length.
Medioventrally the two bars are fused with one another and with
the basibranchial tract. The fore part of the original branchial
pouch has become a long and broad plate on either side. From the
basibranchial tract project backwards a pair of branchials (" thyro-
hyaLj'O ossified as shaft-bones, and sui)portiug the larynx.

Tlie Skull of Plpa.

423. Embryos of the remarkable Surinam Toad, Pipa monstrom,
taken from the dorsal pouches when 9 lines long, have a skull as
highly developed as the earliest described larvae of Bactylethra,

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while as yet they are coiled upon a little differentiated yelk-niaas,
the tail almost meeting the chin. The limbs are well deyeloped and
free externally, and yet the cerebral vesicles are very bulbous, and
the opercular flap is free and small. There are no homy jaws,
no labial cartilages, no traces of branchiae ; although the branchial
and aortic arches are formed. The ear-sacs with the hindbrain
are very large, taking up half the head. In many respects, tadpoles
of the common Frog and Toad only half the length of these embryos
are more advanced ; they are freo and active at one third their length.
But in Pipa the embryo is very unlike a tadpole : it appears never
to have gills; and the metamorphosis is perfect before extrusiun
from the dorsal pouch.

424. The skull is essentially very comparable to that of Dacty-
lethra, with considerable differences in relative size of parts. The
hinder region is predominant, with a long cephalic notochord; the
brain reaches nearly to the anterior extremity of the head. The
cartilage has not united around the notochord, but the intertrabecular
space is already filled in with cartilage, forming a broad trabecular
plate, continued outwards in the mandibular nuspensoria ; the
subocular fenestra is a mere chink. There is a well marked otic
process of the suspensorium. The tegmen tympani is not so con-
spicuous a feature as in Dactylethra. The fenestra ovalis is forming
The trabecular comua are free. The hyoid and branchial cartilages
are on the whole very similar to those of Dactylethra, although
feebler. This is their highest state of development The aortic
arches show no secondary branche&

425. Metamorphosis is complete while the larva remains in the
maternal pouch. In creatures still rolled up, and little more thao
half an inch long, as well as in others showing their faces at the
mouths of the dorsal pouches, seven and a half lines long, the ossi-
fication is already intense. The head and body are only one-fourth
of an inch longer than when the yelk-mass was but little lessened ;
the original ground-plan, so to speak, is filled in with but little
modification in the size of the territories.

42^ Two of the most remarkable features of the chondro-
cranium are, the absorption of most of the cartilage in the floor of
the skull, including some of the postpituitary tract, and the very
great increase in the subocular fenestra. The ' palatopterygoid is

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a slender rod of cartilage. The columella is complete at this early
period, contrasting with its late development in the Frog. The
ceratohyal has become entirely absorbed; the branchial arches are
reduced to a form substantially agreeing with then: condition in

427. The notochord is but little diminished relatively : it lie?
mainly upon the basilar plate behind, and over the parasphenoid
in froni^ free from relation to cartilage. On either side behind,
a fissure marks the division between the almost absorbed auditory
cartilage and the basilar plate ; in front of that chink the auditory,
basilar, trabecular, and suspensorial cartilages are united, the right
and left tracts being widely separated. About the region of the
optic nerve the trabecula is reduced to a normal size. From thence
each bar rapidly widens, running inwards to unite with its fellow
and form the ethmoidal floor, npon which lie the olfactory lobes,
and the anterior fourth of the cerebral hemispheres. The cartila-
ginous cranial wall, continuous with the trabecular base, is a very
thin vertical lamina extending from the auditory capsule to the
ethmoidal region, united with the ethmoidal floor just described,
and forming a narrow anterior ethmoidal wall to the cranium : all
this is entirely unossified. It is continued forwards into a com-
paratively narrow nasal septum with recurrent comua trabeculse,
and is laterally produced into the almost transverse antorbital and
palatine tract There is a small alinasal roof growing on either side
from the top of the septum ; and the nasal investment is completed
by three pairs of upper labial cartilages, two of which extend into
the upper lip.

428. There is a strong prepalatine spur from the ethmopalatine
bar, running forwai'ds parallel with each trabecular comu. Behind
this the main bar is very elongated, joining the fore face of the
suspensorium just within the condyle. The suspensorium is very
different from its primary condition : the pedicle in front of the
auditory capsule is narrow, but joins broadly with the base and
side wall of the cranium ; its direction is foritards and inwards
from the quadrate condyle ; formerly it was directed backwards.
The otic process almost continues the line of the pedicle backwards :
it is not fused with the periotic mass ; it is directly behind the
palatopterygoid bar. The quadrate tract is little extended: it

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furnishes a bilobed cendyle. Instead of being situated at the level
of the anterior end of the subocular fenestra, it is now below its
posterior angle. The columella extends forwards over tlie otic
process to the palatopterygoid bar. The process of the suspensoriom
bearing the condyle for the ceratohyal has disappeared in correlation
to the absorption of that cartilaginous plate.

429. The cranial bones present remarkable features. There ia an
occipito-otic mass, possessing stnall exoccipital tracts, nearly meeting
below in the middle line a strong supraoccipital bridge, beneath
which a cartilaginous plate remains, and very complete otic in-
vestments, which have eaten away most of the cartilage below,
leaving it more extensively above. The extreme external and an-
terior regions of the capsule remain unaffected by bone. The very
large parasphenoid greatly resembles that of an adult perennibran-
chiate or a young caducibranchiate Urodele. It ends behind about
the middle of the auditory region by a sharp spur; and in front
beneath the nasal septum by another sharp spine; it is broadest
in tlie ethmoidal region ; there is no vomer. The cranial fontanelle
is more than covered by a great single parieto-frontal bone, ex-
teuding backwards to the supraoccipital : an anterior median spur
16 due to a supraethmoid ectosteal plate.

430. The nasals are semicircular bones notched in front for
the extenial nares : in front of each is a small preorbital and a
smaller septo-maxillary. The premaxillaries and maxillaries are
flat^ simple bones, lying entirely beneath the anterior cartilaginous
titructures. The maxillaries are falcate, ending behind in sharp
p>ints, a little in front of the pterygoids; the palatopterygoid
cartilage is bowed farther out than the maxillary bone. A delicate
styloid palatine bone extends between the parasphenoid and the
maxillary. The pterygoid lies underneath the corresponding carti-
lage and the suspensorium, almost touching the parasphenoid : its
transverse tract is much the larger. There is no quadrate ossifi-
cation ; the meckelian cartilage is largely invested by the ectosteal
articular, reaching nearly to the symphysis ; the dentary is laigest
at the symphysis. The squamosal consists of a lamina on the
sMspensorium, hidden by the columella ; and a horizontal extension
at the side of the auditory capsule. The columella is fastened
to the simple unossified stapes ; the elongated cartilage is not

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segmented, but it is invested by an iuterstapedial and a raediostapedial
shaft-bone, and is continued forwards into the enlargetl extrastapedial
cartilage, surrounded by the U-shaped tympanic annolus: there
is no suprastapediaL

43 L The skull of the adult Pi pa, although presenting no struc-
tures which cannot be interpreted by the last described form, is
oue of the most remarkable pieces of osseous architecture that can
be imagined. Cartilage is almost entirely absorbed; the bony
tracts have become greatly extended and massive, and by the
alteration of their relative size the skull wears an extremely different
aspect. Its breadth has become half as g^reat again as its length,
the auditory capsules being thrust out on long stalks. The quadrate
condyles are retracted to a position parallel with the occipital
condyles, though at a lower level The cranium is excessively flat-
tened, and the bones of the roof and floor meet laterally. The
maxillaries combine with the pterygoids to bound the upper jaw.

432. The posterior view of the skull presents a bony arc, convex
upwards, bearing numerous bosses for muscular attachment, and
cousisting almost exclusively of the occipito-otic mass. In the basi-
occipital region a small median tract is unossified : the rest of the
foramen magnum is encircled by bone, and no cartilage is left The
occipital mass extends outwards to the squamosal and quadrate
bones, and nearly to the quadrate condyle. The occipital condvle^i
are very outwardly directed. The prootic tract of the occipito-otic
mass reaches out to the tympanic apparatus, which is close to the
quadrate condyle. The parasphenoid is a broad oblong bone, pos-
teriorly filling up an angle between the right and left occipito-otic
masses where they nearly touch below, anteriorly presenting a
median spur beneath the nasal septum, and a pair of palatine wings
reaching to the maxillaries. There is no vomer. The massive
fronto-parietal is twice as broad in the ethmoidal region as it
is behind, where it overlaps the supraoccipital tract It has a
median supraethmoidal spur, and large lateral wings extending on
either side down to the maxillary. There is no cartilage in the
cranial walls: the deficiency is supplied by the bevelling of the
parasphenoid npwards on either side to meet a long orbital plate
of the parietofrontal There is no sphenethmoid.

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433. Cartilage is persistent in the ethmoidal, nasal, and ethmo-
palatine tracts. The prepalatine spur is retained, and also a slender
part of the palatopterygoid bar, not nearly reaching the suspensorium
or the antorbital region. The nasal septum remains, but is veiy
low ; the alinasal cartilage is small ; the recurrent trabecular corana
are long, slender, and straight, diverging from the septum at about
45^ ; a small prenasal spur projects forwards from the septum ; all
but one pair of labial cartilages have vanished. The large triangular
nasals extend forwards to the extremity of the snout, where they
meet; they are notched for the external nares. and overlap the
parietofrontal. Below the narial opening are the preorbital and

434. The premaxilto'es and maxillaries are edentulous palatal
plates. The latter are much the larg^er, have a spur within the
internal nostrils, and extend backwards under the pterygoid bones
for half their length. The anterior part of the pterygoid is of
considerable breadth, bounding the subocular space, which is wider
than the cranium; while the eyeball is quite small, and rests
against the anterior external part of the pterygoid. Posteriorly,
the pterygoid is transversely extended beneath the otic mass so
as to reach the parasphenoid and come very near the middle line;
there is also a smaller process extending outwards and backward;}
beneath the quadrate. The suspensorium is undistinguishable from
the otic mass (although some cartilage remains) except by the &ct
that the quadrate is a distinct considerable bone. The squamosal
clasps the outer edge of the otic mass and the upper and hinder &ce
of the quadrate. The columella has its extrastapedial relatively
dilated, and surrounded by an almost perfect ring of cartilage,
the tympanic annulus. The stapes and the fenestra ovalis are
surrounded by an annular elevation of cartilage. The facial nerve
emerges from the upper surface of the otic mass about half way
towards the quadrate condyle ; it passes over the columella. The
moieties of the edentulous lower jaw have a fibrous symphysis, and
are very rib-like ; the articular bone runs nearly to the chin, and is
covered by the dentary, which is but two-thirds its length.

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{Tropidonotm natrix.)

435. The early phases of the development of the
skull in this form were long ago observed by Rathke,
and his account of them is not by any means superseded.
The first two stages given below are abridged from
Rathke*s Entwickelungsgeschichte der Natter^ 1839 \

First Stage,

436. The trabeculae at their first appearance form
two narrow bands, consisting of the same gelatinous
substance as that constituting the whole investment of the
notochqrd, not sharply differentiated, but only thickened
and more solid parts of that half of the cranial floor
which lies under the anterior cerebral vesicle. Posteriorly
they are separated by a small interval, and thence sweep
in an arch to about the middle of their length, separating
as they pass forwards ; afterwards they converge, so as
to approach one another very closely, or even to come
into contact. Altogether they form, as it were, two
horns, into which the investing mass of the notochord
(parachordal tract) is continued forwards. The elongated

^ See Huxley, Proc, Roy. Soc, Nov. 1858, p. 66, and ElemenU of
Comparative Anatomy, p. 237.

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(intertrabecular) space between them, moderately wide
in the middle, is occupied by softer formative substance,
upon which rests the infundibulum and the forebrain.
Anteriorly the trabeculae reach to the front of the head,
and bend slightly upwards into the frontal wall : but
there is also a small lateral projection or cornu passing
outwards on either side.

437. The middle trabecula^ grows, with the brain, further
and further into the cranial cavity, and as tlie dura mater
begins to be now distinguishable, it becomes more readily
obvious than before, that the middle trabecula raises up a
transverse fold of it, which traverses the cranial cavity trans-
versely. The fold itself passes laterally into the cranial wall ;
it is highest in the middle, where it eucloses the median
trabecula, and becomes lower externally, where it forms, as it
were, a sliort ala proceeding from the trabecula. With increasing
elongation, the (middle) trabecula becomes broader and broader
towards its free end, and, for a short time, its thickness increases.
After this, however, it gradually becomes thinner, without any
change in its tissue, till, at the end of its second period, it is
only a thin lamella, and after a short time (in the third period)
entirely disap^jears.

433. In Mammals, Birds, and Lizards, that is, in those
aniuials in general, in which the middle cerebral vesicle is very
strongly bent up and forms a protuberance, while the base of
the brain exhibits a deep fold between the infundibulum and
the posterior cerebral vesicle, a similar part to this median
trabecula of the skull is found. In these animals, also, at a
certain very early period of embryonic lite, it elevates a fold of
the dura mater, which passes from one future petrous (prootic)
bone to the other, and after a certain time projects strongly
intiO the cranial cavity. Somewhat later, however, it diminishes
in height and thickness, until at last it disappears entirely, the
two layers of the fold which it had raised up coming into
contact. When this has happened, the fold diminishes in
height and eventually vanishes almost completely.

^ This account of Rathke's вАҐ* middle trabecula " is given at length in
order that its siguificancb may be prominently brought forward.

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Second Stage,

439. The trabeculae attain more solidity, acquire
greater distinctness from the surrounding parts, and
assume a more determinate form, becoming in fact fili-
form, so that the further forward, the thinner they appear.
They increase very little in thickness, but far more in
length, during the growth of the head. Quite anteriorly,
they coalesce with one another, forming an intemasal
tract. As soon as the olfactory organs increase markedly
in size, the septal region is moderately elongated and
thickened, without becoming so dense as the hinder part
of the trabeculse. The lateral prolongations which now
proceed from the intemasal tract become little denser,
although considerably enlarged.

440. The lateral parts and upper wall of the cranium,
with the exception of the auditory capsules, remain merely
membranous, consisting in fact only of the cutaneous .
covering, the dura mater, and a little interposed blas-
tema, which is hardly perceptible in the upper part, but
increases in the lateral walls, towards the base of the

Third Stage : Embryo Snakes about an inch and three-
quarters in length.

441. These embryos have recovered from the meso-
cephalic flexure ; but the visceral clefts remain open,
three fairly distinct, the fourth obscure. The head (Fig.
48) has a very monstrous appearance, being covered with
a series of bulbous protuberances. The nasal region is
rounded and beaked : the olfactory cavities are formed,
and the nasal meatus is pervious. The eyes (e.) are very
large, being twice the diameter of the auditory and nasal
sacs. Although the mesocephalic flexure is lost, there
is a remarkable bending of the head upon the neck.

442. The angle of the mouth is directly below the
anterior margin of the auditory saa The inner nares

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are formed, opening together in one common depression
on the mid line of the palate. The maxillopalatine
cheek -mass, from the nasal sacs to the angle of the mouth,
is distinguishable into two tracts, which are better seen
when the lower jaw is removed. There is an outer or
maxillary, and an inner palatopterygoid tract. Th^ lower
jaw is of equivalent length and thickness to the upper;
and between its rami can be seen a pair of rod-like
prominences containing the rudiment of the tongue and
its sheath.

Fig. 48.


Embryo Snake If inch long ; side view of head with facial arches seen

C 1, 2, 3, cerebiral vesicles; oh nasal sac; e. eyeball; au. auditory
mass; m. mouth; p'pg. palatopterygoid tract; mk. meckelian cartilage;
9. quadrate ; tg, rudimeilt of tongue ; hy, hyoid arch ; cl, 1, 2, visceral

443. At this stage the chondrocranium is already
well formed. The cranial notochord {ch Fig. 49) is
short, and twisted upon itself, and does not reach more
than two-fifths of the distance between the occipital end
of the skull and the pituitary body. The parachordal
cartilages are united at two points behind the pituitary
body and in front of the notochord. There is one broad
bridge immediately in front of the notochord ; the other
is narrower, and lies immediately behind the pituitary
body. Between these bridges there is a large oval space
vacant of cartilage {p. b. c,f.). The anterior part of the

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VI.] THE snake: third stage. 191

parachordal cartilage is turned outwards opposite the
postpituitary bridge, to form a hooked process, with a
hinder concave margin. Just outside the same bridge
the carotid artery pierces the cartilage on either side :
this point marks the junction of the parachordal with
the trabecular cartilages.

Fig. 49.

Embryo Snake, about If inch long ; chondrocranium seen from above,
the brain and jaws having been removed.

8.0. supraoccipital tract; au, auditory capsule; a.s.c, h.8,c,y p.s.c,
Ulterior, horizontal, and posterior semicircular canals; ch, notochord;
i.m. basilar plate ; p. b.c.f, posterior basicranial fontanelle ; al.s, ali-
i-pbenoid cartilage ; pty. pituitary body; tr. trabecnla; o.8. orbitosphenoid
cartilage ; II. optic nerve ; na. nasal capsular cartilage.

444. The trabeculse (tr.) pass forwards as rounded
rods, wide apart opposite the pituitary body, rapidly
converging in front, but remaining separated by a distance
almost equal to their own width as far as the nasal region.
The tract in which the trabeculse remain distinct is about
one-third the length of the skull. In front the trabecular
coalesce, forming a considerable internasal plate; and

Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 16 of 31)