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The morphology of the skull online

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tinuous with the posterior forks of the nasal cartilages in
front, and shelves down laterally to the ethmopalatine
region, forming lateral ethmoidal or antorhital plates.
There is no cartilage in the side wall of the cranium in
the region of the tegmen; but each of its postero-
external angles is continuous with a slender rod of carti-

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lage developed in the supraorbital band, which curves
outwards and backwards to join the fore part of the
auditory cartilage (Fig. 17. s, or.). Thus there is a large
heart-shaped membranous fontanelle in the roof of the
chondrocranium, (/o. Fig. 15), bounded in front by the
tegmen (which being produced somewhat backwards in
the middle line causes an emargination in the fontanelle),
laterally by the supraorbital bars and the ear-masses, and
posteriorly by the occipital cartilage.

146. No part of the chondrocranium is ossified in
this stage: but several parostoses have appeared. The
chief of these is the azygous parasphenoid (Fig. 16, pa, $,),
a flat bone lying underneath the cranial floor, extending
from the nasal nearly to the hinder part of the audit.ory
region. It is slightly forked both in front and behind.
A delicate supraethmoid bone overlies the intemasai
septum, and there is a styloid frontal ossification over
each lateral edge of the tegmen. Just beneath the front
extremity of the trabecular plate is a rudiment of the
premaxillary on each side.

147. The first important modification to be noticed
in the facial parts is the coalescence of the primitively
distinct elements, the palatine and the proximal mandibu-
lar segment. The palatine or suborbital cartilage joins a
forward growth from the mandibular, so as to become
expanded behind as well as in front, remaining slender in
the middle. The much enlarged upper mandibular seg-
ment forms the fore part of the wall of the pharynx below
the level of the cranial cavity, and is no longer directly
connected with the skull. Its chief relation to the chondro-
cranium is derived from the arch behind, the upper
and forward portion of which is closely applied to it.
Thus there is now but one continuous cartilage on each
side, extending from the ethmopalatine trabecular process
in front to the hyomandibular behind (Fig. 17). Two pairs
of small nodular cartilages, the upper labials {l^, ZJ are found
in front of the palatine cartilages, underneath the fore
part of the trabeculae. The elongated articulo-meckeliau

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rod, or cartilage of the lower jaw, articulates with the
condyle on the proximal mandibular piece, and is produced
into a thick angle behind the condylar hollow.

Fig. 17.

Salmon fry, second week after hatching ; side view of skull ; parostoses,
eyeballs, and nasal sacs removed.
T.Cr, tegmen cranii; S.Or. supraorbital band; Fo, superior fonta-
nelle; Au. auditory capsule; Pa.Ch. parachordal cartilage; Ch. noto-
chord; Tr, trabecula; above the trabecula, the interorbital septum is
seen, passing into the cranial wall above and reaching the supraorbital
band; II. optic foramen; F. trigeminal foramen; 2. 1, 2, labial cartilages ;
PIPL palatopterygoid bar; M.Pt, metapterygoid tract; Qu. quadrate
region; Mck, meckeUan cartilage; H.M, hyomandibular cartilage; Sy.
symplectic tract; I.Hy, interhyal; C.Hy, ceratohyal; H.Hy, hypoliyal;
G.H}f. glossohyal ; Br, 1. first branchial arch.

148. Bony rudiments are now to be found in relation
with various parts of the mouth. The first is the palatine,
an anterior ectosteal lamina on the palatine cartilage ;
behind this, internally and superiorly, is another ectosteal
lamina, the mesopterygoid ; the proper pterygoid is
embracing the lower edge. In the connective tissue out-
side the pterygoid region is a parostosis, the commencing
maxillary (Fig. 16, mx.). In the lower jaw there is a
ientary parostosis (d.) lying all along the outer edge ; and
near the condyle, on the under edge and inner face, is an
ectostosis, the articular {ar,).

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149. The hyoid arch has grown considerably, but its
essential relations remain the same as in the last stage
(§§ 134, 135)* The hyomandibular (h.m.) is broad above,
where it articulates with the side wall of the ear-capsule,
and styliform below (Fig. 17, sy.), where it is carried for-
wards in contact with the hinder edge of the mandibular
arch, and actually becomes wedged between the quadrate
condyle and the angular process of the articular. The
interhyal cartilage (i. hy.), which connects the hyo-
mandibular with the remainder of the hyoid arch, is
attached to the middle of the hinder edge of the hyoman-
dibular, and curves inwards below, so as to carry the arch
forwards within the two moieties of the lower jaw. There
is a large ceratohyal cartilage (c. Ay.)> flattened above, and
knoblike anteriorly (ventrally), where the hypohyal (h, hy)
fits on to it. The two hypohyals are in close contact,
and bear the median basihyal or glossohyal {ff. hy.), which
projects forwards into the tongue.

150. The branchial arches are still unossified, but
are completely segmented, the pharyngobranchials or
upper segments approaching one another beneath the
floor of the skull and the anterior vertebrae. The prin-
cipal lateral pieces are the epibranchial above and the
ceratobranchial below. The basibranchials have begun
to fuse together.

151. In this most important phase the skull of the
Salmon is elaborately constructed as to cartilage, and
is yet of a shape and structure very distinct from its adult
condition : it answers considerably to the chondrocranium
of several Ganoid types. The occipital ring is complete
superiorly but not inferiorly ; for the basilar plate is not
yet constituted, and a very noticeable space separates
its moieties anteriorly, where the notochord no longer
extends. The fore part of the skull has undergone an
extraordinary development, but is destined to be much
modified by later growth. Yet in certain distinctive
features the adult condition is substantially attained,

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namely, in the elevation of the brain considerably above
the trabecular floor, in the formation of a cartilaginous
cranial roof connected on each side with the periotic
investment, in the development of lateral ethmoidal or
antorbital, and mesonasal growths, which are excellent
landmarks in the study of the skulL

152. The jimction of the palatine bars with forward
expansions of the upper mandibular segment is the chief
event to be noticed in regard to the appendicular pieces.
The relations of the hyomandibular element are further
perfected, and the branchial arches are segmented. Some
membrane ossifications have appeared in proximity to the
axial parts ; the parasphenoid beneath most of the cranial
floor, the premaxillaries in the front part of the roof of
the mouth, the supraethmoid over the nasal septum,
and the frontals at the sides of the cranial tegmen. The
palatine, pterygoid, and mesopterygoid ossificationa are
especially related to the palatopterygoid cartilages, while
the maxillaiy bones arise externally to them. The
dentary and articular bones are found in the lower jaw ;
the hyomandibular, hyoid and branchial pieces are still

Fourth Stage: Toung Salmon of the first Summer.

153. During the growth of the young Salmon from
an inch and a quarter to two inches and a quarter in
length, many of the characters which remind us of Ganoid
fishes are effaced, while those which are specially Telostean
become manifest The cartilaginous skull is much more
massive, and the tegm£n cranii covers the brain to a large
extent; furthermore, many of the cartilage-bones (ect-
odoses) are present. The fore part of the skull has developed
so greatly that the nasal sacs appears only as small re-
cesses on either side of it. The tooth-bearing vom^r is now
found in addition to the parasphenoid, being the inferior
splint of the fore part of the skull. The yelk-sac is entirely
taken into the abdomen, and the tissues generally have
become much more perfect and solid.

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154j. There is no great change to be noted in the
cartilages of the hinder part of the skull. The supra-
occipital cartilage extends forwards as far as the level of
the junction of the anterior and posterior semicircular
canals of the ear. The interior cranial surface of the
auditory capsule is incompletely chondrified, the ampullae
and the greater portions of the arches of the anterior
and posterior canals, as well as the vestibule itself, not
being separated by cartilage from the cranial cavity.

Fig. 18.

Young Salmon of the first summer, about 2 inches long ; side view of
BkuU, excluding branchial arches. The palato-mandibular and hybid
tracts are detached from their proper situations, a line indicating the
position where the hyomandibular is articulated beneath the pterotic

01, olfactory fossa;, trabecular comu; nl\ uP*, upper labial
cartilages ; p,8, presphenoid tract ; tegmen cranii ; s.o.b, supraorbital
band; fo, superior fontanelle; n.c. notoohord; b.o. basilar cartilage;
tr. trabecula ; p.c, condyle for palatine cartilage ; 6, trigeminal foramen ;
7a. facial foramen ; 8, foramen for glossopharyngeal and Vagus nerves ;
mh meckelian cartilage ; op.c. opercular condyle.

Bones : e.o. exoccipital; s.o. supraoccipital ; e,p. epiotic; pt.o, pterotio;
sp.o. sphenotic ; op. opisthotic ; pro, prootic ; b.8. basisphenoid ; aKs.
alisphenoid; o.s, orbitosphenoid; 2.g. ectethmoid or lateral ethmoid;
pa, palatine; pg, pterygoid;, mesopterygoid ;, metapterygoid ;
qu, quadrate; ar, articular; h;m, hyomandibular ; sy. symplectic; i.h.
interhyal; ep.h. epiceratohyal ; c.h, ceratohyal; h.h. hypohyal; g.h. glosso*
or basihyal. . ,

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155. Ossification arises around the notochordal sheath,
laying the foundation of the basioccipital ; and this bony
deposit extends into the surrounding cartilage. A supra-
occipital ectostosis (Fig. 18, 8, o.), and a pair of ossicles,
the exoccipitals {e. o.), in the cartilage bounding the
foramen magnum laterally, complete the hindmost group
of bones in the investment of the brain.

156. Several centres of ossification are present in
connection with the auditory cartilage. There is one on
its antero-inferior margin, behind and above the main
part of the trigeminal nerve, and perforated by its posterior
branch; this is the prootic (pr, o.); a second, the sphen-
otic {sp 0,), is above the ampulla of the anterior semi-
circular canal ; a third, over the ampulla and arch of the
horizontal canal, is the pterotic {pt o.) ; a fourth, the
epiotic (ep,), is above the arch of the posterior canal ; and
a fifth (op.), over the ampulla of the same canal, is the
opisthotic. The sphenotic, pterotic, and opisthotic form
a series on the outer edge of the auditory capsule, over-
hanging the articulation of the hyomandibular ; while the
epiotic extends inwards at right angles to this line at its
posterior extremity. A remnant of the primordial fenestra
between the ear-capsule and the basilar cartilage still

157. The anterior region of the skull has become
remarkably modified. The trabeculae are now separated
only in a very small space surrounding the pituitary
body. Anteriorly to this they have completely fused and
become massive, and have sent up a median ridge in
the interorbital septum, occupying its lower part, below
the level of the optic foramen. The upper part of the
same septum is also chondrified continuously with the
upper part of the anterior cranial wall or median eth-
moidal region, forming a presphenoidal spur (p. «.). Part
of the lateral cranial wall in the interorbital region is
also cartilaginous, in continuity with the tegmen cranii
and the supraorbital tract; while a further portion is
chondrified in connexion with the fore part of the auditory

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capsule, and the hinder part of the supraorbital bar. The
tegmen cranii is thicker and extends farther back over
the midbrain.

158. An ectosteal lamina is found in the fore part of
each side wall of the cranium, extending downwards on
either side so as to press upon the presphenoidal carti-
lage, and upwards to embrace, by a grooved edge, the
downgrowth from the tegmen cranii: this is the orbito-
sphetwid (o. 5.)* The cartilage of the cranial wall in
front of the ear-capsule above the optic and trigeminal
nerves undergoes ossification, and forms the alispkenoid
{al. 8.), Inferiorly it comes into relation with an ectostosis
in thef hinder part of the trabecular crest, lying in the
interorbital septum and projecting backwards in the
middle line into the pituitary space. Thus there is
formed an almost vertical prepituitary basisphenoid bone
(6. «.), Y-shaped, so as to abut against the lower edge of
each alisphenoid. Immediately behind this bone the
pituitary body descends to reach the parasphenoid. There
is no cartilaginous floor at this point : on either side of
the pituitary body one of the internal carotid arteries enters
the cranial cavity. The basisphenoid is partly furnished by
direct ossification of the membranous interorbital septum.

159. The sloping ethmoidal region is high mesially
where it is confluent with the vertical intemasal septum
and the cranial roof, and descends gradually on either side.
An ectethmoid ectostosis arises in the lateral ethmoidal
region (2. e.) near the palatine articulation. The inter-
nasal cartilage is high and massive behind, and rapidly
descends to form the somewhat triangular beak. The
olfactory sacs open laterally by a wide aperture narrowed
by membrane, and are otherwise completely surrounded
by cartilage. Thus the trabecular floor, comparatively
naiTow in the interorbital region, is broadly expanded
in the nasal tract. There are no ectostoses to be noticed
here; but the vomer underlies the intemasal cartilage,
and bears teeth : the supraethmoid plate is its counter-
part above.

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160. The following are the bones related to the
chondrocranium at this stage. Ectostoses: posteriorly,
basioccipital, exoceipitals, supraoccipital ; in the auditory
region, prootic, sphenotic, pterotic, epiotic, opisthotic; near
the pituitary body, basisphenoid, alisphenoids ; related
to the forebrain, orbitosphenoids ; in front of the brain-
cavity, ectethmoids. Parostoses : superiorly and in front,
parietals and froutals, supraethmoid ; inferiorly, para*
sphenoid, vomer ; in front, premaxillaries. The form and
relations of the parasphenoid have become very compli*
cated, owing to the growth of the orbital muscles between
it and the cranial floor behind, the formation of the basi-
temporal wings, and of a median crest between the tra-
becular moieties anteriorly. The fully-developed para-
sphenoid will be described later.

161. Ossification has advanced greatly in the facial
bars, concurrently with modification in the shape of the
cartilages. The cartilage in the pterygoid region has be-
come broadeir and more massive than in the hinder part
of the arch. Cartilage has also extended in front of the
ethmopalatine connection, so as to constitute a pre-
palatine region. The ectostoses are, antero-extemally, the
long Btylif arm pcUatine (^a.), bearing teeth ; behind this,
a small pterygoid below (pg^), and a much larger meso-
pterygoid (m. pg,) internally and above ; and in the hinder
region, abutting on the hyomandibular, a metapterygoid
[mt, pg.) on the cranial side, and a quadrate (qu.) in the
neighbourhood of the condyle for the lower jaw. In the
latter the articular bone has increased very much in
length. The condylar surfaces, both on the quadrate and
articular cartilages, are unossified. The only parostoses
to be noticed are the ma^llary in the edge of the upper
jaw, and the jiyal on the posterior end of the maxillary,
the dentary flanking the meckelian cartilage outside, and
a veiy small angvlar on the angular process.

162. The hyomandibular cartilage remains of mode*
rate size, and is related as' before to the pterotic ridge.
There is a broad hyomandibular ectostosis above, which

fi.M. 5

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extends partially into a cartilaginous knob on the hinder
edge, bearing the opercular bone {op, c). 'The lower styli-
form portion of the hyomandibular cartilage acquires a
8t/mplectic ossification (si/,), which fits into the grooved
hinder edge of the quadrate, thus firmly clamping one
arch to the other. Several parostoses have arisen in the
opercular flap ; but they will be suflBciently described in
the adult skull.

163. The ectostoses in the remaining part of the hyoid
arch are the interhyal above, the eplceratohyal {ep. A.) in
the upper part of the undivided cartilage formerly named
ceratohyal, and the large ceratohyal in the lower part of
the same cartilage; finally, the small hypohyal (arising
by two ossitic centres), and the basi- or glossohyal, which
now bears teeth. The branchial arches are completely
formed ; but their description may be reserved until the
adult skull is dealt with.

The Skull of the adult Salmon.

164. The most important difierences between the
Salmon's skull when a few weeks old and when adult may
be briefly referred to, preliminary to a full description.
The cranial roof and side walls become almost completely
cartilaginous, except where the cartilage is replaced by
ectostoses, or where nerves pass out. The cartilage of
the cranial roof, however, is covered by large parostoses,
conspicuously by the frontals. The precranial region of
the skull has greatly increased in relative length, and
forms a massive cartilaginous beak, extending consider-
ably beyond the nasal sacs. No new ectostoses arise in it
In the base of the brain-case it is noteworthy that the
two orbitosphenoids unite, forming a floor to the anterior
part of the cranial cavity, above the presphenoidal car-
tilage. Another most interesting fact is the growth of
the prootics into the basilar cartilage, and their union to
form part of the base of the cranium.

165. The mouth is almost completely margined by
p«ostoses, which are mostly dentigerous : the ectostoses

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iil] the adult salmon. 67

lie within. The orbit has nearly an entire ring of fibrous
bones. The hyomandibular and metapterygoid regions are
much more massive than they were, increasing the depth
of the framework of the head, and throwing the articula-
tion of the lower jaw farther from the axial parts. The
opercular skin-fold developed from the upper part of the
hyoidarch is provided with a complete series of membrane-
bones ; while a similar membrane proceeding from . the
lower part of the same arch has acquired a large number
of branchiostegal rays. The same thing in miniature has
happened to the branchial arches.

166. The cranio-facial apparatus of the Salmon is
very compUcated. Cartilage is largely persistent in it, and
of the various bones some are but little connected with
the cartilage, some closely invest or encircle portions of
it; while others are massive, although directly continuous
with cartilaginous tracts by their edges. The bones in
the chondrocranium are not very large, and the axial
structures, cartilages, and ectostoses constitute but a small
portion of the entire mass, though the size and outward
extension of the membrane-bones applied to the axial
parts make the latter appear more considerable than they
are. The perfection of the jaws and gill-arches is attained
by a large and highly complex apparatus with many parts
curiously combined. Ectostoses are reinforced by invest-
ing hones laid down in fibrous tissue; yet much cartilage

167. The cartilaginous skull, with its intrinsic ossi-
ft^tions, consists of an elongated brain-case or cranium,
having openings where nerves pass out, or where the
brain is continuous with the spinal cord ; of a strong but-
tress on either side of the brain-case behind, enclosing the
organ of hearing ; and of a precranial mass, nearly as long
as the cranial box, in which the organs of smell are em-
bedded. Thus the auditory region of the chondrocranium
is quadrate as seen from above or below, and is connected
with the precranial mass by a narrow isthmus, at the sides
of which theorbits lie. In front of the isthmus^ the fore


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part of the cartilage is roughly trihedral, with the base
behind, its lateral parts formiDg the antorbital walls. An-
teriorly the cephalic cartilage ends in two short blunt
processes or comua, with an emargination between.

168. On its hinder aspect the skull is lozenge-shaped,
the pterotic bones occupying the lateral angles, and the
supra- and basioccipital the upper and lower angles.
Between the basioccipital and each pterotic are an exoc-
cipital {e. o.) and an opisthotic {op. o). The epiotics {ep, o)
are on the upper edge, between the supraoccipital and
either pterotic. Much cartilage remains in the ear-capsule
and above the foramen magnum, which is relatively small,
and almost entirely surrounded by the exoccipitals.

169. Each exoccipital contributes a supero-lateral facet
to the single occipital condyle, so that the latter is three-
fold, derived from the basioccipital and two exoccipitals.
Between the concave surfaces of the condyle and of the
first vertebra (which are lined with articular cartilage),
there is a mass of pulpy tissue, which is a notochordal
remnant ; but cartilage and bone have closed up the cavity
in the base of the skull, in which the notochord was for-
merly contained. The pterotics, the epiotics, and the supra-
occipital have rounded bosses upon them which project
backwards for the attachment of muscles.

170. Inferiorly, the median region of the skull pro-
jects as a thick double ridge, considerably below the peri-
otic masses; thus the latter overarch a large concavity
on each side, which is walled in externally by the hyo-
mandibular and other facial elements. The median ridge
referred to includes a bifurcated downgrowth from the
basilar cartilage, enclosing a longitudinal channel in which
the orbital muscles take origin ; they diverge right and left
from the anterior end of the channel, on either side of the
basisphenoid. The two laminsB are considerably ossified
by the basioccipital behind, and by the prootics in front
{h, 0., pr.o,y Fig. 20) : while underneath, the channel is
floored by the parasphenoid parostosis (pa. 8.) The under

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surface of the concavity at either side of the skull, beneath
the overarching periotic mass, shows the prootic bone

Fig. 19.

Adnlt Salmon ; lateral view of chondrocraninm with its eotosteal bones ;

the paroBto&es, jaws, and branchial arches having been removed. The

eontinuons cranial cartilage is dotted.

C.Tr. trabecular comu ; 2M*. labials ; Na. nasal fossa ; 11, optio
foramen ; V, trigeminal foramina in prootic bone.

Bones: B,0, basioccipital ; E.O. exoccipital ; Op, 0. opisthotio;
Pr.O. prootic; PtO. pterotic, bearing a cartilaginous facet for the
hyomandibalar, and overarching the concavity in which the preceding
bones are seen; Ep.O. epiotic; S.O, supraoccipital ; Sp,0. sphenotic;
is. alisphenoid ; O.S. orbitosphenoid ; L.Eth, ectethmoid.

anteriorly ; behind this, externally, the pterotic, and the
small opisthotic ; internally, the exoccipital. The extreme
forwardly projecting outer angle is formed by the sphenotic
(% 19).

171. On the external lateral margin of the auditory
region there is the prominent ridge of the pterotic bone,
continued forwards by the sphenotic: underneath the
projecting ledge is the linear longitudinally extended
articular surface for the hyomandibular. .The inner
portion of the ear-cartilage, between the basioccipital
and prootic below, and the supraoccipital above, is quite
unossified, and to a considerable extent the labyrinth and
canals are even unclosed by cartilage on their cranial

172. The prootics have become considerable bones,
extending upwards for some distance in the side wall of
the cranium, and al30 invading the basilar cartilages even

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to the middle line, so as to unite more or less with each
other. They grow down on each side in the wall of the

Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 6 of 31)