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basicranial channel, but do not ossify it to its greatest
depth. The trigeminal nerve (5) passes out of the cranium
in front of the prootic, deeply notching it ; its posterior
division enters a canal in the same bone, through which it
reaches the exterior. The facial nerve penetrates the
prootic in front of its middle region, and passes directly
outwards and downwards. The auditory nerve perforates
the prootic by a large foramen behind the facial nerve ;
the intracranial foramen for the glossopharyngeal and
vagus (9 and 10) is in the same line, between the prootic
and the exoccipital; its external orifice is in the exoc-

173. The anterior part of the investment of the
cranial cavity is of more massive cartilage, much less

Fig. 20.

Adnlt Salmon : median longitudinal section through skull, after remoTal
of jaws and arches.

0. (7. occipital condyle ; «.t./. ethmo-trabecular fissure; Z^. Z*. labials;
m.n.c, mesonasal cavity; II. optic foramen; V. trigeminal foramen;
VII. foramen for -the facial ; IX. X. foramen for the glossopharyngeal
and vagus.

Bones: £.0. basioccipital, Pr.O. prootic, each bone having a de-
scending lamina in the wall of the basilar canal, which is floored by the
parasphenoid, Pa.S. ; the exoccipital, E.O., and the prootic, enter largely
into the side wall of the cranium ; Pt.O. posterior extremity of pterotic,
seen outside the cranium ; S. O. supraoccipital ; B.S. basisphenoid ; A.S.
alisphenoid; 0,S. orbitosphenoid ; Fr, frontals; S.Eth. supraethmoid ;
Vo, vomer,

ossified than the hinder part, and the cavity itself is
considerably smaller proportionallyr Both roof and floor

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are very thick. The interorbital septum is almost entirely
cartilaginous, extending from just beneath the cranial
floor to a level as low as the basilar crests enclosing the
orbital muscles : it includes tlie original trabecul» and
their highly developed crest. It is a thick dividing wall
rather than a mere septum.

174. From the level of the prootics in the basilar
floor, down to the base of the hinder end of the inter-
orbital septum, there is placed a curved basisphenoid bone
(Fig. 20, b. s,), with an arm diverging on either side into
the lateral cranial walL Thus the bone is Y-shaped.
The pituitary body occupies the space immediately
behind ; it is let down between the arms of the Y, and
behind the long leg, to rest only upon the parasphenoid
bone. The line of the basisphenoiaal arms is continueil
by the alisphenoids (a. 5.), which are considerable and
thick bones occupying a large portion of the lateral cranial
wall between the orbitosphenoids and the prootics, and
having the sphenotics on their outer flank, though sepa-
rated by a tract of cartilage.

175. There is only one cartilage-ossification in the
cranial wall in front of the basisphenoid : this is a bone
which may be called orbitosphenoid (o. «.), but which
ossifies more than the orbitosphenoidal region. It con-
tains in a hinder cavity a portion of the forebrain, having
the olfactory crura lying on the floor of the cavity, and
passing forwards in the substance of the bone. Thus the
orbitosphenoid forms the floor and sidewalls and a small
part of the roof of the anterior part of the cranial cavity,
having beneath it the interorbital septum ; and it also
ossifies a considerable portion of the hinder ethmoidal
region. The optic nerves pass out behind and below the
orbitosphenoid on either side, in a gaping chasm between
it and the basisphenoid (2j.

176. The cranial roof is formed of a thick mass of
cartilage, which instead of being gently arched as in
previous stages, is trihedral, having a sharp median longi-

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tudinal ridge 6r culmen cranii above, and rests laterally
upon the orbitosphenoid and the alisphenoids. The
tegmen cranii is complete as far as the supraoccipital bone,
which extends forwards to about the middle of the audi-
tory tract. Iq the alisphenpidal region the tegmen has
sharp external angles; behind this it is somewhat con-
tracted between the auditory masses, and leaves a pair of
small oval parietal fontanelles in the cranial roof just in
front of each antero-external angle of the supraoccipital,

377. The etlimoidal, internasal, and prenasal regions
of the skull have attained an extraordinary development,
quite overshadowing the original nasal septum and sacs.
The trabecular comua (Fig. 19, c. tr.) have grown out in
front of the nasal cavities, and then coalesced ; and behind
these prenasal parts, and yet considerably in front of the
brain-case, is a great mesonasal cavity (Fig. 20, m. n, c),
filled with connective tissue and fat, surrounded on all
sides by cartilage, with the exception of a small opening
on the upper surface of the skull, and irregular breaks in
the cartilage inferiorly in the roof of the mouth. This
space appears to be formed by a splitting of the original
nasal septum into two inner nasal walls with an interval
between; it is covered partly by the forward growth (both
median and lateral) of the ethmoidal region in continua-
tion of the tegmen cranii, and partly by a retral growth of
the upper and back part of the cornuaor prenasal cartilage.
The olfactory crura diverge to their destination in the
hinder region of this cavity.

178. The ethmoidal region, constituting the base of
the gi-eat precranial pyramid, is the widest and deepest
part of it ; it is chiefly formed of massive cartilage, extend-
ing outwards on each side below to give a rounded ethmo-
palatine condyle for the palatine cartilage, and passing
backwards above as great antorbital wings, largely ossified
by the ectethmoid ("prefrontal") bones (Fig. 19, l.eth,),
which however have a forward and downward extension to
the border of the nasal pits.

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179. The latt^er recesses have a high internal wall,
which graduates into low anterior and posterior walla
shelving downwards to the outer edge of the trabeculo-
nasal floor. In front of this are the solid coalesced trabe-
cular cornua, separating again anteriorly and slightly
diverging as rounded bosses, which are tipped by the
remains of the upper labicd cartilages (Z^, /J, which are
united with them by ligament.

180. The cartilaginous palatal roof forms a pointed
arch with its angle upwards, and sides approximately
straight. From being obtuse bohind, in the presphenoidal
region, the angle becomes acute in front : it is filled in by
parostoses. The cornua are rounded bosses with a groove
between them below, in front of these sigmoid ridges.
The labial cartilages attached to the cornua project down-
wards, and on them the premaxillae are moulded.

The palatal roof is bulged and irregularly perforated be-
neath the mesonasal cavity ; and in front of, and external to
this is seen, on each side, a sigmoid cartilaginous ridge passing
from within outwards and backwards to the edge of the carti-
laginous palate. These growths correspond in situation with
the more transverse ridges on the fore end of the palatal aspect
of the trabeculse in the second stage, (§ 130, p. 50), which
subsequently disappeared, to reapi>ear under this form.

181. There are five parostoses, very unequal in size,
especially related to the cranial roof. The two parietals
(pa.) are small, ridged, pubarcuate bones, separated by the
anterior part of the supraoccipital, articulating with it,
and also with the frontal and epiotic of their own side, in
front and behind. The much longer and broader pterotic
is external to them, a tract of cartilage intervening.

182. The frontal bones (/r.) are very large, extending
from the supraoccipital and the middle auditory regions to
the internasal tract, and not merely lying on the compara-
tively narrow cranial roof, but stretching far outwards and
downwards over the huge orbit. They only meet in their
hinder half; being anteriorly separated by the median

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ridge of the cranial roof and ethmoidal cartilage. The
frontals are much the widest in their middle portions
where they extend to the extreme outer limit of the
orbits. A great part of the middle and posterior regions
of the frontals is pitted by excavations for fatty tissue.

183, Anterior to the frontals, and partly overlapping
them by a backward projection on each side, is a con-
siderable median supraethmoid bone (s* eth.), broad and
quadrate in front, covering all the intemasal region and
the back part of the cornua. The proper nasal (na.) is a
small ossification on each nasal roof, external to the supra-
ethmoid in its middle region.

184, The parasphenoid {pa. s.) is the great parostosis
of the cranial floor: it extends from a little distance
behind the palatal ridges mentioned in § 180, almost
to the occipital condyle. Its anterior half is elongated-
oval in shape: its hinder half is narrowed, but sends
up a lamina on either side, outside the lateral walls
of the subcranial canal for the orbital muscles. This
lamina is continuous in the postorbital region with a
basitemporal wing passing in front of the prootic. The
anterior portion of the parasphenoid bears a median
superior keel, and is arched correspondingly with the
palatal roof The vomer is an oblong azygous bone, the
hinder three-fourths of which underlies the parasphenoid ;
it is armed with sharp recurved teeth anteriorly, where it
sends up rather a high crest between the trabecular
cornua, and grooving the cartilage in the region of the
nasal septum (Fig. 20, vo.).

185, The bones and cartilages which remain to be
described are principally related as boundaries to the
mouth and throat. There are, besides, many subcutaneous
bones, one series encircling the orbit, and others developed
in skin-folds connected with the hyoid arch. The position
of the investing bones contributes to unify these various
appended structures with the tracts beneath ; but there are
two important primary connections of the deeper parts,

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namely, the ethmopalatine, where a boss on the lateral
ethmoidal region of the skull is tied by ligament to the
palatine cartiJage ; and the hyomandibular, where the
longitudinally extended upper edge of the hyoid arch
articulates with the periotic cartilage underneath the
projecting eave of the pterotic bone {pt o.).

186. From the ethmopalatine connection a massive
bar, which is for the most part cartilaginous, extends for-
wards and backwards. As it passes backwards it becomes

Fig. 21.

Adult Salmon: side Tiew of sknll with all bones attached; cartilaginous
parts dotted.

Cartilage bones : S.O. sapraoccipital ; Ep.O, epiotic ; PlO, pterotic ;
Pt. palatine ; M8,Pt* mesopterygoid ; M.Pt, metapterygoid ; Qu, qaad-
late ; Art. articular ; H.M, hyomandibular ; Sy. symplectic ; E.Hy. epi-
ceratohyal, partly covered by P. Op. and I.Op. ; CHy. ceratohyal, hinder
part ; H.Hy. hypohyal ; G.Hy. glossohyal.

Membrane bones : Pa. parietal ; Fr. frontal ; S.Etk. supraethmoid ;
Na. nasal ; Lch. lachrymal ; Sb.Or. suborbital; S.Or. supraorbital ; Pmx.
premaxillary ; Mx. maxillary ; Ju. jugal ; P.Op. preopercular ; Op, oper-
cular; I.Op. interopercular ; S.Op. subopercular ; I), dentaiy ; Aug..
angular ; B.Brs. basibranchiostegal ; BrstR. branchiostegal rays.

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much broader, and considerably extended vertically, so as
to descend to a level much lower than that of the cranial
floor. At the outermost inferior angle of this palato-
quadrate tract is a condyle for the mandible. The ex-
tended hinder edge of the bar has a peg from the arch
behind embedded in it in its lower part, while it overlaps
the same arch above. The portion of the palatine car-
tilage in front of the ethmopalatine connection is con-
siderable, and bears a rounded boss looking upwards to
the edge and base of the skull, and articulating with the
overlying maxillary bone.

187. The dentigerous palatine (pi), a rod-like bone,
extends to the forward end of the bar: it has ossified
most of the cartilage in front, but merely invests it
behind ; only one-third of the length of the bone is be-
hind the ethmopalatine conjunction. The mesopterygoid
bone {ms. pt) invests the cartilage behind the palatine,
overlapping the latter by a pointed end for half its length
on the inner side of the bar, and passing back and
broadening to overlap the quadrate and metapterygoid.
It also overlaps the proper pterygoid below, and above
clings round the upper edge of the cartilage and covers
it externally for some extent. The pterygoid embraces
the lower edge of the cartilage from the palatine to the
quadrate, and does not occupy much space on either the
inner or the outer surface, but most on the inner.

188. The metapterygoid (m,pL) is a squarish bone
occupying the postero-superior angle of the palatoquad-
rate bar, and reaching to about half the height of the
hyomandibular [h. m.), which it overlaps externally.
There is a tract of cartilage between the lower edge of
this bone and the upper edge of the rather small trian-
gular quadrate {qu.)y whose inferior angle bears the carti-
laginous mandibular condyle, and whose posterior edge is
grooved for the peg from the next arch.

189. The investing bones related to these regions
belong to the upper jaw. Each premaxillary [pmx) is a

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splint of the corresponding trabecular comu, passing
backwards outside the prepalatine cartilage. It has an
elevated prenasal plate, and a considerable dentary region.
Its Une is continued backwards by the smaller maxillary
(7nx,)y which is overlapped by the premaxillary in front,
and by the jugal (ju,) behind ; it also abuts upon the
quadrate. The facial plate of the maxillary is small and
little elevated; the oval margin is dentigerous in its
whole extent. The jugal is a lanceolate bone not enter-
ing into the edge of the jaw, but lying above, on the
maxillary and the quadrate.

190. The connective tissue encircling the orbit be-
hind the ectethmoid, above the palato-quadrate bar,
below the frontals, and in front of the hyomandibular,
has given rise to about seven membrane-bones, which
form a nearly complete ring. These orbital bones have
some extension within the rim of the orbit and an ex-
pansion on the face, which is more marked in the case
of the hinder ones. The frontals partly interrupt the
ring above, and enter into its circumference.

191. The lower jaw is an elongated bar, very massive
behind, rising high in the coronoid precondylar region,
having a projecting postcondylar or angular process, and
a strong hook on each dentary bone at the symphysis.
The proximal part of the cartilage has been ossified by the
articular (art.)t but anteriorly this bone lies as an outside
splint on the meckelian cartilage, which extends beyond it
almost to the symphysis. The dentary (d,) embraces the
fore end of the cartilage both externally and internally,
but farther backwards it appears principally on the out-
side, overlapping two-thirds of the articular, and terminat-
ing by an upper and a lower fork. It is dentigerous on the
anterior three-fourths of its upper edge, but not behind,
where its upper fork mounts on the coronoid process of the
articular. A small angular (ang,) lies on the angle of the
jaw beneath the condyle, but the postcondylar process is
formed by the articular. The symphysis of the lower jaw

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is of small extent; the moieties are united by fibrous

192. The hyomandibular extends downwards from
its elongated horizontal pterotic articulation behind the
J3ostorbitals and the metapterygoid. It is narrower as it
descends : on its posterior edge in the upper third is a
bony process covered by a knob of cartilage, the opercular
condyle. The cartilage below the hyomandibular bone is
a thick prominent elbow, which turns forwards at a little
more than a right angle into the small peg-like symplectic
bone (sy.)y fitting into the groove on the postero-internal
face of the quadrate.

193. The inferior division of the hyoid arch is attached
to the upper and anterior between its two ossifications,
at the elbow just described. A small partially ossified
interhyal cartilage intervenes, and the lower bar is
directed forwards into the base of the tongue in the inter-
mandibular space, being mainly compnsed of massive
cartilage, flat proximally and thick distally. Its proximal
third is ossified by the epiceratohyal (e, %.), its distal
two-thirds by the ceratohyal (c. Ay.); the fore extremity of
the latter, covered by cartilage, fits into the posterior end
of the smaller hypohyal mass (h, hy.), which has two
distinct ossifications. An azygous basi- or glossohyal
{g. hy) passes forwards into the tongue, having the hypo-
hyals on either side behind, and being joined in the
middle line posteriorly to the first basibranchial. The
long glossohyal is ossified superiorly, and bears a row of
teeth which project in the middle of the floor of the

194. The opercular membrane is a great semi-oval
skin-flap extending backwards from the hinder edge of the
hyomandibular and quadrate regions, and covering the
upper part of the gill-arches. Four large membrane-bones
are developed in this flap. The anterior, the preopercular
{p. op.), occupies nearly its whole vertical extent, and is
crescentic in shape, with two horns and a broad median

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region; the lower horn almost reaches the postcondylar
process of the mandible. The preopercular is burrowed
in a radiating manner by mucous glands ; and to its upper
hom is attached a small supratemporal bone, one of the
series of mucous bones which is continued by the scales of
the lateral line. The bones of the orbital ring and the
nasals also belong to this category.

195. The principal and largest opercular (op,) is sub-
quadrate with very rounded angles ; it is overlapped in
front by the upper hom of the preopercular, and has a
cup on its upper anterior angle, for the opercular knob of
the hyomandibular. There is a postero-mferior suboper-
cular («. op.), lying within the opercular, which overlaps its
upper edge, and also within the next bone to be described,
which is outside its forward region. The interopercular
[iop.) overlaps the lower anterior angle of the opercular
and the anterior portion of the subopercular. It is itself
overlapped largely by the preopercular, and tied to the
angle of the lower jaw by a strong ligament Both sub-
and iuteroperculars are very thin bones^ of elongated oval

196. The lower division of the hyoid arch has attach-
ed to its imder edge on the outside a series of twelve flat
subfalcate bones, the branchiostegal rays (6r. «. r.). Ihe
posterior is the outermost and largest, and overlaps the
next, and so on to the front and smallest. A continuous
elastic membrane encloses the whole, passing from the
hinder edge of one ray to the tore edge of the one behind
it. Where the folds of opposite sides meet in front there
is an azygous membrane bone, or basibranchiostegal,
(5. br. 8.), small anteriorly, with a high median crest and a
broad hinder basal plate.

197. The five branchial arches form a series in the
walls of the throat The first four bear gills, and are bent
backwards at a right angle, the upper portions being
about two-thirds the length of the lower and anterior
divisigns. The upper parts of the arches are also bent

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inwards under the hinder region of the cranial floor and
the anterior vertebrae: and they present two divisions,
a superior and smaller pharyngobranchial and a larger
epibranchial, with a fibrous joint between. The fourth
pharyngobranchial is attached to the lower hinder surface
of the third. All four pharyngobranchials are more or
less ossified and project almost directly inwards; they
bear no teeth in this type. The forward epibranchials
are the larger, and are shaft-like bones tipped with
cartilage at both ends; they bear an upper external
pointed process. The fourth epibranchial is short and

198. The lower divisions of the branchial arches are
attached to the upper by very perfect hinge-joints. From
this joint the long shaft-like osseous ceratobranchial is
directed forwards ; and the three first arches have a smaller
hypobranchial piece segmented off* and considerably
ossified. The fifth branchial arch has only the lower
division developed, and ossified as ceratobranchial; it
bears teeth on its inner edge, and is the inferior pharyngeal
bono of Cuvier. The bones of the arches are solid in the
middle and usually surround a cone of cartilage at either

199. The first and fourth branchial arches have pro-
jecting from their antero-extemal margin about a score of
small denticular bones, curving inwards ; the second and
third arches have two series of these, one directed forwards
and the other backwards. These little bones are so
arranged as to form a colander through which the water is
strained as it pours through the branchial clefts. The
postero-external face of each arch is grooved from the top
of the epibranchial to the bottom of the hypobranchial.
In this groove lie the branchial vessels ; from it the gill
laminae project

200. The gill arches of opposite sides are bound
together mid-ventrally by a contmuous series of bones and
cartilages. Each basibranchial^ like the basihyal, runs

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hi] the salmon: summary. 81

forwards in front of its arch. There is no segmentation of
the cartilage except behind the third basibranchial : and
the cartilage behind tliis point is unossilied, and bears
both the fourth and the fifth branchial arches. Each of
the three solid basibranchial bones is continuous by a
cuusiderable tract of cartilage with the next.

201. Summary. Considering first the history of the
cartilaginous elements, we find parachordals and trabe-
culae in the floor of the cranium very early, and the union
of trabecula with parachordal is complete before the para-
chordals have united with one another. The latter quickly
fuse with the adjacent otic cartilages, and an occipital ring
is completed in continuity with these masses. The para-
chordals during growth become separated by a consider-
able distance in their postpituitary region, and the trabe-
culae bend outwards very definitely where they bound the
pituitary space. In front they are speedily confluent, and
become developed beyond the brain-case into lateral eth-
moidal and prenasal or comual regions, between which
the nasal capsules lie.

202. The formation of a median interorhital septum
above the coalesced trabeculse elevates the fore part of the
brain. The septum is at first membranous, but afterwards
chondrifies continuously with the trabeculse. The pre-
cranial cartilage grows to a height corresponding with that
of the septum, and differentiates into massive lateral eth-
moidal, subnasal, internasal, and prenasal regions. Upper
labial cartilages become connected with its comual termi-

203. The supraorbital tract is early chondrified, and
is confluent in front with the lateral ethmoidal region, and
behind with the auditory capsule. A strong cartilaginous
cranial roof is developed from before backwards until it
joins the supraoccipital growth, and only leaves a pair of
small membranous fontanelles in the parietal region. The
side walls of the cranial cavity anteriorly to the otic
masses are not extensively chondrified.

B. m. 6

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204. The mandibular, hyoid, and branchial arches
arise as simple rods on each side ; and a palatine or sub-
ocular bar is at the same time developed, having a definite
attachment to the lateral ethmoidal region. The postoral
arches undergo a segmentation by which they are divided
into upper and lower portions. The inferior piece of the
mandibular arch is elongat/cd to constitute the axis of the
lower jaw ; the upper division grows forwards to join the
palatine bar, forming a continuous palato- quadrate tract.
It gradually descends from its proximity to the cranial
wall, and is finally supported at a much lower level by the
upper division of the next arch.

205. The segmentation of the hyoid arch occurs
throughout its entire length, the anterior tract becoming
superior, and the posterior inferior. The upper part is

Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 7 of 31)