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or antorbital region. The extremal nostrils are almost
directly above the internal, and are protected by a lunule
of cartilage which lies over the opening. This cartilage

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is the upper two-thirds of the original upper labial
(Fig. 40, w. /'.). The lower third has become a rounded
nucleus which lies between the premaxillary and the
nasal capsule in front. Within the nasal opening the

Fig. 43.

Adult Frog : under view of skull, with lower jaw removed.

e.o. exoccipital; p,o, prootio; par, parasphenoid ; e. sphenethmoid ;
V. vomer ; pm. premaxillary ; mx. maxillary ; q. quadrato-jugal : sus. sua-
pensorium; pt. pterygoid; 1, optic foramen; 2, trigeminal foramen;
3, glossopharyngeal and vagus foramen.

septum sends out from its upper part a broad thin lamina
of cartilage, which runs outwards to the outer and ante-
rior wall of the nose, and unites with the subnasal
floor ; half way outwards it becomes subdivided into two
laminae, so that at this region the nasal cavity has three
horizontal floors. The nasal passage, from external to
internal nostril, is completely lined by the little subcylin-
drical septo-maxillary.

394. On either side of the cranium is a large oval
suborbital membranous space. It is bounded in front by
the flat ethmopalatine bar, passing directly outwards and
sending forwards a prepalatine spur (Fig. 40, p. pa), while
posteriorly it is continued into the slender pterygoid bar.
Behind, this passes into the mandibular suspensorium,.
marked by three principal regions, (1) an antero-inferior,
the pedicle, directed inwards, and articulating by a

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thickened condyle with the auditory capsule in front of the
fenestra ovalis; (2) a supero-posterior, the otic process,
attached to the fore part of the tegmen tympani ; (3) the
quadrate or main suspensorium, directed backwards,
downwards, apd outwards, so as to reach nearly as far
back as do the occipital condyles.

Fig. 44.

Adult (edible) Frog : under view of skiiU ; investing bones remoyed
on right side. (Huxley.)

O.C. occipital condyle ; p.n. posterior (inferior) nostril ; gti. quadrate
tract ; it. hy. stylohyal, partially shown ; eu. eustachian opening ; IE. op-
tic foramen ; Y. trigeminal foramen ; X. glossopharyngeal and yagus

Bones: «.o. exoccipital; pr.o, prootic; pa.8. parasphenoid (left half) ;
g. sphenethmoid ; vo. Yomer; pmx. premaxillary ; nuc. maxillary ; pi.
palatine; pt. pterygoid; quju, quadrato-jugaL

395. The palatine or antorbital bone invests the
hinder surface of the transverse ethmopalatine bar; it is
slender and almost crescentic, and slightly underlies the
sphenethmoid. The pterygoid is elongated and more
massive ; it has transformed much of the cartilage of the
subocular bar, and just touches the palatine bone in
front. Behind, it is forked, passing inwards by a square
(metapterygoid) process to support the pedicle, and back-
wards as a narrow parosteal spur to clamp the under surface
of the suspensorium almost as far as the condyle. The

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quadrate cartilaginous region is only very slightly ossified
by the hinder end of the quadrato-jugal.

396. The T-shaped squamosal {sq. Fig. 45) lies on
the suspensorium and the external auditory region. Its
horizontal tract is outside the prootic behind, and sends
forwards over the pterygoid a considerable postorbital
spur. At more than a right angle with this is set the
descending portion, lying on the suspensorium and over-
lapping the quadrate part of the quadrato-jugal. It is
separated from the cartilage by perichondrium, and from
the quadrate by periosteum. It lies beneath the tym-
panic annulus.

397. The investing bones of the nasal and maxillary
tracts have now to be described. The nasals are coDsi-
derable bones, transversely placed on the nasal roofs,
broad within, narrow externally, not nearly meeting in
the middle line. In the middle of their anterior margin
th3y are notched for the external nostril, and then ex-
tend backwards and outwards along the nasal lamina of
the maxillary to end in a sharp preorbital spur. The
premaxillaries are of considerable breadth, are apposed
but unanchylosed in the middle line, and possess both
dentary and nasal plates. The latter project upwards and
backwards by a curved spur in their middle region,
touching the nasals. Mesially a tract of cartilage is
uncovered by bone between the sphenethmoid, the nasals,
and the nasal plates of the premaxillaries (see Fig. 42).
The palatal plates of the latter are thin and quadrate,
with a somewhat irregular surface.

398. The maxillaries are elongated, with extensive
nasal laminae in their anterior third; the septo-maxillary
{ Fig. 45) appears between the inner and anterior
angle of this plate and the nasal process of the premaxil-
lary; the external nostril notches it behind this point
The nasal plate of the maxillary dies away, at first by a
sudden decrease in the antorbital region, and then gradu-
ally, the bone becoming a fine pointed style, largely over-
lapping the quadrato-jugaL It is adjacent to the anterior

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third of the pterygoid, but diverges outwards from it,
while the pterygoid curves inwards (Fig. 44). The palatal
plate of the maxillary is but slight and ranges with that
of the premaxillary, soon becoming insignificant.

399. The vomers are elegant trifoliate plates, wide
apart like the nasals. The middle leaf, which is emar-
ginate, and the narrow falcate posterior leaf, together
largely surround the interaal nostril The pointed ante-
rior leaf nearly reaches the suture between the maxillary
and the premaxillary. The rounded stalk converges to-
wards its fellow, lies partially under the sphenethmoid,
and almost touches the parasphenoid ; this tract of the
vomer is dentigerous. The septo-maxillary can just be
seen between the middle and posterior leaflets of the vomer.

Fig. 46.


Adult Frog : side view of skuU, dissected.

o.c. occipital condyle ; st, stapes ; ty. tympanic membrane ; a.*, trm-
panic annolus ; a.n. anterior nostril ; qu. quadrate condyle ; mck. mecke-
lian cartUage; sLhy. stylohyal cartilage; h.hy., basi-hyobranchial
plate ; IL optic foramen ; Y. trigeminal foramen.

Bones : e.o, exoccipital ; sq. squamosal, partly ooyered by tympanic
ammlus; pr.o. prootic; parietofrontal; g. sphenethmoid; na, na-
sal; pmx, premaxillary ; ».mx, septomazillary ; mx. maxillary ; pa, 8, para-
sphenoid; |>t. pterygoid; quju. quadrato-jugal ; m.iiic/e. mento-meckelian;
h,br. Lypobranchial or thyrohyaL

400. The lower jaw (Fig. 45) is comparatively simple,
and very elongated, with its rami widely separated behind,
and approaching each other rapidly in front The meck-
elian cartilage persists except in the symphysial region,

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where it is replaced by a small cylinder of bone, the
mento-meckelian, which however is partially derived from
the remains of the lower labial cartilage. The articular
region has an elevated coronoid part; the condyle is a
smooth egg-like mass, with its long axis longitudinal; it
plays very freely beneath the smoothly-scooped base of
the quadrate. The dentary, bearing no teeth, and conti-
nuous with the mento-meckelian, runs back two-fifths of
the length of the jaw, surrounding the cartilage ; the arti-
cular* ensheaths the inner side completely, but very much
of the cartilage is bare of bone externally.

401. It appears unnecessary again to describe the
columella and tympanic annulus: the essential arrange-
ment is given in §§ 379, 380. The anterior boundary of
the tympanic cavity is traversed by the main facial nerve,
which lies above the pedicle. The stapes itself is attached

Fig. 46.

Adult Frog: colnmella anris.
it, stapes; %M. interstapedial ; m.«t. mediostapedial ; eAU eztra-
stapedial; 8.8^ suprastapedial.

to the margin of the fenestral fossa by a delicate band of
fibrous tissue ; but much of the inner face is in immediate
contact with the cavity of the vestibule. This fenestral
fossa is worthy of some note; it is an egg-shaped
fossa of considerable depth, into which the prootic extends
slightly; the exoccipital does not reach to its posterior
rim. The fenestra ovalis merely occupies the postero-
inferior third of the bottom of this fossa ; it is uniform,
with the concave edge looking upwards and forwards ; the
main otoconial mass can be seen through it. At an earlier
period the fenestra was as large as the stapes, but the

1 Prof. Huxley says this bone represents the angular, coronary, and
splenial elements ; but it is here named articular from a consideration of
transitional cases. See Huxley, art. * Amphibia,^ Encyc, Brit. Vol. I.
p. 765.

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latter has grown, and the Burrounding cartilage has
enlarged to embrace it

402. The slender curved hyoid arch (st hy.) is at-
tached to the periotic capsule a little way below the rim
of the stapedial fossa at its anterior third. It has changed
little since the Frog was two or three months old; it has
not become ossified. Ventrally it passes into the antero-
external angle of the broad hyobranchial plate, which is a
great cartilaginous escutcheon with irregular patches of
superficial endostosis on its surface (Fig. 47). It has lost

Fig. 47.

Adult Frog : hyobranchial plate.
ttX hyoid arch or stylohyal; e,h. oeratohyal tract; h.hy, basihyal; basihyobranchial ; ir, remainB of third branchial arch ; th, fourth
branchial or thyrohyal.

the symmetrical foramina it formerly had; and its various
outgrowths are unossified. Its upper surface is gently
concave ; and it is rendered exquisitely mobile by reason
of its suspension by the sigmoid hyoid bars. The antero-
extemal angle is produced into a leafy hypohyal lobe
in front of the attachment of the hyoid cornu ; there
is another broad lamina, the remnant of the first two
branchial arches, just behind this attachment, and a
third more elongated process (6r.), representing the third
branchial arch, extends almost from the postero-extemal
extremity of the plate. The fourth branchials {th,) are
ossified as shaft-bones, a core of cartilage remaining, and

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attached near one another at the posterior extremity of
the hyobranchial plate; they are slender in the middle
and expanded behind.

403. Summary. The primordial skull of the Frog,
arising very early in development, possesses the typical
elements quite distinct from one another; but there is
nothing to represent the palatopterygoid arch, and the
suctorial mouth is situated far forwards. The mesocephalic
flexure is early overcome, and at a subsequent period
considerable labial cartilages form the functional jaws.
When the external gills are at their fullest development,
the face is enlarged proportionally to the brain-case, and
the head is comparatively flat, the parts of the brain
being arranged almost in a straight line.

404. By the time the tadpoles are half an inch long,
extensive coalescence of the primitively distinct elements
has occurred. A cranial floor is formed by the union of
parachordals and trabeculse ; the latter give rise both to
an intemasal plate and to nasal floors. The mandibular
arch unites with the trabecular cartilage both in front
of and behind the orbit The meckelian bar is cut off,
and the .definite suspensorium has articulated to it the
unusually massive hyoid bar, which possesses no dorsal
region. Subsequent metamorphosis puts the tadpole of
an inch long in possession of a boat-shaped cartilaginous
cranium united behind with the ball-like otic masses;
while the mouth remains entirely in front of the brain-case,
the suspensorium of the mandible (still functionally in
abeyance) is a large subocular bar, bearing a superior
lamina roofing the t.emporal muscle. The branchial arches
have coalesced into a most remarkable basket-work,
which is folded almost double in the sides of the throat

406. As the tail is disappearing the cranium is found
much more extensively chondrified, and nasal capsules
are well formed. The character of the mouth is changed ;
the labial cartilages become subordinate, and the two
jaws functional The mouth is elongated by the gradual

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retraction of the suspensorium, concurrently with a growth
of (palatopterygoid) cartilage from the anterior confluence
of the suspensorium with the trabecular cartilage, and
the meckelian bar is correspondingly lengthened. The
hyoid and branchial arches gradually dwindle, and ulti-
mately form a rod-like arch and a broad basal plate.

406. During this great transformation most of the
membrane-bones of the skull appear, and also the ex-
oecipitals and prootics. Some most characteristic bones
are particularly late in development, for instance the
sphenethmoid and the mento-meckelian. The series of
parts which is identified as the upper part of the hyoid
arch, forming the columella, does not arise till after the
young Frogs have taken to the land; while another
structure, which is related to the organs of hearing, the
tympanic annulus, becomes connected with them, having
been originally a postero-superior leaf cut off from the
mandibular suspensorium.

407. The contrasts between the Elasmobranch skull,
destitute of bones, the Salmon's, with its multitude of
osseous plates little compacted together, and the Frog's,
80 highly specialised, and with bones so accurately moulded
on cartilage, are very obvious, and need not be detailed
here. But it may be serviceable to emphasize some of
the features in which the Frog is unlike the Axolotl, both
having skulls presenting at first sight many resemblances.
The Axolotl's suspensorium is not a subocular arch at
first ; its palatopterygoid cartilage is comparatively scanty,
and very slight in the adult. The Axolotl has no labials,
uo orbitar process, no columella. The Frog developes a
more complex branchial framework, which is more com-
pletely absorbed than in the Axolotl. The retraction
of the suspensorium is a special feature in the Frog, as
well as its giving rise to a tympanic annulus. The early
history of the trabeculse differs much in the two types,
especially as regards the parachordal extension, so marked
in the Axolotl, and the trabecular comua, which are
early formed and of notable size in the Frog,

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408. The skull of the Axolotl is ossified by more
bones, appearing earlier, and has less of permanent
cartilage than the Frog. Among bones found in the
former and not in the latter are the pterotic, opisthotic,
and ectethmoid. The mento-meckelian and columellar
ossifications of the Frog however are not represented in
the AxolotL


409. The skull of Rana pipiens (the Bull-frog) presents marked
distinctions from that of the common Frog. It is both highly
generalized, having more features looking back to the Fii^es, and
very specialised, for no creature turns its suspensorium so far bacL
The larvae are much more Petromyzine in aspect than those of
the Frog. The cranial box is very narrow in proportion to the
width of the skulL There are many additional bones, frequently,
minute, and having the character of ganoid scales. Multiplication
of bones takes place in several of the b^t defined tracts; the
metapterygoid and mesopterygoid are distinct from the pterygoid;
the palatine tract is divided into two bones ; there is an anterior
parasphenoid distinct from the vomers; a series of small bones
represents the septo-maxillary, and another series covers the sus-
pensorium. The tympanic annulus is of great size, and forms a
complete circle.

410. In the small Frog, Pseudis paradoxa, the skull is pro-
portionate to the body, and its details are of marvellous interest
In one stage of development the whole of the occipital and auditory
regions is ossified by one continuous bone on either side. Later,
each occipito-otic is cut into two bones ; and subsequently the hinder
pair of bones (mainly occipital) unite into one, while the anterior
pair remain separate as prootics. The parasphenoid is split down
the middle in front, and has no posterior handle (or projecting
spur behind the basitempond vnngs). The parieto-frontals arise
as one bone on either side, and are subsequently segmented into
parietal and frontal. The intemasal cartilage is of great extent^ but

1 For much valuable information see Prof. Huxley, art. 'Amphibia,'
Encyc, Brit,, ninth edition, YoL i.

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the nasal cayities are siualL The prepalatine region is considerable ;
the antorbital cartilage is almost severed from its ethmoidal
attachment. There are large suctorial cartilages, or labials.

411. The skull of the Toad {Btrfb vulgaris) differs from that
of the common Frog in the following respects. The orbitar process
in the Tadpole forms a perfect arch over the temporal muscle ; the
second upper labial coalesces in the adult with the intemasal plate
in front, and with the antorbital bar behind ; the palatine cartilage
is segmented off both from the antorbital and from the pterygoid ;
the upper part of the suspensorium in the adult rises above the
"condyle of the pedicle f there is no separate metapterygoid bone ;
the culumellar cartilage is not segmented, subdivision taking place
by the formation of two separate bony shafts, whose meeting-point
is distal, not near the stapes ; the extrastapedial is a large semioval
leaf, and the suprastapedial coalesces with the tegmen tympani ;
the lower hyoid bar coalesces with the auditory capsule ; the car-
tilaginous tympanic annulus forms only two-thirds of a circle;
the continuous nasal roof is only one-fourth the sise of the same
tract in the Frog ; the prorhinals are much smaller.

Th^ Skull qf Daetylethra\

412. The skull in this remarkable form, the aglossal Gape
Toad, is as notable in its history and adult structure as any among
the Anura. The larvae are very divergent in character from those
of ordinary Frogs and Toads ; they present much general resem-
blance in outward form to the extinct Coccosteus and Pterichthys,
The mouth is suctorial, and small, but very wide, like that of
Siluroid Fishes and Lophius. The lower jaw protrudes beyond
the upper; a very long tentacle proceeds on either side from the
upper lip, and there is no trace of the homy plates possessed in
isarly life by most Frogs and Toads. In conformity with these
characters the whole head is extremely flat or depressed, instead
of being high and thick. There are no claspers beneath the
chm. The branchial orifice is not confined to the left side, but
exists ako on the right The tail, like the skull, is very Chimseroid,
the whole caudal region bemg narrow and elongated, and terminating

^ See Phil Tram. 1876 for figures and detailed description.
B. M. 12

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in a long, thin, pointed lash. The fore limbs are not bidden beneath
the opercular fold.

413. The youngest larvae examined were an inch and a quarter
in length, three-fifths of this being caudal. The fore and hind limbs
at this stage are nearly equal in size, although subsequent growth
makes them very unequal. The grade of development of the skull
corresponds to that described in the third stage of the Frog. The
cranial structures are much more flattened even than an external
view would suggest; for the outer skin is very loose, and the
subcutaneous stroma copious and gelatinous. Unlike the larvae of
the ordinary Frogs and Toads, the tissues are very transparent,
although richly supplied with brown pigment.

414. The cartilaginous cranial floor has a very astonishing aspect ;
as in the Skate*s skull, there is a long, common, cranio-nasal valley,
formed behind of the parachordal cartilages and notochord, and
in front, for twice the extent, of the coalesced tral)ecula5, to whose
transversely extended anterior edge a transverse labial cartilage is
united. The periotic masses are confluent with the parachordal
and trabecular cartilage, but the cranial floor extends considerably
behind them, and gives rise to exoccipital walls. The trabecule
form blunt side walls to the cranium in front of the auditory masses,
and these elevations converge, narrowing the cranial cavity. The
brain proper ends at a point in front of the periotic cartilages only
one-fourth of their length, and the diverging olfactory crura are
as long as the brain. The precranial valley is embanked by the
continued trabecular elevations, which again recede from one another,
forming no transverse ethmoidal wall The trabecular laminae in
the sides of the precranial valley are but thin, and arched out-
wards over a considerable infero-lateral coneavity on either side.
The precranial valley is filled with a watery tissue through which
the olfactory crura run.

415. In the auditory capsule the fenestra ovalis is opening, but
is merely filled with indifferent tissue. The tegmen tympaui projects
as a considerable lobe from the hinder part of the capsule, is narrower
laterally, and then extends broadly from the whole anterior and
antero-lateral margin. It runs forwards and outwards as a broad
ribbon, equal in length and breadth to the capsule, and then is
confluent with a footsliaped flap having a pointed end forwards,

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nearly reaching the orbitar plate of the snspensorium. This lobe
is to be transformed into the tympanic aunulus. The facial nerve
runs beneath the t^men.

416. The mandibular sospensoriam is broadly conflaent behind
with the trabecular cartilage in the whole of that tract where it
forms a wall for the brain, and then diverges gently from it so as to
leave a narrow falcate membranous space, the subocular fenestra,
rounded in front, pointed behind. The suspensorium broadens as
it passes forwards, and has a good-sized orbitar procesM, in a region
not far behind the nasal openings. The quadrate condyle is just
at the same level as these apertures; and at that region the sus-
pensorium is confluent with the axial cartilage by a broad postnarial
or palatine lamina and by a narrower prenarial lamina, which takei^
the place of the ligament uniting the comu trabeculae of the Toad
to the quadrate. The eyeball rests on the suspensorium somewhat
outside the subocular fenestra.

417. The meckelian cartilage is small as compared with the
existing portion of ttie hyoid arch. The symphysial end is pointed,
and between the opposite points of the two bars is found a pair
of short inferior labial cartilages continuing their line (very unlike
the lower labials of the Frog's tadpole). The upper labials, as has
been mentioned, are very different; the moieties have coalesced
witli each other, and to some extent with the anterior trabecnlnr
edge ; but on either side there is a sharp fissure between the labial
and the comu trabecule, and the labial runs, as a gradually at-
tenuating thread of true cartilage, to the extremity of the long
tentacle, so tliat at this stage it reaches to the end of the abdominal

418. The hyoid arch shows a ceratohyal piece, with no segmen-
tation; it is broad and massive, yet sinuous, and articulates definitely
with the suspensorium beneath the orbitar process. A basi-hyo-
branchial plate separates the two hyoid moieties. There is no
iipper or columellar region at present There are four branchial
arclies, all confluent above and below, separated laterally by three
narrow oblique slits. The first arch is a large wide bag of cartilage,
with sinuous walls, through which can be seen radiating I'ows of
internal tufted gills. The second and third arches are comparatively
narrow; the fourth is much wider and somewhat like the first.

12 2

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Through the clefts can be seen the interdi^tating papilke from
which the dendritic gills grow.

419. In Tadpoles of Dactylethra at their largest size the
proportionate width of the hinder part of the skull has greatly
increased; the precranial valley has become filled with cartilage
in which the olfactory crura run ; and from the auditory to the
nasal sacs the floor of the skull is one wide sheet of cartilage (as
in Sharks and Bays), gently cfmvex in the middle, concave near the

Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 15 of 31)