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(various Urodeles) ; and either of these regions may coalesce
with the cranial cartilage. As a result of transformation, or
as a primary relation in higher types, the mandibular arch
may be brought into connection with the capsular cartilage
of the ear, may develope a surface for articulation with
it (Frog), or may unite with it A backward prolongation
of the upper region of the arch (otic process) may lie

^ Huxley, CeratoduSf l.o.

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along the side of the capsule or coalesce with it. Further-
more, a hinder and superficial leaf of the suspensorium
is cut oflf in many Anura, and becomes adapted to the
tympanic membrane as the annulus.

744. The suspension of the lower jaw in hyostylic
skulls is effected by a smaller or greater modification of
the primitive form of the hyoid arch. The upper part of
the arch either arises separately, or is cut off from the
rest of the arch; being carried forwards at its lower end,
it is firmly attached to the lower part of the suspensorium
and to the jaw. In a primitive condition (Shark) the
upper end of this hyomandibular element is articulated
both with the basis cranii and the auditory capsule, under-
neath the pterotic ridge.

745. The segmentation of the hyoid arch to form
this hyomandibular may either be transverse or oblique,
in a position comparable to that of the joint between the
epi- and ceratobranchials; or it may be longitudinal
(Salmon), affecting nearly the whole length of the arch,
the anterior (hyomandibular) piece becoming superior, and
the posterior inferior. In am phi- and autostylic skulls
the hyomandibular piece may be comparatively small, or
may not exist.

746. In many Anura and Sauropsida, when the hyoid
arch chondrifies, it appears to be only the lower portion
of an arch. Sometimes at an early, sometimes at a
comparatively late period of metamorphosis, a series of
structures originates near the auditory capsule and behind
the mandibular suspensorium, in precisely the same situa-
tion as the hyomandibular, the nerve and cleft relations
being identical. They failed formerly to be recognized
as portions of the hyoid arch, because they did not
originate contiiiuously or contemporaneously with its
lower segments, or because in the adult they may not
be continuous, their directions being different. These
structures form the columella auris, and always enter into
connection, sometimes coalesce, with the stapes belonging

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to the auditory capsule. One or more segments may arise
in this upper hyoid tract; and its outer end is usually
in relation with the tympanic membrane, and helps to
support it.

747. In Mammalia, while the primitive condition is
more complete, the metamorphosis is in some respects
more extraordinary. Both the mandibular and the hyoid
arches are of full length and development at first; and
their upper ends become related to each other and to the
tympanic cavity. The mandibular arch is never seg-
mented; but its meckelian region does not form part of
the functional lower jaw. Its upper tract, remaining small,
is specially moulded for purposes connected with hearing,
lies in the tympanic cavity, and supports by one of its
processes the tympanic membrane. The upper part of the
hyoid arch is bent inwards to be fastened to the stapes,
and then the hook, apposed by its elbow to the top of
the mandibular arch, is cut off, becoming the incus; the
main part of the arch is carried backwards and coalesces
with the auditory capsule. The processes of the incus
are exactly paralleled by the processes of the columella in

748. Retrogression in the cartilaginous cranium of
particular forms can hardly be said to exist, except in the
Urodeles, where some of the axial basicranial cartilage is
absorbed. But as we pass from branchiate to higher types
the proportionate amount of cartilage that occurs in the
cranial investment is diminished.

749. The mandibular arch can scarcely be said to
undergo retrogression : at a varying period of development
it ceases to grow, and becomes a less important factor
in the lower jaw in each ascent of type. The hyoid and
branchial arches may however be said to retrograde, but
the change is related to new functional adaptations. These
phenomena occur especially in the caducibranchiate Am-
phibia. The main (lower) part of the hyoid arch, from
being short and massive, and attached to the suspensorium.

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becomes long and slender and attached to the ear-capsule;
in some it is entirely absorbed. In Frogs the branchial
arches are originally quite distinct from one another, but
become united into a basket-work by transverse bars both
above and below; and then gradually during the loss of
the water-breathing function, the whole branchial struc-
ture is almost completely absorbed, leaving rudiments
which are related to the larynx, and which unite with a
broad basihyobranchial plate very much resembling a
sternum. In other cases the branchial arches in becoming
modified are variously tied to one another; and the rem-
nants are of very different patterns. Frequently what
looks like a process of transformation is merely a cessation
of growth combined with adherence to a basal attachment.

750. In the abranchiate forms the early hyoid arch
may elongate, remaining slender, and there may be con-
tinuous cartilage between the basihyal region and the
ear-capsule; or a portion of the arch becomes merely
ligamentous^ leaving cartilage proximally and distally.
One pair of branchial cartilages is found, ceasing to grow
at an early date; they mostly do not disappear, but
become applied to, and support the larynx: a relation to
the orifice of the breathing apparatus in air-breathers
parallel with that of the branchial arches to the respi-
ratory orifices in water-breathers. In the Frog and in
Mammals paired branchial rudiments partially embrace
the larynx. In Birds it is the posterior end of the basi-
branchial bar that is applied to the larynx: in Urodeles it
is the same element which is thus adapted, but it is com-
pletely segmented off from the remaining parts. In
Lizards there is no application of the branchial arches
to the larynx; and there is a considerable first branchial
arch. In Woodpeckers there is no basibranchial, but an
immensely long first branchial arch is extended completely,
in other birds partially, round the head, and lies over the
nasal roof. .

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The Cranial Nerves,

751. The relations of the brain-case to the foramina
of exit of the cranial nerves are of much importance iu
the study of the skull. The positions of these nerves
assuredly serve as landmarks; but their precise significance
cannot yet be definitely expressed. A certain order is al-
ways observed in their arrangement; but the distances of
the foramina from one another vary greatly, and two of the
nerves (trigeminal and vagus) often perforate the cranium
as two or more tninks. The. order in which the nerves pass
out from before backwards is the following: olfactory, optic,
oculo-motor (several), trigeminal, facial, auditory, glosso-
pharyngeal, vagus, hypoglossal. These nerves are certainly
not of equal morphological value. How far they correspond,
in relation to segments of the body, with the ordinary
nerves, e,g, of the dorsal region, is not yet ascertained \
Consequently deductions from them with regard to the
composition of the cranium are premature. Those who
hold that the olfactory and optic nerves are of so special
a nature as to be quite unconnected with the ordinary
segmental arrangement of nerves, are certainly not en-
titled, from the position of these foramina, to make any'
dogmatic conclusions respecting the cranial segments and
their homology with vertebral segments. We believe that
these nerves will ultimately be found to have some definite
connection with primordial segments and their nerves,

752. The position in which the cranial nerves emerge
may be generally defined to be the lateral region of the
basis cranii or the lower part of the side-walls. The
emergence is never in the middle line, the nearest approach
to it being in the case of the olfactory nerves. These
mostly pass out of the primordial cranium through a large
fenestra in the back part of the nasal walls, the fibres

1 See Mr BalfourV very able discussion of this question in his Deve-
lopment of Elasmobranch Fishes^ Joum. Anat. Vol. xi., 'Part 3, April,
1877, and also Mr MarshaU's valuable paper on the cranial nerves of the
Fowl in the s^me journal.

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being not closely packed, but more or less widely dis-
tributed in the fenestra. The olfactory fenestra is at or
very near the anterior end of the cranium.

753. The optic nerves enter the hinder or middle
region of the orbits by a foramen frequently larger than
the nerve. The motor nerves of the eyeball may enter it
by separate small foramina behind the optic foramen, or
in common with the anterior division of the trigeminal.
The latter is always situated near the junction of the
cranial wall with the ear-capsule. Severed branches pro-
ceed from it, becoming distinct before emergence. They
may all pass out of the brain-case by a common foramen,
or by as many as three distinct foramina, corresponding
to the three main divisions of the nerve. The facial nerve
is more or less closely connected with the auditory, and
in higher forms one foramen may be common to the two
nerves In many Anura the facial emerges in front of the
ear-capsule, in the same foramen with the trigeminal. In
other forms it bores into the front part of the ear-capsule
and has a winding courae within its anterior and lower
region. In Sharks and Axolotls the facial foramen is in
the line of junction between the true basilar cartilage and
the ear-capsule. The auditory neiTe enters its capsule
directly. The glossopharyngeal and vagus foramina are
always found in or near the line of junction of the oc-
cipital and auditory cartilage; being more in the base
or side-walls according to the position of this junction.
They may emerge by a common foramen; or by two
foramina not far from one another; or the vagus, separated
into a number of strands, may have a foramen for each,
as in Notidanus. The glossopharyngeal appears frequently
as if included in the auditory cartilage: the vagus not so.
The hypoglossal nerve (where it exists) comes out more
internally than these nerves, just in front of or near the

754. The greater part of the tracts in front of and
above the mouth are supplied by branches of the tri-
geminal nerve. The main anterior branch (ophthalmic

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or orbitonasal) has a general distribution throughout much
of this territory, especially around and in the orbits and
nasal cavities. In Birds its nasal branch becomes specially
related to the prenasal cartilage: and it may pierce
through the anterior facial tissue and appear on the ex-
treme fore part of the face in them and in mammals. There
are several indications that this nerve is a dorsal branch,
whose distribution is extended far forwards*. The co-
extension of the orbitonasal and the tmbecular growths
is very remarkable, whatever may be its interpretation.

755. Of the oculomotor nerves it may be said that
they supply a series of muscles which are developed, at
least in lower forms (up to the Frog), from the first
muscular segment of the body, appearing primarily on
either side of the anterior part of the notochord; and
permanently taking origin from the basicranial canal in
the Salmon. The oculomotor nerves are more or less
clearly divisible into an anterior and a posterior division.

756. The orbitonasal nerve runs along the side-wall
of the skull in its orbital path ; frequently it ascends high
up under the supraorbital ridge. In every case where
an ascending process of the mandibular suspensorium is
developed, this lies above the orbitonasal at its divergence
from the remaining trigeminal trunks. The two other
divisions of the trigeminal are the superior and inferior
maxillary. They diverge almost directly outwards in most
cases over the suspensorium in autostylic skulls. The
distribution of the superior maxillary corresponds tolerably
closely with the extent of the maxillopalatine process:
that of the inferior maxillary with the lower jaw.

757. The facial nerve in almost all cases sends for-
wards a notable branch, soon after its exit from the cranial
cavity, outside and at a little lower level than the orbito-
nasal. This is the palatine or vidian nerve, and it is
supplied to preoral parts like the last, but does not extend

1 Mr Balfour has brought forward ibis view prominently in his paper
above cited.

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SO far forwards morphologically: it has nothing to do with
the prenasal region. The facial trunk further divides
sooner or later into two branches, one of which (known as
the chorda tympani, in mammals), turns forwards and
unites more or less closely with the inferior maxillary
nerve. The forking of the facial is always above the first
visceral cleft, and is often very closely related to it. Con-
sequently also this branching always takes place in or in
close proximity to the ear-capsule. The course of the
facial in the periotic cartilage is notable for its great
concavity looking backwards вАФ which may be matched by
the concavity looking forwards, frequently seen in the
course of the glossopharyngeal. The posterior branch of
the facial is primarily related to the fore edge of the
hyoid arch. The distribution of the facial is very compli-
cated and little explained in higher animals ; it spreads
over a large territory on the face, and is the motor nerve
connected with the development of facial expression.

758. The glossopharyngeal nerve is not so much
implicated in the ear-capsule as the facial, but yet may
be united by branches both with the facial and the vagus.
It supplies the hinder part of the hyoid and the fore part
of the first branchial arch, the branches being separated
by the second visceral cleft where that exists. The vagus
is distributed to the branchial arches, its branches (like
so many distinct nerves) forking above the arches after
the manner of the glossopharyngeal. Besides this the
vagus is distributed to many tracts of the body, and its
whole significance cannot be entered upon here. In
abranchiate forms it supplies those parts which are in any
way recognizable as corresponding to branchial arches:
but this is but a very small fraction of the whole distribu-
tion of the nerve. The hypoglossal is the nerve of several
muscles behind the hyoid arch as well as in front of it.

759. It should be noticed how remarkably several of
the cranial nerve3 pass forwards, and appear to go beyond
any simple segmental distribution. The orbitonasal, the
vidian, the glossopharyngeal, and the hypoglossal are all of

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this nature, while we see the vagus distributed far
backwards, even in the simplest forms: thus, supposing
evolution to have occurred, the lowest types referred to in
this book have passed very far beyond any primordial ar-
rangement of segments and nerves that can be conceived.

Plan and Segmentation of the CaHilaginous Skull.

760. We are thus *led naturally to consider what
conception can be formed of the real structure of the
cartilaginous skull. This must necessarily involve refer-
ence to the cartilaginous vertebral column, which shares
with the brain-case the function of surrounding and pro-
tecting the axial nervous system and emitting nerves for
distribution over the body.

761. We do not conceive of the skull as being com-
posed of a number of coalesced vertebrae; not having
perceived indications of any process of coalescence in the
embryo, and being unaware of any evidence of the past
occurrence of such a transformation in ancient times. A
large portion of each vertebral centrum is due to chondri-
fication of a tube of tissue around the notochord. Su^ h
chondrification takes place distinctly around the cranial
part of the notochord in two types examined; and this
cartilage is no doubt homologous with the spinal cartilage
just mentioned. As evidence relating to its segmental
value in the chondrocranium we have merely the constric-
tion of the cranial notochord in two places in the Fowl,
giving the appearance of three notochordal segments; and
one constriction in the Urodeles. The notochord and its
enveloping cartilage are never found in the anterior part of
the cranium, and consequently, judging by ordinary stand-
ards of homology, the prepituitary structures contain
nothing resembling the main part of the vertebral centra.

762. The brain becomes enveloped in lower verte-
brates by almost continuous cartilage, except where nervous

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structures pass out. The origin of this investment from
two paired bars, the trabeculae and parachordals, which
have different histories, do not originate contemporaneously
in some types, and lie in the base of the anterior and
posterior regions of the cranium, is apparently unlike any-
thing in the history of the vertebral column. What then
is the meaning of these bars ? Are they similar to one
another ?

763. Viewing embryos at the stage when the meso-
cephalic flexure is strongest, gives rise to the idea that
the trabeculae and parachordals are not of the same order.
The early anterior coalescence of the trabeculae presents
some resemblances to the coalescence of the ventral ex-
tremities of a visceral arch. The curved outline of the
trabeculae in various forms further gives an appearance
of possibility to the view that the trabeculae might be
forwardly turned visceral arches supporting the brain;
but the same fact has made it conceivable by some that
the trabeculae are a pair of down-turned neural arches.

764. Yet when we consider that the trabeculae, like
the parachordals, arise in the mesoblast in the floor of
the cranium; that the occurrence of the meaocephalic
flexure does not change these relations, and would appear
to be due to the great expansion of the brain at an early
stage, producing a discontinuity of the basal bars at the
region of the curvature ; that the flexure becomes oblite-
rated, and the basal cartilages coalesce into one on either
side, and then by junction of the right and left bars a
continuous floor arises; that the side walls of the fore
part of the cranium are formed by growth from the trabe-

.culae. just as, posteriorly, the walls are formed from the
parachordals; that nerves are similarly emitted through
the trabecular and the occipital walls; when it is seen, in
short, that the trabeculae are neural in all their relations,
as completely as and in similar fashion to the para-
chordals; it seems impossible to resist the conclusion
that the trabeculae and the parachordals must be placed
in one and the same category.

B. M. 22

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765. If we further consider in what way the structure
of a wide neural tube will differ from that of a simple
cylindrical one, it will appear that the formation of a
broad floor is a feature of unlikeness, but only in degree.
If then the trabeculae and parachordals are looked upon
as representing in a continuous form the basal parts of a
series of cartilages forming the neural arches of vertebraR,
flattened and widened, we think some advance is made
in comprehending the skull; and we are then prepared
to view the lateral parts of the cranium as continuous
neural arches protecting the brain, extending into the
roof and uniting more or less extensively with each other.
And just as in certain forms of vertebrae the bases of
the neural arches nearly or quite meet above the centrum
surrounding the notochord ; similarly in the part of the
cranium destitute of notochord, the bases of the neural
investment meet and form a basal tract, but without any
proper centrum beneath.

766. The most complete brain-case of this kind is
found in forms which do not develope bones; in other
types the cranium is mostly never perfected in cartilage;
often the roof is very incomplete, and even the side
walls are considerably deficient. The occipital region
always however remains more or less perfect ; sometimes
it is imperfect basally in the notochordal tract.

767. There is no definite evidence of segmentation
in the history of the highly-perfected chondrocranium of
the Elasmobranchs, unless the separate origin of the tra-
beculae and parachordals is regarded as such, which we
cannot think. It has been said that it is still very doubt-
ful what is the real significance of the position of exit
of nerves from the cranium as regards segmentation. Until
the doubts as to the homology or equivalence of the cra-
nial nerves are cleared up, it would be rash to frame a
theory of skull structure upon their disposition. K we
trusted to them, we should be led to consider that at
fewest six segments are represented in the simplest skull,

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without reckoning more than one as related to the
vagus \

768. In Elasmobranchs there is a precranial floor
perfectly continuous with the cranium, but it is destitute
of a roof, and its side- walls unite with the nasal capsules.
Where shall we mark off the termination of the cranium
and the commencement of that which is not cranium ?
We cannot tell. We see no logical ground for separating
into two categories the cartilage between the nasal sacs of
a Skate and the cranial floor: the scooping of the- rostrum
in the same skull as well as its continuity of origin from
the trabecula appears to indicate that it is not to be
dissociated from the cranial floor. These parts then
must have originally had some relation to body-segments.
The long precranial valley found in an early stage of
Dactylethra may be considered as throwing light on this

769. There appears to be no reason to doubt that
the Elasmobranch skull is in many ways the lowest and
simplest type we have examined ; and it leads us to specu-
late on the former existence of a still simpler form of
skull, related to a large number of segments of the body,
in which the brain was less concentrated and little ex-
panded, and extended along a lengthy brain-case. Ascent
of type appears to correspond to expansion of the capacity
of the cranium together with concentration of the brain,
its retraction from between the nasal capsules, and in
higher forms its very great diminution in the interorbital
region, or almost total retraction behind the orbits.

770. Thus we view the chondrocranium as having had
a long history ; and it may be impossible to discover and

^ This mode of oompntation is adopted proTisionally, in preference to
adding the fuU number of branchial arches to the cranial segments. V^e
do not see that it foUowfl, because a nerve is suppHed to several seg-
ments, that the portion of the skull to which it is related must be itself
a condensation of several segments. The emergence of the vagus by
several cords in Notidauus may be an indication that a series of cranial
segments corresponding to the branchial arches has disappeared.

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formulate accurately the number of segments of the pri-
mitive vertebral body to which the present chondrocranium
corresponds. It appears to have grown beyond the pri-
mordial simple construction, and to have arisen into a
higher and more complicated form, more perfected and
specialised in relation to conditions of life, combining all
kinds of potentiality, of adaptability to different uses. If
these views are read in connection with those of Professor
Huxley on Amphioxus (Proc. Roy. Soc. 1874), their great
agreement will be perceived.

771. Then in those higher types in which bone occurs,
we get a more or less distinct origination of alisphenoid and
orbitosphenoid pieces in the cranial wall ; but we do not
find any warrant for saying that each of these is homolo-
gous with a neural arch. It does not appear to us that
the cranium is a structure made up of such pieces soldered
together; their distinctness is like that of disconnected por-
tions of a continuous structure ; they belong to the higher
type into which the brain-case grows as an osteocranium.

Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 28 of 31)