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develope a low nasal septum which is continuous at its
upper edge with the nasal capsules. There are lateral

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processes at the sides of the fore end of the septum —
the trabecular horns ; and a median anterior downgrowth,
the prenasal cartilage.

445. The nasal glands lie right and left of the septum,
but on a lower plane, in the anterior part of the palate.
The whole nasal wall has chondrified ; seen from above
each nasal roof (na.) appears as a reniform cartilage, the
two being in contact back to back, and the external nasal
opening occupying the hilus. The nasal cartilages are
relatively large as compared with the trabeculse, and have
coalesced with nearly the whole extent of the nasal septum.

446. The posterior region of the basicranial cartilage
has two mammillary condyloid projections, one on either
side of the notochord ; and a little in front of these are
the anterior and posterior (or rather external and internal)
condyloid foramina. The chondrocranium is completed
in the occipital region laterally and superiorly. The
supraoccipital region (s. o.) forms a prominent boss in the
middle line behind, and anteriorly is shaped hke a
rectangular wedge fitting between the auditory capsules ;
but the union of the original supraoccipital moieties is
not yet complete, and their lateral union with the auditory
capsules is also imperfect.

447. The periotie masses have a remarkable tri-
angular shape ; the long base of the triangle is external,
while its apex is situated internally, at the fore part
of the supraoccipital wedge : the two capsules nearly
meet in the cranial roof. The front part of the capsule
diverges much farther from the middle line than the
hinder portion : it is also longer and reaches to the level
of the middle of the posterior basicranial fontanelle. The
semicircular canals are relatively very large and distinct;
the anterior and posterior meet by less than a right
angle at the inner (supraoccipital) part of the capsule;
and the ampullae are all well seen from above. The
great supracranial fontanelle is margined behind almost
entirely by ihe somewhat concave inner and upper edges

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of the auditory capsules, along which margin the large
anterior canals run.

448. Opposite the apex of the notochord in the lower
edge of each capsule the membranous labyrinth is budding
into a rudimentary cochlea lying just mesiad of the extern^
canal. Behind and above this swelling, on the infero-
lateral face of the capsule, is the fenestra ovalis, containing
a membranous plate which is chondrifying continuously
with the hyoid arch. A diverticulum can be seen growing
backwards from the fenestra ovalis; this is the commence-
ment of the fenestra rotunda, which afterwards becomes
separated from the former by a bar of cartilaga Behind
these and internal to them the glossopharyngeal and vagus
nerves are seen passing out in the boundary between the
otic mass and the occipital cartilage.

449. Bet;veen the out-turned anterior termination of
the basilar cartilage and the side- wall of the otic mass is a
large rounded notch, just opposite to the postpituitary
fontanelle and of similar size. Part of this notch is occupied
by a small independent ear-shaped cartilage, having a
narrow end forwards, and its edge convex inwards; this
is the alisphenoid cartilage (al a,). The orbitonasal branch
of the trigeminal nerve passes out between this cartilage
and the auditory capsule, and then forwards into the
orbit. The second and third divisions of the trigeminal
also pass out behind this alisphenoid cartilage.

450. The only chondrified tracts in the visceral arches
are found in the mandibular and upper part of the hyoid
(Fig. 48). The quadrate (j.) is segmented from the
meckelian rod, and lies foosely external to the upper and
outer side of the auditory capsule, its posterior otic process,
which is sub-bifid, nearly reaching to the posterior canal.
The part answering to the pedicle in the frog is a short
rounded lobe. The shaft of the quadrate is directed
forwards and downwards, and the condylar articulation is
on as high a level as the external semicircular canal. The
mandible (mk,) is a slender sigmoid rod, continuing the
line of the quadrate shaft, and having a long angular

B. M. 13

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process ascending behind the condyle. The two mandibular
bars nearly meet in the chin.

451. The whole of the hyoid arch Qiy,) is only about
half the length of the meckelian cartilage. It is a small
rib-like bar, with a bilobate proximal extremity similar to
that of the quadrate; and a curved distal arc turned
backwards. The proximal part is closely applied to the
infero-lateral surface of the auditory capsule, and the
hinder and inner of the two lobes coalesces with the stape-
dial plate filling the fenestra ovalis. The rounded outer
tubercle is free from the otic mass and is directed forwards.
The posterior region of the hinder part of the hyoid is
ready to be cut off as a free piece homologous with the
mandible ; it is separate a little later, and diverges further

452. At this period the chondrocranium of the Snake
is fairly complete, without bony deposit. The notochord
occupies only the hinder part of its original territory;
the trabeculse and parachordals have united, and while
union has taken place at two places behind the pituitary
body, there is a considerable posterior basicranial fontanelle.
An occipital arch and alisphenoid cartilages complete the
list of proper cranial elements. Anteriorly the trabeculae
are parallel for a considerable distance without uniting,
but in the nasal region they coalesce with each other and
with the cartilages of the nasal capsules: comual and
prenasal tracts are likewise present. The auditory capsules
have a notable triangular form, and already possess a
rudimentary cochlea. The mandibular arch is divided
into upper and lower pieces; the tissue in the maxillo-
palatine process has not chrondrified. The palatal surface
possesses median, submedi an, and lateral longitudinal thict
enings, which are nearly prepared for osseous deposit. The
simple hyoid bar is continuous with the stapedial plate in
the fenestra ovalis.

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Faurth Stage: Ernhryos about two and a half
inches long,

453. The head is more elongated, and much leas
monstrous. The cerebral vesicles are still well marked
from above, but not nearly so protuberant as before (Fig,
50). The jaws have lengthened correspondingly with the
rest of the head. The visceral clefts are entirely obliterated.
Ossification is considerably advanced in several regions.

454. The general form oi the basis cranii has altered
very little from that of the more advanced individuals of
the last stage. The fontanelle behind the pituitary body
is larger, and the space between it and the end of the
notochord is less than it was. The two moieties of the
occipital roof have completely coalesced. The cranial part
of the notochord lies on the united parachordals or basilar
plate, and is surrounded by a bony sheath which is spreading
into the substance of the cartilage right and left, forming
the basiocdpital (6. o.). In the cartilage on either side of
this is a bone (e. o.) pierced by the foramen for the
hypoglossal nerve and extending as far as that for the
vagus, which it is beginning to enclose. These exoccipitals
can be seen postero-laterally on the occipital roof. The
auditory capsules have altered their external shape very
little; they are still unossified. The diverticulum to form
the cochlea is a rounded bud extending downwards and
forwards below and in front of the fenestra ovalis. It
has no definite fenestra (rotunda) in the auditory wall as yet.

455. The small alisphenoid cartilage (oZ. 8.) is still
earshaped, and is placed somewhat longitudinally. Its
hinder dilated extremity is applied to the front of the
auditory capsule below, and its anterior end lies along the
edge of the basilar plate, behind the region of the pituitary
body. A notch in the hinder part of the alisphenoid
embraces the posterior division of the trigeminal nerve
(5), which passes backwards; and the remainder of the
trigeminal emerges from the cranial cavity in the space
between the concave notch of the front aspect of the


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basilar plate and the concave edge of the alisphenoid look-
ing inwards.

456. The postpituitary bridge of cartilage separating
the pituitary space from the basicranial fontanelle remains
as it was in the last stage ; the external carotid artery
pierces the cartilage on either side, and on the outer edge
of this region is a bony rudiment ossifying the cartilage,

Fig. 50.

Embryo Snake, about 2 J innhes long; side view of head, dissected;
several membrane bones having been removed.

(71, 2, 8, fore-, mid-, and hind-brain; oe.e. ocdpital condyle; t.o.
snpraoccipital cartilage;, h,8c., p,8c, anterior, horizontal and
posterior semicircular canals; al.8, ^sphenoid cartilage; o.«. orbito-
sphenoid cartilage; tr» trabecula; «.n. nasal septum; pn. prenasal
cartilage ; oL upper part of nasal capsule ; 2, optic foramen ; 5, trigeminal
foramina; /«o. fenestra ovalis ; st.h. stylohyid cartilage; co, columella;
mk. meckelian cartilage ; tg, tongue.

Bones: bu), basioccipital ; e,o. exoccipital ; 5.«. basisphenoid; ;>'
parietal; /. frontal; mx. maxillary; pa, palatine; pg, pterygoid;
t,pa, transpalatine ; q, quadrate.

the two ossifications coalescing later to form the hasi-
sphenoid (6. s.). The long uncoalesced portions of the
trabeculae (tr,) in front of the pituitary body remain
closely apposed; and between them is appearing a deli-
cate styloid bone, enlarged into a spatulate plate just m

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front of the pituitary body, beneath the broader part of
the intertrabecular space. This is the origin of the para-
sphenoid. The small infero-lateral granular tracts of the
cranial (orbitosphenoidal) wall above and in front of the
optic foramina have not yet chondrified. The nasal
cartilages have not changed in form, but the septum {$. n.)
is higher. The trabecular cornua are now indistinguishable
from the nasal capsules ; and the prenasal spur (p. n.) is
curved downwards and backwards in front of them.

457. On either side of the midbrain in its lower
region a thin film of bony matter is giving rise to the
parietal (p.) ; and the frontals (/.) are similar bony plates
on the sides of the forebrain, nearly meeting beneath the
cranial cavity, above the trabeculae. Also on the inner face
of each olfactory roof there is a thin shell of bone, the
naad; and these two shells lie back to back above the low
septum. In front of the nasal cavities a bilunate plate of
bone has appeared, the azygous pretnaxillary.

458. On the side and below the level of the nasal
septum is a large reniform nasal gland with a duct which
passes downwards and outwards. This gland is covered
above by a delicate film of bone, which enlarges inwards
into a vertical plate near the septum; this is the septo-
maxillary. Mesiad of the duct, the lower surface of the
gland on either side is invested by a bone, the vomer,
having a convex surface downwards. Two small (labial)
cartilages are attached to the duct of each nasal gland.

459. Along the anterior region of the palate is a
pair of bony styles lying external to the trabeculsB, and
partly underneath the nasal cavities. In its middle region
each of these palatine bones (pa.) sends inwards a process
(the ethmopalatine) towards the trabeculae, just opposite
the pointed end of the parasphenoid : it curves over the
posterior nasal passage, which opens in the middle of the
palate. A pair of slender gently-curved bones begin
immediately beneath the hinder end of the palatines, and
pass outwards and backwards to the inner face of the
quadrate cartilage. These are the pterygoids (|>5^.)* Neither

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palatines nor pterygoids arise in cartilage. In the margin
of the cheek and upper lip there is a splinter of bone
reaching from the outer angle of the premaxillary to
opposite the posterior end of the palatine — the maxillary
(mx,). Between the posterior end of the maxillary and
the middle of the pterygoid, lying very obliquely, is
another slender bone with a broad end towards the
maxillary : this is the transpalaiine [t pa,).

460. The quadrate, instead of being a rib-shaped carti-
lage in the same line as the meckelian, passing downwards
and forwards, is broad and fan- shaped, and its narrow lower
part is rapidly ossifying as the quadrate bone {q). The
curved upper edge is applied loosely to the side of the
auditory capsule, and is separated from it already by a
delicate sickle-shaped plate, the squamosal^ whose pointed
anterior end is attached to the fore part of the capsule.

461. The meckelian cartilage, now very elongated, is
gently sigmoid, and prolonged into an angular knob behind
the quadrate articulation. The rods of oppasite sides are
widely separated by connective tissue. Films of parosteal
bone have appeared in relation to it ; the dentary in front
and outside, forking behind, and exposing a considerable
tract of cartilage; behind it, applied to the outer face of
the jaw, is the surangular, reaching to the condyle. On
the inner surface there are three splints, enlarging from
before backwards — the splenial, the coronoidy the angular,
the latter reaching nearly to the angle of the jaw. Ectostosis
has arisen in relation to the cartilage near the articulation,
and extending into the angular process; this is the articular.

462. The backward direction of the hyoid arch is
now much increased, and the rod has become slender
and less curved (co.). The part resembling the tubercle
of a rib, instead of turning forwards, is directed back-
wards, and is applying itseLf to the posterior edge of the
stapedial plate. A lamina has been separated from the
hinder and upper edge of the bar, forming a distinct
stylohyal cartilage (st L), which is somewhat heart-shaped
with the. apex directed backwards.

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VI.] THE snake: fifth stage. 199

463. We have here no record of great transformation
in cartilaginous parts; and the principal changes of
relation are those by which the upper mandibular segment
and the hyoid arch are carried backwards, the former
becoming at the same time more loosely connected with
the brain-case. The bones which have appeared in
cartilage are the basioccipital, exoccipital, basisphenoid,
quadrate, and articular. The principal membrane-bones
are also present, the low lateral position of the frontals
and parietals, the azygous condition of the premaxillary,
and the occurrence of the septo-maxillary and transpalatine
being points of interest. The auditory capsule is as yet

Fifth Stage: Embryo Snakes near and up to the time
of Hatching.

464. The skull has made great progress towards its
adult conformation ; the brain-case is very straight, and
the original protuberances are scarcely distinguishable on
the upper surface. Ossification has extended rapidly,
but is still far from complete : many new centres are

465. The basioccipital has become a large heart-shaped
bone, the notochord with its bony sheath (cephalostyle)
still lying above it, though united with it ; the bone is
separated by considerable tracts of cartilage from the sur-
rounding elements. The pointed end of the basioccipital lies
backwards, and is flanked on either side by a tubercle of
cartilage, forming the two somewhat projecting lobes of the
median condyle. In front the bone is extending beyond the
original cartilage into the posterior basicranial fontanelle.
The paired basisphenoidal ossifications described in the
last stage have coalesced into one bone, ossifying the pre-
chordal (postpituitary) bridge of cartilage, and beginning
to extend backwards into the fontanelle. It passes for-
wards also on either side of the pituitary body, as far
as the posterior end of the parasphenoid, with which it
ultimately coalesces. The internal carotid arteries perforate

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the bone. There is no ossification of the trabecular
cartilage in front of the basisphenoid.

466. The exoccipitals have extended, so as to meet
by suture over the foramen magnum. The remainder of
the supraoccipital region is ossified by a distinct supra-
occipital J which is five-sided, broadest transversely between
the highest part of the auditory capsules. Its concave
anterior margin is the hinder border of the great supra-
cranial fontanelle : posteriorly it is conterminous with
the exoccipitals, and laterally with the epiotics. The
glossophaiyngeal nerve pierces the hinder end of the ear-
cartilage ; the vagus passes out between the capsule and
the exoccipital ; the hypoglossal pierces the exoccipital.

467. The auditory capsule has acquired its three
most constant ossifications. The eptotic lies at the side
of the supraoccipital, and occupies the most elevated part

, of the capsule, where the anterior and posterior canals
unite : it is the smallest of the periotic bones. The
opisthotic is related to the posterior part of the horizontal
canal and the lower part, with its ampulla, of the posterior
canal. It is of considerable vertical extent, and passes
downwards as a wedge between the fenestra ovalis and
the fenestra rotunda (of the cochlea), also passing behind
the latter so as almost to encircle it. Its anterior process
nearly meets the prootic beneath the fenestra ovalis,
over the neck of the cochlea, whose bulb is still unossified.

468. The rest of the capsule is enclosed by the
prootic y which includes most of the anterior and horizontal
canals with their ampullar, and surrounds the meatus
inteinus. This opening lies in front of the lower ray of
a triiadiate synchondrosis between the three periotic bones.
Beneath the two ampullae the prootic sends down a
wedge-like process into the floor of the cranium between
the basioccipital and the basisphenoid. In this wedge
there are three foramina, two large ones, anterior and
inferior, giving passage to the anterior and the posterior
divisions of the trigeminal nerve; and a smaller above

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between them and the meatus intemus, for the facial

469. External to this lower part of the prootic,
standing on the oblique, hinder, and outer edge of the
basisphenoid, and underpropping the anterior part of the
prootic, is a small four-sided bone which has ossified the
alisphenoid cartilage, as well as an additional band of
membrane bridging over the notch which previously
existed in the cartilage, (§ 455, p. 195). Through the
foramen thus formed the anterior portion of the tri-
geminal nerve passes after its exit from the prootic. The
concave hinder part of the alisphenoid is in front of
the posterior division of the trigeminal, which passes
almost directly backwards : the facial nerve is immediately
above this. There is one other cartilage bone in the
cranial wall, the m^bitosphenoid ; it is triradiate, wedged
in between the frontal and parietal, above and in front
01 the optic foramen.

470. The parietals, very solid and thick, now occupy
a large portion of the side-walls of the brain-case, and
reach inferiorly to the lateral edges of the basisphenoid
(anterior part) and the front of the alisphenoid and
prootic; they send backwards a horn along the upper
edge of each prootic and epiotic, reaching the antero-
lateral angle of the supraoccipital. In front of the anterior
ampulla the parietals also send a vertical ridge inwards
as a partial boundary between the midbrain and the
hindbrain. At present they have not roofed over the
brain-case, but simply lie in its sides. The frontals are
more complete above, and below nearly meet, above the
parasphenoid. Their side-wall is concave externally, the
parietals being convex.

471. The nasals extend much further over the nasal
capsules than they did; and the vomers and septo-
maxillaries are becoming solid, and taking the shape
which they possess in the adult. The small azygous
(edentulous) premaxillary has developed two distinct
palatal processes, one on either side of the median prq-

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nasal spur. The parasphenoid has grown large after
the pattern described in the last stage.

472. The palatines are hatchet-shaped : the blade of
the hatchet is very concave below where the bone arches
over the posterior nasal meatus. There is a small external
and a larger anterior process ; all these parts are related
to the ethmoidal region of the skull, (between the eye
and the nose). The handle of the palatine passes directly
backwards, and lies above the fore end of the pterygoid.
This latter is very much enlarged ; it is a spindle-shaped
bone extending semewhat outwards and far backwards
to the inner side of the quadrate ; its middle region is
broad, and supports the curious transpalatine, which is
forked anteriorly, receiving the posterior end of the
maxillary. This is now much elongated and dentigerous,
bounding the anterior half of the gape, and towards its
binder end having a horizontal expansion which supports
the inner fork of the transpalatine.

473. The squamosal is an oblong splint-like bone
attached loosely by its fore ^nd to the auditory capsule
over the anterior ampulla ; behind, it diverges backwards
and outwards. On its inner face is a much smaller
splint, the supratemporaL The handle of the fan-shaped
quadrate is much longer than it was, and its extended
upper edge is still cartilaginous: it applies itself over
and outside the posterior end of the squamosal. Thus
it has lost all direct connexion with the cranium, and is
also directed definitely backwards. The quadrate condyle
k now behind the level of the basioccipital region; it
forms a hinge-joint, with an angular process (like an
olecranon) behind, and a raised rim in front.

474. The posterior part of the meckelian cartilage
is well ossified by the articular ; anteriorly the cartilage
persists to the front of the jaw, covered on its outer
side by the dentary. In front of the middle of the jaw
the broad ends of two bones meet in a. vertical line : one
of these, directed forwards, is the splenial, the other,
lying behind, is the coronoid; both end by a point. A

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wuTow angular invests the lower edge of much of the
articular, extending back to the region of the joint. The
surangular is on the outer side of the same region, but
is twice as large.

475. The colmnella is now composed of a bony
stapedial plate and a shaft continuous with it, all but the
distal third of the original cartilage being ossified. The
stapedial plate itself is nearly straight behind, and convex
in front; convex externally and scooped within. The
ascending (tubercular) process of the shaft is above and
behind, the capitulum below and in front. To the curved
posterior half of the sigmoid shaft a thick subcrescentic
[stylohyaV) bone is attached. It has ossified all but the
extremities and the free under edge of its cartilage : and
it is very tubercular and rough. Its outer face is attached
to the inner surface of the quadrate, and both it and the
columella extend behind the posterior edge of that bone.

476. The condition of the skull above described
requires careful attention to details for its full compre-
hension ; the growth of the parietals and frontals is
especially noteworthy. The anterior halves of the trabecular
remain unossified; but supraoccipital, pro-, epi-, and
opisthotic, alisphenoid, and orbitosphenoid centres have
appeared. The quadrate condyle is now carried behind
the basioccipital region, and it is connected with the side
of the cranium by the intervention of the squamosal, — a
very novel feature. The columella and the stylohyal have
become ossified.

The Shdl of the AduU Snake.

4iJ7, The cranial investment constitutes an exceed-
ingly strong box, the bones being united by even sutures,
sometimes slightly squamous. The bony substance is very
dense, almost like ivory, with very little diploe; and a
large number of the sutures that were present at the time
of hatching have disappeared. The bones of the face are

Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 17 of 31)