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portion being the pterygoid. Beneath the intertral)ecular
space and the hinder parachordal part of the trabecular
is a median lanceolate parostosis, the parasphenoid, form-
ing a support for a considerable portion of the cranium,
and lying in the roof of the mouth.

258. The primary relations of the chief cranial nerves
are distinctly manifest in this stage. Instead of the large
anterior ganglionic mass formerly described (§ 231, p. 96),
there are now two gangliA at the fore end of the auditory
capsule, in contact with the posterior margin of the tra-

1 This relation is precisely similar to that of the same processes
in Birds.



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106 MORPHOLOGY OF THE SKULL. [CHAP.

becular crest. The first or Gasserian (5) gives off three
branches, constituting the trigeminal nerve. The orbito-
nasal branch passes forwards beneath the ascending process
of the suspensorium. The other two pass over the sanae
process lower down, gently diverging as they pass out-
wards and forwards.

259. Beneath the Gasserian ganglion, and well seen
on the under surface of the skull, is the ganglion of the
facial (7) ; the two principal nerves given off from it are
(1) the anterior or Vidian, passing forwards heneath the
ascending process, below and a little external to the orbito-
nasal ; (2) the facial, directed outwards and backwards,
and then forwards, skirting the auditory capsule, and
crossing beneath the otic process of the suspensorium. On
the inner side of the auditory capsule behind, wedged
between the occipital parachordal and the periotic car-
tilage, is the ganglion of the glossopharyngeal nerve, and
the vagus passes out close behind it. The auditory nerve
enters the capsule behind the facial ganglion. The optic
nerve pierces the trabecular crest near its middle. The
olfactory cms passes over the anterior part of the trabecula,
where it gives off the lateral horn.

2G0. This stage is marked in the chondrocranium by
the development of the proper occipital parachordals, by
the complete chondrification of the ear-capsules and the
formation of the fenestra ovalis, and by the union of the
trabeculse anteriorly and posteriorly. The internasal plate
and trabecular cornua become distinct. The mandibular
suspensorium has coalesced by its ascending process with
the sphenoidal wall (rf the cranium, and possesses a dis-
tinct otic process. The ossification of the apex of the
notochordal sheath is very interesting; other new bony
developments (parostoses) are the paras phenoid, parietals,
frontals, squamosals, and articulars ; the nasal processes
of the premaxillaries, and the pterygoid portions of the
palatopterygoids.



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IV.] THE axolotl: fifth stage. 107

Fiftk Stage: Young Axoloth of\\, 2 J, and 3J inches long^.

261. In the following account, observations made at
three different periods of growth are combined for the
sake of brevity. The changes described occupy several
months, and are of great importance. The most notable
phenomena are the growth and straightening of the tra-
beculse, the formation of the stapes, the appearance of a
palatopterygoid series of cartilages, and the subdivision
of the palatopterygoid bone.

262. The cartilaginous bridge formed by the tra-
beculae in front of the apex of the notochord increases
considerably ; and the cartilage gradually creeps over the
notochord above and below, so as nearly to enclose it.
The anterior half of the cranial notochord becomes en-
sheathed by the growth of the bony matter at its apex
(Fig. 26, c. St.), described in the last stage (§ 251, p. 103).

263. The parachordal portions of the trabeculaB unite
completely with the occipital parachordals ; and the whole
basilar cartilage thus constituted coalesces with the
periotic capsules. The occipital condyles are formed,
immediately external to the notochord, in A, Lateral
occipital walls arise from the hinder part of the basilar
cartilage, in continuity, however, with the ear-capsules,
but extending some distance behind them. These curve
inwards above, and have coalesced to form an occipital
roof in B (Fig. 27, s. o.). In the same stage a small patch
of bone is developed around the foramen of exit of the
glossopharyngeal and vagus nerves (Fig. 26, e. o.). Later
on, this exoccipital bone grows into the base of the con-
dyle, which has increased in size and appears shortly
pedunculated.

264. The trabeculse lengthen considerably, so as to
outstrip the other cranial structures in growth. Thus the
intertrabecular* space becomes a parallelogram with rounded
angles. The trabecular crest, or sphenoidal wall, is con-

- 1 These will be referred to as J, B, and C.



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108 MORPHOLOGY OF THE SKULL. [CHAP.

tinuous behind with the periotic cartilage over the Gasse-
rian ganglion ; in front it extends to the nasal region. A
transverse section in any part where the crest is developed
presents a crescent standing erect with the convexity out-
wards ; there is no distinct basal part : the base of the tra-
becula does not develope beyond its first condition, and
consequently it becomes insignificant by the greater growth
of other parts. Thus the cranium has its roof and base
very largely membranous, while its lateral walls are com-
pletely cartilaginous.

265. Instead of the intemasal plate projecting a little
beyond the lateral horns of the trabeculae, these latter
grow into considerable fan-shaped lobes (c. tr,), which by
their inward approximation bound three-fourths of a cir-
cular space in front of the intemasal plate. This circle is
completed in B by the premaxillaries (Fig. 26, px.) growing
transversely to meet each other.

266. A new structure appears in the nasal region.
The nasal sac is floored in front by the trabecular comua,
but its roof is formed above and behind by a distinct cres^
centic cartilage (Fig. 27, o/.). The superior nasal opening
lies in the concavity of the nasal cartilage, but is narrowed
by membranous folds. In G the trabecular horns have
grown very much, and the capsular cartilages have united
with them by their anterior and inferior edges, the hinder
part remaining free.

267. In By the cleft which is to form the fenestra
ovalis in the periotic capsule, is more lateral than in the
earlier stages ; and the irregular cartilage previously de-
scribed on its antero-internal margin has developed into
a broad thin lamina which forms a lid qj^er the fenestra.
This lamina is connected by a broad isthmus antero-exter-
nally with the general periotic cartilage, but it is com-
mencing to be segmented off (Fig. 26, st). In G this is
completely separated as the stapes, lanceolate in shape,
with the point behind. From its outer surface a fibrous
ligament passes to be inserted into the hinder and under



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rv.] THE AXOLOTL: FIFTH STAGE. 109

surface of the otic process of the suspensorium : this is the
suspensorio-stapedial ligament.

Fig. 26.



p



Young Axolotl, 2^ inches long ; under Tiew of skoll, dissected, the

lower jaw and gill arches having been removed.

nc. notochord ; oc.c, occipital condyle ; /.o. fenestra ovalis ; tX, stapes;

tr. trabecolar cartilage; i.n, internal nares ; ctr. coma trabecala; yd,

pedicle of suspensorium ; q. quadrate region ; under j^. outline of ptfriy-

goid cartilage ; 5', orbitonasal nerve ; 7, facial nerve.

Bones : 'pas, parasphenoid ; c,%%» oephalostyle, part of the para-
sphenoid having been removed to shew it ; t.o, exoccipital ; v. yomer ;
j>a;. premaxillary ; mx, maxillary ; pa. palatine ; pg, pterygoid.

268. The quadrate extremity of the suspensorium (9.)
gets progressively turned outwards, making the mouth
broader. The ascending process continues coalesced with
the cranial wall ; and a lower process in the same region,
the pedicle (Fig. 26, pd.), becomes distinct as a rounded
bud which articulates with the antero-inferior edge of the
auditory capsule. Above the condylar facet for the man-
dible a considerable mass of bone arises in the suspenso-
rium, forming the quadrate Bone.

It is worth notuig that this bone and the exoccipital, while
arising at first as ectostoses, rapidly metamorphose the deep
cartilage and appear very like true endostoses.

269. In A there is no pterygoid process from the
anterior edge of the suspensoiial cartilage: but in the



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110 MORPHOLOGY OF THE SKULL. [CHAP.

antorbital region a small transverse rod of cartilage has
kppeared, attached by its inner end to the trabecula some-
what behind the internal, nares, above the broad tooth-
bearing part of the palatine bone. In B the suspensorium
sends forward from the middle of its anterior border a large
(pterygoid) tongue of cartilage, gradually narrowing to its
termination half-way between its origin and the nasal
capsule. In front of this is a distinct but much smaller (post-
palatine) cartilage, tending to attach itself to the antorbital
rod. The latter has now a broad base towards the tra-
becula, and narrows as it proceeds outwards and forwards,
nearly reaching the posterior angle of the maxillary bone.
In C the small post-palatine cartilage has completely
coalesced with the fore end of the pterygoid process, and
is attached by a short ligament to the antorbital.

270. The parasphenoid in A extends from the hinder
end of the intemasal plate to the basioccipital region.
Opposite the apex of the notochord it gives oflf a small
angular basitemporal process directed outwards. Later,
the parasphenoid (Fig. 26, pa. s.) has become broader and
longer, underlying the intemasal plate to a considerable
extent, and behind reaching nearly to the extremity of the
skull. Still later, the apex of the shrunken cranial noto-
chord is found lying in a groove on the upper surface of
the parasphenoid, aiid the bony matter which surrounded
the notochordal apex cannot be distinguished from that
bone.

271. The vomers in A and B are oval plates directed
forwards and inwards, close to the fore end of the para-
sphenoid. In (7, harmonising with the greatly increased
breadth of the front parts of the skull, the vomers have
become elongated, narrow, and subcrescentic, the concavity
looking backwards. They extend from the emargination
of the front of the intemasal plate to the inner side of the
internal nares.

272. In A the pterygoid (hinder) part of the palato-
pterygoid bone is much elongated, being proportionately
broader behind, where it partly underlies the quadrate



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IV.] THE AXOLOTL: FIFTH STAGE. Ill

region of the suspensorium. In B the dentigeroiis pala-
tine {pa,) is nearly segmented from the edentulous ptery-
goid ( pg-)* They tend to separate obliquely, so as to over-
lap one another by a blunt point. In C the separation is
complete, and on the right side a space intervenes between
them. The axes of the two bones are coincident, and the
vomer continues the line, but with a curve inwards. In B
the hinder part of the pterygoid is still broader, and
imderlaps the quadrate more and more : but in C it has
become extended almost to the hinder margin of the
suspensorium.

Fig. 27.



Toang Axolotl, 2J inclies loDg ; upper view of bkull ; lower jaw
removed.

nc. notocbord; oc.c. occipital condyle; s.o. snpraoccipital cartilage;
a.g.c.t p'.s.c. anterior and posterior semicircnlar canals; c.tr, cornu trabe«
culae ; ol. nasal capsular cartilage ; a.o, antorbital cartilage ; pt.pa. post-
palatine cartilage ; pp. pterygoid process ; a.p. ascending process of sus-
pensorium ; beneath it, 5', the orbitonasal nerve ; 5, hinder part of tri-
geminal nerve.

Bones : p. parietal ; /. frontal ; px. premaxillary ; pa. palatine ; the
pterygoid is partially seen under pg. ; q. quadrate ; sq. squamosal.

273. In A the premaxillaries do not meet at the
middle line; in B they meet; in C their nasal processes
have become broad. A slight maxillary style, bearing
teeth, appears in Ay continuing the arcuate line of the



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112 MORPHOLOGY OF THE SKULL, [CHAP.

premaxillaries, so that the four bones (two on each side)
form nearly a semicircle. In (7 a small ascending plate is
found on the upper margin of the maxillaries, lying out-
side the capsular cartilage of the nose. Attached to this
process is a small bone lying on the nasal roof, the ecteth-
moid ; and between this and the nasal process of the pre-
maxillary, is another small bony plate, the nasal. The
external nostril is now bounded, in front by the pre-
maxillary, externally and below by the maxillary, behind
by the ectethmoid and nasal, and internally by the nasal
and nasal process of premaxillary. This relation is sub-
stantially maintained in subsequent stages,

A bone in a precisely similar position to the ectethmoid
here descrihed arises as an ectostosis in the Salmon.

In the Axolotl and Urodeles generally it appears to be
always a parostosis,

274. The frontals (Fig. 27,/.) and parietals (p.), about
equal in size, continually enlarge, gradually covering the
cranial fontanelle; and in B and G the parietals reach
back to the supraoccipital cartilage. They also send out an
external process which strongly clamps the auditory capsule
close behind the anterior ampulla and the exit of the two
hinder divisions of the trigeminal nerve. The squamosal
(Fig. 27, sq,) becomes much broader above, where it spreads
over the otic process and the antero-extemal part of the
auditory capsule. Below, it extends in a pointed shape
nearly to the quadrate hinge. The bones of the mandible
thoroughly invest the meckelian cartilage. There is no
change of importance to notice at present in the remaining
arches.

275. The conclusion of this series of changes leaves
the chondrocranium fairly complete. We have the basilar
plate formed out of trabecular and occipital elements, and
the occipital investment united with the periotic car-
tilages. The cranial cavity is bounded by cartilaginous
side walls derived from the trabeculae, while the para-
sphenoid and the frontals and parietals protect the brain,



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IV J THE ADULT AXOLOTL. 113

where it lacks a chondrified floor and roof. In front of the
cranial cavity is a thick internasal plate, with large tra-
J>ecular horns, and a cartilaginous nasal roof. The forma-
tion of the stapes, at first in continuity with the periotic
cartilage, and afterwards separating from it ; the develop-
ment of a considerable pterygoid process from the sus-
pensorial cartilage ; and the appearance erf" antorbital and
post-palatine cartilages, are the remaining principal events
of chondrification. The exoccipitals and quadrates are the
only bones which are at present transforming cartilage.
The maxillaries, ectethmoids, and nasals are new parosteal
centres. The transformations which the vomers, squa-
mosals, and palatopterygoids pass through are of very great
interest.

ITie Skull of the adult Axolotl

276. Comparing this form with C in the last stage,
the changes that have occurred are seen to be on the
whole simple, principally consisting in the addition of car-
jbilage-bones, and alterations in the relative size of parts
during the increase (about threefold) of actual dimensions.
Thus the precranial region, including the internasal plate,
nasal capsules, and trabecular comua, has increased to
twice its former relative size ; whilst the auditory capsules
are not much more than half as large proportionally ; and
the same is the case with the basilar cartilage. The latter
is now only about one-sixth of the length of the skull :
while in the fourth stage (§ 250) the parachordal car-
tilages are one-half its length. The new Dones developed
are the sphenethmoid, the prootic, the pterotic, and the
stapes: all paired bones.

277. The cranial cartilage and its bones will now be
described in detail. The chondrocranium as a whole may
be characterised as an elongated flattened box, deficient in
the middle below, and in a large portion of its length
above. In front it bears a much broader flat nasal ex-
pansion; while behind, its breadth is extended by the
r9.ther small ear-sacs, to which the outspread forwardly-

B. M. 8

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114 MORPHOLOGY OF THE SKULL. [CHiP.

directed suspensoria of the mandible are firmly fixed. A
bar on each side, parallel to the axial box, extends from
the front part of the suspensorium to the hinder region of
the nasal expansion.

278. The notochord is not visible on an external exa-
mination of the cartilaginous basilar plate. The. groove
for its reception on the upper surface of the parasphenoid
has disappeared, and the latter comes clean away from
the cartilage. The basilar plate as it passes into the pre-
chordal region consists of more solid and massive cartilage,
and is continuous with the cranial walls on either side
(Fig. 28, tr.). In their anterior third the cranial walls are
converted into solid bones, which widen anteriorly, and
in front of the cranial cavity are thrust like thick wedges
into the sides of the intemasal plate, to one-third of its
extent; thus we have a pair oi sphenethmoid bones {G),

279. The projecting occipital condyles are somewhat
reniform, and their cartilaginous articular face is principally
inferior. They are largely ossified by the exoccipitals {e. o.) ;
but a basioccipita,l region, equal in width to the exoccipi-
tals, remains unossified. Tbese bones occupy the side walls
of the hindmost region of the skull, and come nearer to-
gether above than below. The obliquity of the foramen
magnum causes the supraoccipital plate to have a scooped
margin behind ; its front outline is also concave forwards*
The whole of the broad supraoccipital region except the
small part occupied by the exoccipitals behind, is entirely
unossified ; as is also a projecting angle in the auditory
cartilage on each side, over the junction of the anterior
and posterior semicircular canals ^tlie epiotic region).

280. On the inferior surface of the ear-capsule, mesiad
of the fenestra ovalis and not reaching to it, is a shell-like
opisthotic plate of bone, semidistinct from the exoccipital,
underlying the main portion of the vestibule, and extend-
ing partially behind it. On the front region of the vesti-
bule below, a narrow tract of bone lies transversely, parallel
with the scooped surface in which the pedicle of the sus-



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IV.]



THE ADULT AXOLOTL.



11.^



pensorium fits. This is part of the prootic ossification
(Fig. 29, pr. o.), which is more largely seen from above,
fitted, shell-like, over the neck of the anterior ampulla, and
running down on the antero-intemal face of the ear-capsule.
Above, it grows forwards along the upper edge of the cranial
side-wall, nearly reaching to the sphenethmoid. This may
be called the alisphenoidal spur of tlie prootic.

Fig. 28.




Adult Axolotl ; under view of skull, the lower jaw and arches being
removed, and also the investing bones on the right side.

b.o. basilar cartilage ; tr, trabecular cartilage ; i.n. intemasal plate;
<.«./. sttspensorio-stapedial ligam^it ; p. pedicle, a. ascending process,
0. otic process, pt. pterygoid process of suspensorium ; the antorbital
cartilage is not shown.

Bones : e.o. exoccipital ; o.c. occipital condyle ; pa.s, half of para-
sphenoid; g, sphenethmoid; pmx, premaxillary ; mx. maxillary; vo.
vomer ; pi. palatine ; pt. pterygoid ; qu, quadrate ; sU stapes.

281. The contiguous ampullae of the anterior and
horizontal canals are partially uncovered by bone; but
behind this bare space, covering the horizontal canal and
most of the roof of the capsule, is a massive bone in the

8—2



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116 MORPHOLOGY OP THE SKULL. [CHAP.

cartilage, corresponding to tbe pterotic of th^ Salmon.
Above, it articulates by its antero-intemal angle with the
prootic, and behind with the outer half of the front margin
of the exoccipital. It extends through the substance of
the external longitudinal ledge (pterotic ridge) of the cap-
sule. The cartilage is bevelled inwards below this ridge as
far as the stapes {st)y which is now a somewhat cunred
plug of bone with its broad end inwards, fitting into the
fenestra ovalis, and its narrow end looking outwards and
forwards. From its narrowed extremity there extends a
strong fan-like ligament, which spreads to be attached
anteriorly to the posterior margin of the suspensorium
(suspensorio-stapedial ligament, s.sA.). Thus we have,
almost perfectly distinct, four bony elements in the ear-
capsule, prootic, pterotic, opisthotic, and stapes. The sphen-
otic and epiotic regions have no ossification ; and below,
the lower part of the prootic and the outer part of the
opisthotic tracts are also unossified. In all, about two-
thirds of the superficies of the capsule is bony.

282. The broad flat intemasal tract of cartilage (t. n.)
occupies about one-third of the longitudinal axis of the
skull, and its breadth is considerably greater than this. It
has a concave margin before and behind. The hinder con-
cavity lodges the fore part of the brain ; the anterior is
the emargination separating the broad trabecular cornua.
The latter are subquadrate, and project further forwards
from the intemasal plate than in the last stage. They
are thinned away from below forwards, terminating by a
sharp edge.

283. The height of the intemasal plate behind is
equal to that of the fore part of the cranial cavity ; it
is scooped in such a manner that the superior cranial fon-
tanelle extends further forwards than the inferior fon-
tanelle. Each sphenethmoid bone sends down a process
which covers the upper surface of the cartilage bounding
this inferior fontanelle in front, and these processes nearly
meet The olfactory crura pierce the shelving front waU
of the cranium (in the intemasal plate) just on the inner



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IV.] THE ADULT AXOLOTL. 117

surface of the spfaenethmoids near their upper edge. The
nasal capsules are relatively twice their size in the last
stage. The cartilage is more than crescentic, the two horns
of the crescent nearly meeting externally; and the internal
part of the cartilage is broadly confluent with the inter-
nasal plate and trabecular comu. The external nares
(Fig. 29, a. n,) are bounded in front by the incurved ante-
rior cornu, and posteriorly by a valvular fold of fibrous
tissue.

284. The broad parasphenoid {pcua,) underlies al-
most the whole of the skull. Its sides are nearly parallel
in the greater part of its extent ; but it sends out a small
process on either side in the basitemporal region; and
each antero-extemal angle projects prominently so as to
touch the inner extremity of the corresponding vomer.
The latter crescentic bone {vo) is just behind the median
notch terminating the basicranial axis: it extends out-
wards and backwards to the fore part of the palatine. The
anterior bones (premaxillaries and maxillaries, pmx. mx,)
being apposed, form a very regular semicircular margin to
the mouth, having no palatal plates ; the arcs of the two
vomers are parts of another semicircle behind this, and
almost concentric with it.

285. On the upper surface of the skull, the premaxil-
laries send a long nasal process backwards, overlapping the
frontals. The two processes converge, but do not meet, part
of the internasal cartilage being visible between them. In
front of the cartilage, the emargination between the lateral
comua is left open as a round membranous space, bounded
anteriorly by the apposed premaxillaries. The maxillary
bones have also a short nasal process extending upwards and
backwards, attached to the anterior and outer end of the
ectethmoid. Thus the superior narial opening is bounded
on one side by the nasal process of the premaxillary, on
the other by the corresponding process of the maxillary.
Above and behind, the ectethmoid (2. eth.) and the nasal
bound the nostril, the latter bone being wedged in between
the former and the nasal process of the premaxillary.



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118 MORPHOLOGY OF THE SKULL [cHAP.

286. The frontals (Fig. 29, fr) appear to be larger
than the .parietals {pd-), but the latter are considerably
overlapped by the frontals: they really reach nearly to
the antorbital region. The frontals are partly covered in
front by the nasal processes of the premaxillaries, by the
nasals, and by the lateral ethmoids. The frontals and pari-
etals together form a nearly regular oblong, much resem-
bling the parasphenoid, but broader.

The parietals just in front of the auditory capsule are
curved downwards on the side walls of the cranium, and
their lateral boundary lies alongside of the alisphenoidal



Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 10 of 31)