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The morphology of the skull online

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parietals. The squamosals are long, lying on the side of the ear-
capsules and running down on the suspensorium, griving off a long
process over the facial nerve to reach the stapes. There are no nar
sals or maxillaries; the dentary part of the premaxillaries is very short
Behind these ate the two large and long vomers, with dentigerous
outer margins. The palatopterygoids resemble those of the larval
Axolotl three quarters of an inch long ; they are broad spatulas in
front, with a dentigerous tract carrying on the line of the vomers ; they
end at the middle of the inside of the suspensorium by a blunt point:
there is no pterygoid cartilage. In the mandible cartilage is largely
persistent ; the ossifications are dentary and articular ; there are no
.splenials, but the dentary ossifies a tract of cartilage in front after
the manner of the meutomeckdian of the Frog.

The Skull of Siren.

312. In Siren there is a postpituitary trabecular bridge of carti-
lage, but behind it the parachordals are widely separate, forming a
triangular posterior fontanelle ; they are scarcely united even in the
condylar region. The supraoccipital arch is of considerable extent ;
the exoccipital bones (which include an opisthotic region) nearly meet
above, and pass partially into the cranial floor below. The auditory
capsules are largely ossified, especially by the prootics, which extend
into the cranial roof, apd to a small extent into its floor. In the
top of the capsule is a large permanent aqtiediictus vestibtdi; the
stapes is considerable, but unossified. In front of the ear-masses
the cranial walls are steep, with cartilage behind continuotis with the
auditory capsules and the postpituitary bridge. From near the.tri-
geminal foramen to the middle of the internasal region the cranial
walls are ossified by sphenethmoids, solid and broad in front, high
behind; they do not unite with one another, but the olfactory crura
perforate them. The internasal plate is short and broad, ending in
three triangular processes, preuasal and comual. There is a pair of
supraethmoid cartilage-bones. The nasal capsule has a small carti-
lage in its posterior boundary; and the ethmoidal region has a

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considerable antorbital attached to it, and connected by ligament with
a large sphenotic process from the front of the auditory capsule : the
small eyeball is also fixed to the antorbital

313. The premaxillaries extend on to the upper surface of the
skull outside the supraetbmoids : there are frontals and parietals,
but neither nasals nor ectethmoids. The maxillaries are minute;
the vomers are long and arcuate, with many teeth in seyeral rows,
which are continued on the short palatine; there is no pterygoid
ossification or cartilage. The parasphenoid is much like that of
Proteus. The short broad suspensorium is wholly unossified; it
is directed nearly outwards, the condyle looking somewhat for-
wards; the pedicle passes into the trabecular cartilage beneath the
trigeminal nerve, and there is an ascending process above its
orbitonasal division. The squamosal lies upon the suspensorium,
and has no backward process over the &cial nerve as in Proteus.
The lower jaw is of great depth, with a solid dentary and articular,
and a considerable meckelian cartilage extending nearly to the middle
of the ramus ; the splenial is a very delicate long dentigerous spicule.
The hyomandibular is a small oval cartilage attached by ligament to
the stapes, the suspensorium, and the angle of the jaw. The large
hyoid bar below is covered with a bony sheath in its lower three-
fifths : there is no hyiK>hyal. The long first basibranchial is largely
ossified ; it carries, in its hinder part, two pairs of branchial arches,
the first large, with nearly the lower half ossified ; the second with a
ceratobranchial piece bearing three short cartilaginous epibranchials.
The second basibranchial is a remarkable bony mass^ termin;itiug
behind in three flat wings of bone*

The Skull qf Menopoma,
314. This is a veiy flat broad skull, with comparatively little
cartilage persistmg in it. The exoccipitals pass far forwards, ossi-
fying the hinder part of the ear-capsule. The prootics are distinct
and large in front : the region surrounding the vestibule where the
stapes fits is unossified. The hinder part of the cranial wall near the
auditory mass is cartilaginous, but there is a strong sphenoid bone
from the ethmoidal region to near the trigeminal foramen. The inter-
nasal region is flattened, and there are alinasal cartilages over the
large nasal sacs; the external nares are small, and far forwards. The
dentary margin of the upper jaw is very extensive, and semicircular


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iu shape. In the middle line aretwo triangular premaxillaries with
a amall round supraethmoid between them in front. Two wedge-
shaped nasals are related to the external nostrils, while oyer the
upper edge of each nasal sac lies an irregular lateral ethmoid.
Between this bone and the nasal the frontal curves forwards to the
maxillary close to the nostril ; it extends far backwards in contact
with its fellow between the parietals ; which stretch from the eth-
moidal region nearly to the foramen magnum.

315. The maxillaries occupy a much greater region of the gape than
the premaxillaries. Both these bones haye palatal plates, and within
them are the broad flat vomers, having dentary regions parallel with
the external series of teeth. The parasphenoid extends partially
between the vomers, and passes back to the end of the skull as a broad
flat solid bone with basitemporal wings. The suspensorium has a
large oblong quadrate ossification with a condyle looking forwards.
Internally and above is a long tract of cartilage passing beneath the
trigeminal nerve and coalescing with the unossified trabecular carti-
lage close behind the sphenoid bone. Another tract of cartilage
(ascending process) passes upwards over the orbitonasal and vidian
nerves, runs into the front of the prootic, and is even ossified from
it The squamosal is oblong, directed outwards, with a short pos-
terior process. The palatine and "pterygoid are represented by one
big flat no9-dentigerous bone, stretching far forwards, touching by its
forward angle the ethmoidal cartilage, and by an outer fork nearly
reaching the maxillary. The palatopterygoid narrows behind, passing
sharp and wedge-like to the inner and hinder part of the quadrate. The
lower jaw is composed of very soh'd bones, the dentary being pre-
dominant ; its teeth fit between the maxillary and vomerine series
above. The dentary is grooved for the little persistent meckelian
cartilage, which is covered internally by the thick and large spleniaL
The articular is small ; and the condyloid facet ovaL

316. The large stapes is well ossified, and attached by its an-
terior third to a thick triangular cartilage fitting against the suspen-
sorium and lying over the facial nerve; by this relation it is shown to
belong to the same category as the cartilage which forms the tympanic
annulus of the Frog. Behind the quadrate, attached to it at a
deeper level, is the pyriform hyomandibular, tied by its point to the
main hyoid cartilage which is massive, ossified ^t its upper third, bent

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outwards, and nnossified in its lower portiou. Mid-yeDtrallj there
are three pairs of basal cartilages, one pair only in contact, with a
small median piece projecting backwards. The first basibrancliial is
a broad leaf of cartilage in front, and a thick stalk behind, where it
bears two branchial arches, of which the first, unsegmented, is ossi-
fied in its middle third. Nearly the whole of the second branchial
bar (attached to the extremity of the basibranchial) is ossified, and it
bears at its lateral termination three more branchial bars, all more or
less ossified. All that remains to indicate a second basibranchial is
a pair of small cartilages applied to the larynx.

Tie SkuU €f MenobranchuiK

317. The adult skull, which is much depressed, has no proper
cartilaginous flour: the low side walls are cartilaginous in front of the
auditory c^ipsules: an antorbital cartilage is united by fibrous tissue to
the cranial wall, which is continuous with an ethmoidal, intemasal, and
prenasal tract, extending to the end of the snout. The conical ex-
tremity of the notochord is embedded in fibrous tissue in the basi-
occipital regioa The suspensorium is inclined downwards and forwards
at an acute angle with tJie cranial axis ; it has a pedicle continuous
mth the cranial wall in front of and below the level of the exit of the
trigeminal nerve; an ascending process above the orbitonasal branch
of that nerve ; and an otic process lying above the main facial nerve.
There is a short triangular pterygoid process on the middle of the
front edge of the suspensorium. The meckelian cartilage is largely
persistent, stout behind, tapering forwards. The hyoid arch includes
a small hypohyal and a long straight ceratohyal, united by ligament
to the posterior edge of the suspensorium, to the angle of the mandi-
ble, and to the stapes. The first basibranchial bears posteriorly at
an acute angle with its axis a pair of ceratobranchials, which them-
selves bear three other branchial pieces from their hinder ends, an
outer large and much curved, a middle bar less curved, and an inner
nearly straight, attached to the last. These are called epibranchials,
by Prof Huxley, a little nodule at the base of the middle piece beinif
denominated ceratobranchiaL An osseous second basibranchial,
rod-like, projects from behind the connection of the two principal
ceratobianchials with the first basibranchial.

1 Huxley, Proc, ZooL Soc, 1874, p. 186.

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318. The cartilage bones in the skull of Menobranchns are
(1) the exoccipitals, (2) the epiotics, ossifying also the opisthotic
region ; they are conical caps of bone distinct from the ezoccipitals,
separated by cartilage from (3) the prootics, covered above by the
parietals ; (4) the quadrates, in the distal ends of the suspensoria ;
(5) the second basibranchial. The hyoid and branchial arches are
uuossified. The parostoses are (1) the parietals, of remarkable
shape; broad behind, covering almost the extremity of the exocci-
pitals and epiotics ; meeting in the middle line by suture, passing
forwards between the frontals ; extending forwards also outside the
frontals as far as to the antorbital region by a rod-shaped process which
supfiorts the side wall of the cranium ; and having another short
process lying on the ascending process of the suspensorium ; (2) the
frontals; they project posteriorly in the angle between the median and
lateral regions of each parietal (united by suture in the greater part
of their extent) ; they send a small spur l>etween the nasal proce^se8
of the premaxillaries, and are also extended far forwards outside
those processes ; further, they surround the olfactory foramen and
reach behind it to the antorbital cartilage ; (3) the premaxillaries,
l)Oun^ng the gape, having a narrow dentigerous part not reaching the
internal nares, and a nasal process ascending at an acute angle with
the rest of the bone on the surface of the skull ; (4) the vomers, with
a dental series parallel to the premaxillary teeth, broad behind and
within the teeth ; separated mesially by the anterior region of the
parasphenoid, and abutting externally against the palatopterygoids ;

(5) the parasphenoid, partly underlying the exoccipitals and the ear-
capsules behind, and in front of them wider than the cranial cavity ;

(6) the palatopterygoids, flattened plates, dentigerous in front, not
widening behind where they underlie the suspensorium ; (7) the
squamosals, long slender curved bones reaching from the epiotic
region along the hinder face of the suspensorium to the condyle,
with a backward process from the middle towards the stapes ; (8) the
dentaries, as long as the jaw, having the meckelian cartilages lying
within them in a grooved shelf; (9) the splenials, much smaller, but
also dentigerous.

The Salarnandrine Skull.

319. In high Salarnandrine forms, such as Salamandra, Triton,
and ISeironota, nearly the whole cranium consists of solid bone.
There is a tract of cartilage between the prootic and the sphenoidal

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wall ; with a prootic postpituitary bridiire in Salamandra but not in
Triton. The exoccipitals are large, and nearly meet above and
below. The antorbital cartilage has coalesced with the large olfac-
tory capsules. The suspensorinm has its pedicle attached by fibrous
tissue to the cranium, while the large ascending process has coalesced
with it. There is a small pterygoid process of cartilage : the ptery-
goid bone, which is small and outwardly directed, fits closely to the
pedicle, forming a joint with the basb cranii and the auditory cap-
sule. Usually the quadrate is ossified, the otic process unossified.
The palatines are long and dentigerous, coalesced with the yomers ;
they reach nearly to the auditory capsule, and undergird the edges
of the parasphenoid. The vomers, maxillaries, and premaxillaries
are broad, forcaiug the broad anterior part of the palate ; the pre-
maxillaries may be anchylosed. The ectethmoids and nasals are of
considerable size, and there are frequently septo-maxillaries. The
frontals in some (e.g. Seironota) have strong supraorbital ridges.
The lower jaw has largo dentaries and articulars, with nanrow
dentigerous splenials. An ossified (spiracular) leaf behind the
sospensorium occurs in Desmognathus and Spelerpes. The stapes
ia ossified : there is no hyomandibular ; a considerable upper tract
of the hyoid arch is bony, and the two remaining branchial arches
are more or less ossified. The second arch loses its epibranchial
piece entirely. Mostly the remnants of the primitive long second
basibranchial are found attached to the larynx.

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First Stage: Embryos from two to three lines long, about
the time of hatching,

320. In these embiyos the cerebral vesicles are nei-
ther so large relatively, nor so prominent as in forms pre-
viously described. The upper surface of the head is not
especially broad; it is gently convex, and curves down-
wards in front. The first vesicle occupies the fore part
of the head; but is partly surmounted by the second;
thus a mesocephalic flexure is established. The frontal
cerebral prominence is separated from the face by a dis-
tinct transverse elevation of the skin, extending from the
upper border of one eyeball round to that of the other.
Below this frontal ridge, and above the oral opening, is
the broad nasofrontal process, emarginate in the middle
line Laterally the outer margins of the nasofrontal
process bound the olfactory rudiments. Immediately
above and behind these, and projecting more externally,
the primitive eyeballs are seen as similar involutions, with
a rim deficient in front and below. In the same lateral
line, at some distance behind the rudiments of the eyes,
are the ear-masses, elongated oval in shape, and covered
with a continuous integument.

321. The mouth is a small lozenge- shaped depression
behind the nasofrontal process; bounded in front b}' its
right and left horns and median emargination, and he-
hind by the rounded prominences of the mandibular
tracts. The swelling of the cheeks is due to the presence

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of a mass of cells surrounding the fore part of the primary
embryonic cavity. This aggregation of cells gives rise
to most of the mucous membrane of the mouth and pha-
rynx, and passes into the cutaneous system a little way

Tadpole four lines long, foor or fiye days after hatching ; side yiew
of head.

ol. olfactory sac ; e. eyeball ; Ip, maxillopalatine process ; m. month ;
el. 1 to 6, visceral clefts ; op. opercnlnm (the line is carried a little too
far) ; 6r. 1, 2, external branchisd ; cp, clasper.

within the oral cavity. Below and behind the cheeks are
the sucker-like organs called claspers (cp.), which together
form a horse-shoe shaped prominence by which the little
tadpole adheres to water-weeds.

322. The lateral facial wall presents four vertically-
placed elevations, one behind another, occupying the tract
between the mouth and the yelk-mass of the body-
cavity : but there is no complete fissure between any of
them. The first or mandibular is the largest, being
swollen by the cell-mass in the pharynx; it is very
strongly turned inwards below to the median ventral line.
The succeeding arches recede more and more from the
median line beneath, and are only well marked laterally.
The hyoid arch has no distinctive feature ; but each of
tbe two branchial arches behind bears a small papilla,
the rudiment of the external gills. Behind the branchial
arches on either side there is a cleft opening into the body-

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323. On dissection, the gelatinous notochord, enclosed
in a distinct sheath, is found underlying the hinder part of
the cranium, hardly reaching to the fore part of the third
vesicle. Immediately in front of the apex of the notochord
is the relatively large pituitary body ; and a considerable
part of the frontal region of the cranial cavity is below the
line of the notochord. The mesocephalic flexure, however,
very early begins to be obliterated ; in embryos only two
days after emergence from the egg, much progress has
been made in the straightening of the cranial floor.

324. Small patches of mesoblast, approaching a carti-
laginous consistency, are found by the sides of the apex of
the notochord ; these soon become identified with the tra-
beculse. The parachordal patches are much smaller and
less definite than the series of rapidly chondrifying paired
rods which begins in the fore part of the cranial floor and

Fig. 32.

BnLSjr ^

Embryo Frog, just before hatching ; side view of head, with skin


Na. olfactory sac ; e. involution for eyeball ; au. auditory investment;

tr. trabecula ; wn. mandibular, hy. hyoid, hr. 1, first branchial, arches ;

the gill-buds are seen on the first two branchial arches ; L labial car*


is continued in the ridges in the side walls of the mouth
and throat. But the first pair, the trabeculse {tr,), are
distinguished from the others by their forward direction.
The visceral arches, although the first two are parallel
at their proximal ends with the trabeculae, descend
vertically in the wall of the throat, and turn somewhat
backwards below. The trabeculae and the arches are all

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absolutely distinct, ending proximally by blunt points. The
former are shorter and thicker than the latter, and lie in
the base of the prechordal part of the cranium. The oral
opening is included between the anterior extremities of
the trabeculae and the mandibular arches.

325. The visceral rods resemble one another very
considerably, the two foremost, however, being stouter
and extending farther both proximally and distally than
the branchials ; nevertheless, the mandibular and hyoid bars
of opposite sides are separated by some distance in the
middle line below. The third branchial is very slender,
and the fourth is but indistinct. The heart in its peri-
cardium occupies the ventral, line of the throat between
and below the distal ends of the branchial arches. Proxi-
mally all three branchial rods approach closely to the apex
of the notochord.

326. Only one of the sense-capsules, the auditory,
has a wall of condensed tissue. This is continuous except
on the supero-extemal face, where there is an area of soft
cells : but there is no channel between the interior and
the exterior. Two granular elements are found in the
upper and lower lips. One of these rudiments of labial
cartilages is found beneath the fore end of each trabecula;
and another below and internally to the distal end of each
mandibulai- rod (fig. 32, L).

327. In the oldest tadpoles of this stage the meso-
cephalic flexure is almost unobservable ; the distal clubbed
ends of the mandibular bars are preparing for segmenta-
tion ; the rudimentary tongue lies between and upon the
ventral ends of the right and left hyoid arches ; and the
gill-buds have enlarged.

328. This stage presents us with an exceedingly sim-
ple view of the primary elements of the skull. Para-
chordals, trabeculae, mandibular, hyoid, and branchial
arches are all separate from one another; the auditory
investment is solidifying ; and labial rudiments exist. No
palatopterygoid region or arch is yet discernible.

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Second Stage : Tadpoles from 5 to 6 lines long.

329. At this period the external gills are at their
fullest development, having tertiary papillaj ; the opercu-
lum has not yet covered and enclosed them. The brain-
case has greatly diminished relatively to the face, and the
component parts of the brain lie nearly in a straight line.
The sense-organs are subequal in size, the ear-sacs and
eyeballs not acquiring such a preponderance over the
olfactory organs as in the embryos described previously.
The fore part of the face has grown very much faster than
the branchial region; and the mouth is now a rounded
cavity of considerable size.

330. Parachordal cartilages extend from the apex
of the notochord to the first cervical vertebra ; they are
very early found perfectly continuous with the trabeculae,
which become nearly horizontal instead of being placed at
an angle of 45* with the notochord. The trabeculae form a
commissural union in front of the notochord and behind
the pituitary body; and a second immediately in front of
the same organ; the intertrabecular space is rounded.
The anterior confluence extends to the intemasal region,
where the fore ends of the trabeculae diverge in the form
of broad cornua, lying within and in front of the inferior
nares (already distinct at this early stage) and their con-
nection with the olfactory sacs (Fig. 33,

331. The auditory capsules are well-chondrified ovoid
masses lying in the hinder cranial region, having the
mandibular arch directly in front of them, and the
branchial arches immediately behind and beneath. They
are as yet distinct from other cartilaginous elements, but
the mandibular arch is clinging very close to them. When
the tadpoles have attained a length of about seven lines,
a slit appears on the infero-lateral face of the auditory
capsule, just mesiad of the projection of the external
(horizontal) canal. This slit widens into an irregularly
lanceolate space, and is filled with delicate cells of in-
different tissue; by the time the tadpole is eight lines

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v.] THE frog: second stage. 141

in length this has chondrified to form the simple stapes,
lying in the fenestra ovalis which has thus arisen. No
lurtber development in connection with the stapes takes
place until a month or more after metamorphosis is com-
plete (see pp. 160, 161).

Fig. 33.

Tadpole of Common Toad, one-third of an inoh long ; cranial and mandi-
bular cartilages seen from aboye ; the parachordal cartilages are not
yet definite.

ne. notochord; m.«. mnscnlar segments; au. auditory capsule; py. in
region of pituitary body ; tr, trabecula; e,tr, eomu trabecule;
pidatopterygoid conjunction; pd. pedicle, q, quadrate condyle^ ml,
meckelian piece of mandibular arch ; s.o.f. subocular fenestra ; u,L upper
labial cartUage. The dotted circle within the quadrate region indiiMttes
the position of the internal nostrlL

332. The mandibular arch has become greatly en-
larged, and much broader ; and its distal end is segmented
off as a small free meckelian bar (mk.), which is directed for-
wards and upwards in the lower lip at an acute angle with
the main part, which may now be called the mandibular
suapenscyrium. In its proximal fourth, the suspensorium is
almost at a right angle with the trabecula, and is confluent

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with it. It then turns forwards at a somewhat lower
level, and is roughly parallel with the trabecula, a meta-
branous space, the subocular fenestra, intervening. More
anteriorly, just behind the nasal sacs, the suspensoriuoi is
united with the trabecula by a bar similar to the hinder

Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 12 of 31)