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772. The intimate relation of the three sense-capsules
to the cranium has already been noticed at length. Each
pair is built into the cranium in its own style, being vari-
ously and very perfectly protected or supported by it. A
partial floor, inner wall, and roof may be provided for
each organ by cranial cartilage, in addition to its own chon-
drifications. In ascending from the lower to the higher
types we are struck by the fact that each organ appears
to press continually closer to the middle line, and either
to compress or get beneath the cranial cavity or cartilage.
In the Salmon we find a very simple state of the inter-
and prenasal cartilage, which is expanded into a thick
mass not directly comparable with the internasal region
of a Dogfish. There is a simplification of this condition
in the IJrodeles, the inner walls of the proper nasal cap-
sules and the median tissue coalescing. But in higher
forms a process occurs which appears to be intelligible
by imagining the lateral parts of the internasal plate of



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JX.] THE CARTILAGINOUS SKULL. 341

the Dogfish, and the inner nasal wall, to be elevated and
constituted into vertical walls, which being approximated
unite to form the nasal septum. In these types also it
becomes increasingly difficult to distinguish what is inter-
nasal (trabecular) cartilage and what belongs to the proper
capsular wall. The floor of the nasal sac in the Frog and
other forms is made out of the primary cornua, and appears
very comparable to the basilar cartilage underlying the
ear-capsule in the Elasmobranchs and in the Axolotl, and
the trabecular shelf supporting the eyeballs in the former
group.

773. Approach of the orbits to the median line of the
head occurs in many forms, concurrently with development

*of an interorbital septum : and the brain is either elevated
above them or much retracted. The approach of the
ear-capsules to the middle line in Birds and Mammals
appears to be a phenomenon of the same order : but in
them the organs are largely covered by the brain in its
region of highest development.

774. The fenestration of the cranial cartilage by the
sense-capsules has also been alluded to : the capsules
appear to be related to three tracts of cranial cartilage,
and at any rate in some forms cartilage is aborted where
they abut. This is exemplified in the fenestration of
intemasal, interorbital, and orbitosphenoidal tracts, and the
frequently greater size of the olfactory and optic foramina
than the nerves which pass through them; as well a$
by the non-formation of lateral cranial cartilage internal
to the ear-capsules in Elasmobranchs and others, a fact
less perceptible in higher forms in consequence of the con-
tinuity of cranial and capsular cartilage m them from the
first. »

775. It does not appear that the sense-capsules bear
any sure testimony to the segmental structure of the
skull. They were probably originally situated in definite
segments, but their large size may have extended them
over the tracts due to several primordial segments. Seer



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842 MORPHOLOGY OF TfllB SKULL. [CHAP.

ing what is our uncertainty about the composition of the
brain-case, we cannot tell what segments of it especially
correspond to the capsules: only the regions can be roughly
indicated.

776. Of what value are the visceral arches as indica-
tions of the composition of the skull? The mandibular
arch is constantly related to the basis cranii just in front
of the trigeminal foramen; the hyoid to the basilar plate
where it extends outwards beneath the ear-capsule, and
in other cases to the median region of the same capsule ;
the branchial arches are behind these, and not directly
connected with the cranium. Bo all the branchial arches
belong to the segments which the cranium represents?
They are certainly all supplied with nerves from the
cranium; but to relate all the branchial arches to cranial
segments requires the assumption of the disappearance
or condensation of a series of body segments so far as
their axial and neural parts are concerned ; and our reckon-
ing of the number of segments that have thus disappeared
will vary according to the view taken of the perfect quota
of branchial arches. If we view the first two branchial
arches as specially related to the cranium (namely, those
supplied by the glossopharyngeal and the first branch of
the vagus), and the hinder branchial arches as belonging
really to the neck, we have four arches related to the
cranium at the side of and behind the pituitary body\
The bearing of this view on the structure of the osteo-
cranium must be considered later.

777. We believe that it is justifiable to regard the
comua trabeculoe of the Dogfish and Frog, (recurrent
cartilages of Pig, Passerine Bird,) and the antorbital plate
or bar, as preoral representatives of visceral arches; and
the method of segmentation of certain palatine tracts in
Urodeles, Lizards, and Birds gives some colour to the
imagination that another segmental piece is there to be

1 We have no wish to oast any discredit on the opposed yiew, of
the disappearance or condensation of segments in the hinder part of
the ttkoU. The question cannot jet be decided*



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IX.] THE OSSEOUS SKULL. 343

traced ; but the light is very uncertain at present. There
is no certainty that all the body segments concerned in
tJie cranium are provided with representatives <rf arches.
But on this view, derived from arches or their representa-
tives, not fewer than seven segments are o(mcerned in the
formation of the cartilaginous skull,

778. The meaning of the labials and other superficial
cartilages is very obscure. No definite relations between
the labials and the arches can be made out at present:
yet their almost constant occurrence, their predominance
in the Lam|Hr«iy« and Eiasmobranchs, in the early stages
of Frogs, and the importance of the changes in which they
take part, lead to the idea that they must have some
intelligible position in reference to the skull. The extra-
branchials of the Dogfish are at ajiy rate superficial
cartilages related to the branchial arches, and they appear
to be homologous with the scapulo-coracoid cartilages.



The Osseous Skull.

779. Calcareous deposit occurs in vertebrates in the
following tracts; epidermis or epithelium (enamel of teeth,
outer layer of Ganoid scales): dermis (dentine of teeth.
Ganoid and Teleostean scales) : subcutaneous fibrous mesh ;
immediately outside the perichondrium of a cartilaginous
tract (parostosis) ; immediately within perichondrium, and
eating into cartilage, (ectostosis) : a little way beneath the
surface of cartilage (superficial endostosis) : and deep in its
substance (true endostosis, central or subcentral). In most
of these tracts the calcification may be such as not to gain
the title of bone; but in all except the first, true bones
may result from the process.

780. In various embryos it has been noticed that the
calcification of teeth occurs before the bony plate that
bears them and unites a series has appeared. The prin-
cipal membrane-bones arise previous to any ossification of
cartilage. On the other hand^ some types very deficient



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344 MORPHOLOGY OF THE SKULL. [CHAP,

or entirely wanting in ectostoses have superficial calcifica-
tions of cartilage ; and in others, deep endosteal deposit
takes place before ectostosis, but it is soon either absorbed
or superseded by the latter.

781. Any of the cakified tracts mentioned above may
unite with the next so as to be undistinguishable from it.
Dermosteal and parosteal tracts are frequently conjoined:
again and again do bones developed parosteally graft
themselves on cartilage as ectostoses, or ectostoses extend
into outer tracts of fibrous tissue : and ectostoses similarly
unite with endostoses.

782. These facts, combined with our present know-
ledge of fossil forms, seem to furnish a clue to the pro-
cesses by which the bony skull has become what -it is
in various types. We conceive of primordial vertebrates
which possessed a calcified exoskeleton and a cartilaginous
brain-case; these calcifications becoming true bones, and
continually ossifying deep tracts; the chondrocranium
gradually acquiring its proper bony centres; the parostoses
extending more deeply and being applied to and moulded
upon the cartilaginous parts, in many cases uniting with
their proper bones, or aborting the cartilage.

783. Scales and dermal bones in vertebrates have
probably a relation to the segmentation of the body.
The bony armour of Callichthys, the lateral line series of
scales each with its mucous dudt, of Teleosteans, the bony
scutes of the Sturgeon and the Crocodile, are but a few out
of many facts suggestive of this idea. The dermal bones and
parostoses of the head are representatives of the same
thing, occurring serially and in rows which can be traced
more or less clearly. Ihis relation has been proved with
regard to the parostoses related to the cartilaginous parts
of the sternum and shoulder -girdle*; and if a student
desired to be prepared in the best way for understanding
the difficulties and complicated relations of tracts of bone
and cartilage in the skull, he could not do better than

I On the Shoulder-Girdle and Breast Bone, Ray Society, 1866.



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IX.] THE OSSEOUS SKULL. 34jS

master the simpler problem of the sternum and shoulder-
girdle.

784. The body of Callichthys is enveloped in a right
and left series of elongated supero-lateral and infero-lateral
(ossified) dermal plates. The supero-lateral bones are
directed downwards and forwards, the infero-lateral down-
wards and backwards. The upper and lower series are
very much alike; only the upper plates are pierced
at their base in front and notched behind, for mucous
glands. If this portion were cut off by a suture in each
upper plate we should have a series comparable to the
mucous scales of the lateral line of typical Teleostei.
This does in fact take place in the first cincture behind
the head, forming a post-temporal bone. The cinctures
are not perfect above or below; space is left for small
dermo-spinal plates and spines in the fins; and for dermo-
ventrals and ventral spines.

785. In the work referred to it is shown, in the
case of Callichthys, that the two supraoccipital derm-
bones, the single parietal, the symmetrical frontals, the
single dermo-ethmoid are all serial homologues, whether
azygous or symmetrical, of the upper three-fourths of the
supero-lateral dermal plates clothing the body. The lower
part of the first body-plate (containing the mucous duct)
is cut oflf as a supratemporal. Then this series becomes
double around the eye, and we have the dermal post-
frontal, and postorbital, and supraorbital, the suborbital and
lachrymal, and then the nasal. The infero-lateral plate is
subdivided where it is in relation with the shoulder-girdle,
into three ; supraclavicle, clavicle, and interclavicle. The
opercular repeats the supraclavicle, the subopercular (not
found in Callichthys) the clavicle; but the hyoid dermo-
cincture is completed by the many branchiostegals (three
in Callichthys); the basibranchiostegal is a lower spine-
bone, and seems to correspond with the interclavicular
region. The squamosal, in relation to the mandibular pier,
carries on the series of supraclavicle and opercular; the
clavicle and subopercular are represented by the inter-



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846 MORPHOLOGY OF THE SKULL. [CHAP.

opercular; the dermal bones of the lower hyoid and
interclavicular regions are represented by the jugulars,
and the splints of the mandible. The maxillary, jugal,
and quadrato-jugal, homologous with part or the whole of
the squamosal, belong to the infero-lateral series in the
fore part of the head.

786. The ossifications which occur in the cartilaginous
brain-case and its anterior continuations, independently of
the proper capsular bones, are comparatively few, and on
the whole simple in character so long as we have to do
merely with cartilage. The most constant ectostoses in
the cranium are the exoccipitals, appearing on either side
of the foramen magnum, .enclosing the vagus and often
the glossopharyngeal and hypoglossal nerves, bearing the
occipital condyles where i^offre mae two, tsr jawitrilwiaMg
the lateral portions when the condyle is single. In every
skull which is truly ossified these bones occur ; and when
teither basi- nor supraoccipital exists, the exoccipitals
extend largely into the floor and roof of the cranium, very
much after the manner of the neural arches of vertebrae
in various animals.

787. The orbitosphenoid is, next to the exoccipital,
the most constant bone of the same category. It is the
ossification of the lateral cranial wall anteriorly, or of
so much of it as developes cartilage ; it is perforated by,
or is anterior to the exit of the optic nerve. Very fre-
quently it is conjoined in development with the similar
tract in front (ethmoidal), beyond the brain-case: and the
lateral ossifications may become united by median bone,
forming the complicated sphenethmoid. It is noteworthy
that the orbitosphenoid may arise by two centres on each
side. The alisphenoid is the only other bone which can
be classed with these : it is in the lateral cranial wall,
or so much of it as is cartilaginous, between the exit of
the optic and the trigeminal nerve, or partially or entirely
enclosing the latter. It also in some cases arises by two
centres on each side. Both the orbito- and alispheuoicb



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IX.] THE OSSEOUS SKULL. 347

may lie in the lateral part of the floor of the cranium in
higher types with large brains.

788. The basiocci^ntal, rarely found in Amphibia, but
occurring in all higher types, is an expression of ossifica-
tion arising immediately around the notochord, after the
manner of the main part of a vertebral centrum, and
extending on either side into the proper parachordal car-
tilage. Developing at first in the Chick around the hinder
portion of the cranial notochord, it subsequently grows
so as to enclose the whole of its three segments, when the
notochord has relatively retired and the posterior basi-
cranial fontanelle is established. In the Axolotl, where
no basioccipital is formed, there is an apical ossification
around the notochord (cephalostyle), which afterwards
loses its identity in the parasphenoid, or is absorbed. We
thus see that the basioccipital corresponds very closely
with a vertebral centrum, but its relation to the whole of
the cranial notochord makes it doubtful whether it is not
equivalent to two or three vertebral centra.

789. The nfext bone, the basisphenoid, corresponds
to a certain extent with the basioccipital, in being more
or less laid down in cartilage which is the result of the
junction of lateral masses, the trabeculae, as the basi-
occipital is largely due to the parachordals. But the basi-
sphenoid has no relation to the notochord, or to anything
like a vertebral centrum. A basisphenoid formed from
cartilage occurs less frequently than a basioccipital; in
most cases it arises from a pair of centres on the antero-
lateral margin of the pituitary body, which afterwards
join. It is not till we reach the Birds and Mammals
that a basisphenoid arising from a single median centre
is found, but even in the highest form, in Man, we find
a pair of centres. The basisphenoid may be increased in
bulk very largely by ossification in adjacent membrane.

790. A rudiment of an anterior median centre, the
presphenoid, is found in Lizards, and more markedly in
^irds: it is iiL the upper part of the interorbital septum^



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348 MORPHOLOaY OF THE SKULL. [CHAP.

the expression of conjoined and compressed trabeculse. A
distinct presphenoid is marked in various Mammals, most
definitely in rodents; but in many cases the region is
ossified by the inward extension and union of the orbito-
sphenoids. Anteriorly to this, the median cartilage is
ossified by a vertical mesethmoid where a true nasal
septum is found : it occupies the hinder part of this region
and the fore part of the interorbital. The cribriform plate
in Mammals is simply a lateral ossification continuous
with the top of the back part of the mesethmoid, and
extending through the interspaces between the olfactory
fibres where they pass out of the cranium. The meseth-
moid has a definite extent forwards, not passing beyond
the cranio-facial fenestra (Birds), or into the aliseptal
region as formerly defined : but in various birds two or
three centres may ossify part of the anterior region of the
septum.

791. Other forms of ossification of precranial cartilage
are found in the Cod tribe, where there is an anterior
median ethmoid, and in the Pike, where small paired ossi-
fications invade the cartilage. One lateral ectostosis has
to be mentioned, the ectethmoid (prefrontal) in the ant-
orbital wall, occurring in the Salmon and again in Mam-
mals. In the adult Pig a prenasal bone is found in the
extreme anterior cartilage of the head (prenasal and ali-
nasal). Ossifications in the turbinal ingrowths of the nasal
capsules arise only in Birds and Mammals. They present
no important phenomena beyond those of the cartilaginous
structures in which they take their origin.

792. In the ear-capsule we have a series of bones
very definitely related, and very persistent. The prootic
is about the anterior semicircular canal, and is related to
the antero-inferior region of the capsule ; the epiotic is
over the junction of the anterior and posterior canals ; the
opisthotic is postero-inferior, and frequently furnishes the
hinder part of the margin of the fenestra ovalis. These
are the most constant ossific centres, remaining distinct
^om one another either throughout life or till an advanced



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IX.] THE OSSEOUS SKULL. 849

stage, in Fishes, Amphibians, and Reptiles. In Fishes
there is a pterotic over the external canal, frequently
bearing the horizontal articular facet for the hyoman-
dibular : this centre reappears in the Axolotl, but is not
found in the higher types. There may be another centre
in front of this, the sphenotic, occupying the antero-
extemal region of the capsule ; but the cartilage in which
it arises is always of a composite character, being due to
a confluence of proper cranial cartilage with that of the
capsule,

793. There is a strong tendency for the epiotic centre
to unite with the supraoccipital, and the opisthotic with
the exoccipital ; and in some forms the periotic bones do
not arise separately, but the supraoccipital and exoccipitals
extend into the epiotic and opisthotic regions respectively.
Where there is no supraoccipital, the exoccipital may
ossify the whole posterior portion of the capsule. In Birds
the epiotic and opisthotic are quite small, and the adjacent
bones grow into the auditory cartilage and then annex
these centres to themselves. The prootic on the other
hand has much more independence and persistence; it
is the most constant periotic bone, and is never aborted
by adjacent bones. It may grow far into the floor of
the cranium, and may even occupy a considerable portion
of its roof in Amphibians ; sometimes it further developes
a process which lies in the side wall of the skull anteriorly
to the capsule, and is comparable to an alisphenoid.

794. The cartilage-bones of the outworks of the
skull have in their way, as curious a persistence as those
of the cranium proper. But some regions comparatively
early in the ascent of types become affected by mem-
brane ossification, and cease to develope cartilage, or
develope it only when the ossifying force is exhausted,
or develope it in the embryo and abort it under the influ-
ence of the growth of neighbouring parostoses. The parts
which persist as cartilage-bones are selected, as we ascend
the scale, for specially important functions, being usually



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350 MORPHOLOGY OF THE SKULL. [CHAP.

reduced in relative size and becoming more elegantly
shaped.

795. It appears that the palatine of the osseous Fish
represents the antorbital or palatine of Amphibia, and
partially the os uncinatum of a Bird. It cannot be de-
finitely stated whether the pterygoid and mesopterygoid
of an osseous Fish belong to a proper subocular arch or
to the forward growth of the upper mandibular segment :
probably the latter is the case. There is no doubt that the
metapterygoid and quadrate of Fishes belong to the upper
mandibular segment ; and they correspond regionally with
the epi- and pharyngobranchials. In the classes above
Fishes the palatine and pterygoid are very largely formed
out of membrane, the cartilage being either ossified in
addition, or more or less aborted as in Amphibia; or only
coming into existence at a late period, in various tracts,
which remain permanently unossified, as in Birds. A
specialised portion of the pterygoid tract may however
exist in cartilage within and above the membrane-ptery-
goid, and ossification of this developes the epipterygoid of
Lizard and Turtles.

796. In Reptiles and Birds the whole upper mandi-
bular segment has but one bone, the quadrate ; and this
may be said to answer in a general way to both quadrate
and metapterygoid of the lower forms; but it is more
highly specialised, and itself becopaes, like the palatine
and pterygoid tracts, increasingly unified with the cranium.
The mandible usually has but one centre, the articular,
corresponding to a ceratobranchial; in the Frog and in
Man there is an inferior or anterior (mentomeckelian)
centre, apparently like a hypobranchial. But the whole
meckelian cartilage never becomes proportionally so much
ossified as the branchial cartilages. In ascending the
scale the articular is very persistent, but becomes small in
the Birds (largest in Struthionidae) and specialised in
form. In Mammals no articular is formed, and the repre-
sentative of the quadrate (the malleus) is very small.



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IX.] THE OSSEOUS SKULL, 351

highly specialised and intimately involved in the cranial
structure.

797. The upper segment of the hyoid arch is ossified
by two centres, the hyomandibular and the symplectic,
in osseous Fishes. Where it exists in higher forms it
is from the first specialised for the service of the ear,
and in some is ossified by two centres corresponding
to the larger ichthyic centres. Of course the part that
ossifies the proper stapedial plate, due to the auditory
capsule, is not to be considered as representing any por-
tion of the hyoid arch: but there is often no distinction
between the stapedial and the hyoid centres. In the
mammalian type, however, the stapes is quite distinct.
In most columellas there is but one bone, which may be
considered as a specialised hyomandibular. The incus is
its mammalian representative, and is very highly special-
ised, the parts however being developed according to the
type foreshadowed in inferior forms. The two main ossifi-
cations of the lower part of the Salmon's hyoid arch, the
ceratohyal and the epiceratohyal, must be regarded as
answering on the whole to the ceratobranchials. The
hypohyal of course represents the hypobranchials ; the
ba^ihyal and the basibranchials are homologous.

798. The homologue of any ossification found in the
Mammalia between the incus and the anterior comu of
the hyoid bone must be sought in the cerato- and epi-
ceratohyal of the Salmon. The anterior comu itself is
homologous with the hypohyal. The ossified pieces of
the branchial arches which persist in XJrodeles and in other
types are mostly to be considered as ceratobranchials :
there are two segments on either side in many Birds and
Newts; they may be cerato- and epibranchials ; but it is
not very necessary to formulate any precise homology
between them and the branchial bones of Fishes,

799. We see then in the ichthyic branchial arches
with their four ossifications, the fullest development, ac-
cording to a simple type, of the lateral rods of the



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352 MORPHOLOGY OP THE SKULL. [CHAP.

pharynx; tLey have a principal region of segmentation,
between epi- and ceratobranchials, comparable to the
division between sternal and vertebral ribs. The anterior
arches, so much modified and specialised, nevertheless
agree in being dominated by this segmentation into a
supero-lateral and an infero-Iateral tract. In almost all
cases the mandibular and hyoid arches manifest a distinc-
tion into these two regions, this being seen even in



Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 29 of 31)