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The morphology of the skull online

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Mammals in the ossification of the proximal and the non-
ossification and dwindling of the distal part. In the hyoid
arch of Anura these two portions become developed more
and more independently of one another, as they are
specialised, the one for the ear, the other for the service
of the tongue ; yet after they are definitely formed they
grow towards one another, and repeat their primary rela-
tions in the Fishes: while in the Mammal the same
development is carried to its highest pitch, and yet the
whole hyoid arises continuously from the first.

800. The tracing of scales and membrane-bones froni
their original external and superficial position into their
ultimate complete combination with the primitive endo-
skeletal structures of the skull is a study of high
interest. The very numerous superficial bones become
diminished in number, certain of them being selected, so
to speak, for specialisation, others being dispensed with;
and this occurs more and more as the bones are found
deeper and deeper, and become modelled on the cartilagi-
nous parts. An intermediate condition is seen in the
Sturgeon, where a great number of small bones remain,
together with several large ossifications which represent
definite membrane-bones of other forms. The regional
names of parietal, frontal, &c. have been given to these
bones; they are not, however, in the Sturgeon, merely
representatives of scales belonging to single segments of
the head, but have encroached beyond their primitive mor-
phological territories like the scutes covering the rest of the
body. If they were originally segmeiital ossifications, the
precise segment to which they belong cannot be deter-

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mined, nor can the true composition of the skull be
deduced from them.

801. In those Ganoids which resemble the Teleosteans
in their superficial ossifications, as well as in the Teleos-
teans themselves, while a number of unspecialised mem-
brane-bones remain (in the orbital ring, for instance), we
see an advancing specialisation of certain membrane-bones
which are recognizable more or less throughout the higher
vertebrates. But they often include ossifications of very
superficial strata, and are but loosely connected with the
cartilaginous parts, which have an independent complete-
ness. The parietals, frontals, nasals, and squamosals
(= preopercular + supratemporal) are the bones which be-
come most constant and definite ; and if to them we add
the ectethmoids (prefrontals) and lachrymals (the latter
being specialised from the orbital ring) we have included
all the principal membrane elements on the surface and
sides of the skull, omitting those which margin the jaws.
These bones cannot be assigned to particular segments of
the body ; only their order and regional position can be
predicated ; one pair, the frontals, often represented by a
single bone, acquire a great predominance ; the parietals
attain almost a similar extension in many Mammalia, and
even in some Amphibia.

802. The ingrowth of epiblastic tissues into the
mouth and nares makes it not surprising that membrane-
bones should be found therein. But these are even less
specialised for segments than the outer bones. The para-
sphenoid, never found double excepting where the basi-
temporal wings arise separately at first, as in Birds, is the
basal bone underlying the entire cranial cavity; the
vomers are a pair related to the internal nares : the pre-
maxillaries are in front of these. At the junction of inner
and outer surfaces, the line of the premaxillary is con-
tinued by the maxillary, the jugal, and often the quadrato-
jugal. All these bones are frequently found more or less
specialised as splints to cartilage, or moulded upon its
surface, whether cranial, nasal, prenasal, or palatoptery-

B. 21. 23

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gold; but this does not appear to be the primary condition,
and the bones in the higher forms are in more than one
of these tracts quite independent of the cartilaginous
tracts. The splenial is the corresponding internal bone of
the lower jaw.

803. The external series of membrane-bones belong-
ing to the arches only become definite splints in relation
to the lower jaw, the angular and the dentary being the
two principal bones. In the other tracts the bones are
less accurately or not at all moulded on the cartilage.
The related tracts and membrane-bones are as follows:
upper mandibular region, preopercular and interopercular ;
upper hyoid, opercular and subopercular ; lower hyoid,
branchiostegal rays; branchial arches, the denticular

804. When we reach the Amphibia these membrane-
bones are found much reduced in number and more
closely applied to the cartilage; and at the same time
there is a diminution of the cranial cartilage, especially
above. The squamosal, representing preopercular and
supratemporal, becomes accurately moulded on the man-
dibular suspensorium and on the side of the auditory
capsule. The membrane-bones related to the hyoid arch
do not occur.

805. In the Reptiles the membrane-bones attain
greater perfection and specialisation, with a smaller pro-
portionate development of cartilage; and the external
series still form prominent outworks distinct from the
cranium proper. The frontals and parietals attain a
high degree of perfection, dipping considerably at the
sides, forming cranial walls, and even sending laminae
beneath the brain.

806. In Birds and Mammals a much higher condition
exists; the membrane-bones are greatly expanded in
relation to the increased size of the brain, and more of
them are assumed into its walls, while others are closely

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engrafted upon them, or unified with the cartilage-bones
in the face. The addition of the squamosal to the cranial
wall is most important. The frontal retains its old pre-
dominance, while the parietals gain a greatly increased
influ^ice. Nasals and maxillaries tend more and more
to determine the special characters of the face : but the
lachrymal is a curiously persistent relic of the circum-
orbital bones. The squamosal retains its facial relations
while becoming engrafted upon the cranial wall; the inter-
opercular element crops up as the tympanic bone. Thus
we ultimately get a smoothly compacted skull in which
all the principal elements in the heterogeneous skull of
the Salmon are used up, but which presents the greatest
possible contrast to the Salmon's. Sutures and anchylosis
take the place of apposition or overlapping; the brain-
case is very large proportionately and contains a brain
as large. The parts originating in cartilage, and those
due to membrane, those which have a history looking
back to ancient scales and scutes join harmoniously with
the elements related to the vertebral series, or to the pro-
tection of sense-capsules, so that a complex mass is pro-
duced which seems to defy analysis, and looks as if it had
been straightway made as it appears, instead of being
the result of a prolonged accumulation of morphological
change in the individual, the species, the group, and the

807. A few remarks may be made about the seg-
mentation of the bony skull. In the first place, there is
no difference caused by ossification in the postoral arches
which modifies the views expressed in § 766. Secondly,
only one bony segment, the occipital, can be said to be
clearly manifest in the skulls of Fishes and Amphibians.
And in these forms there are no good grounds for as-
signing to the cranial bones special names indicating a
correspondence to particular parts of vertebrae. From a
study of adult structures in the mammalian groups, skull-
theories have been devised, lacking the basis of embry-
ology ; and granting that they express some of the truth


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respecting the highest forms of skull, there is only injury
to knowledge in arbitrarily interpreting the lower forms
by them. In Reptiles the skull becomes much more per-
fect, but with wide variations in the different groups,
such that they cannot be merely subordinated to and
explained by the mammalian type. A careful study of
the growth of the Bird's skull, again, will show that it
is impossible to express its composition in a simple for-
mula (lerived from vertebral structure. But from the
lower to the higher forms of vertebrates we can discern a
growing away from the primordial type of skull, towards
and into a loftier development. One feature or another
of elevation is manifest in each great group, culmi-
nating in the Birds, where a beautifully-finished and
compact skull results from the union of elements of very
varied primary significance; and iii the Mammals, where
a stability of skull-constiniction seems to be reached, the
full development of a higher type superimposed on a
lower one, and capable of indefinite variation. Here and
here only, with various degrees of perfection, may the
cranium be said in a certain sense to be made up of
segments, enclosing a portion of the neural tube. There
are three of these segments, the occipital, the parietal, the
frontal ; the ethmoidal and nasal tracts cannot be brought
within the category of " segments." The squamosal and
the periotic bones are not explicable by any simple seg-
mental theory of the skulL

808. Then, if it be granted that in the Mammalia
we may perceive with some distinctness three cranial seg-
ments, it does not follow that they constitute three cranial
vertebrae. The considerations we have advanced pre-
viously, in discussing the cartilaginous skull, indicate what
difficulties there are in the way of regarding even the
basioccipital as equivalent to one vertebral centrum, and
the reasons for thinking that it represents ossification in
several primitive body-segments. Consequently we cannot
admit that our investigations give any reason for de-
scribing the skull as constructed by the modification of a

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ixl] thb osseous skull. 357

series of vertebrae, still less for viewing it as directly made
up of a number of cranial vertebrae. This short and easy
road to the comprehension of the skull is really neither
short nor easy, for it does not supply the means of group-
ing together under one conception all the known facts
of adult structure ; it leads to inconsistency in assigning
importance to facts or in framing a nomenclature, and it
fails to give a hint of the existence of those clianges in
growth which we have been tracing ; it makes no pretence
to supply an explanation of them. The longer and more
difficult studies of embryology furnish clues which lead to
a conception of the structure of the skull by which a firm
grasp of facts can be attained, With a sense that there is
no distortion of Nature in the process. The cartilaginous
structures are no longer omitted from consideration ; the
history of the skull is no longer a sealed book.

809L What is the import of these things? What
is their place in our conception of Nature ?

We find that every form of skull that has been inves-
tigated, every stage in development, contributes to one
idea, which becomes simpler, more intelligible, more har-
monious, by the pursuit of a right process of investigation.
There is a unity of structure in the skeleton of the head,
a fundamental formal unity which may always be per-
ceived; and an adaptability to the most varied conditions
of life in water, on land, in air, which becomes more and
not less astonishing as knowledge slowly and surely in-
creases. The illustrations, retrospective to lower condi-
tions, prospective to higher groups, which individual
forms present to us, are an assurance that each life-history
carries its own abundant lessons, that subjects for investi-
gation are inexhaustible, that each worker may win some-
thing for himself towards the comprehension of the creation,
to be his own peculiar possession.

810. In these researches the man of sound mind and
right spirit walks with bated breath, and is charged with
something higher than curiosity as he watches the long-

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concealed operations by which the germ, so simple to the
eye, so similar in all forms, is in continuance fashioned
into the likeness of a vertebrated animal and into the
special representation of its ancestry. And he is filled
with joy, not merely because he is permitted to discover
or to comprehend anything, but because the operation,
the plan, the construction that he sees is so beautiful,
so infinitely beyond human planning. The little mass of
protoplasm and food-material in the egg or the womb is
by unseen power, noiselessly, unceasingly, unhastily driven
onwards in a growth and differentiation which not merely
show us how the individual is built up, but in addition
link it with its fellow-creatures. The embryo is not for
the sake of the imdividual only : it expresses a condensed
history, a manifest relationship ; it is for the sake of those
who can study and learn about nature.

811. We are necessarily led to see that this unity of
structure, this relationship, includes extinct creatures as
well as those now living. And the student cannot but
seek for some further light than is involved in the esta-
blishment of the fact that there is a unity in the structure
of all vertebrate skeletons. An explanation is required ;
we want to comprehend how this unity in diversity has
come about. Morphology studied in the history of embryos
reveals to us an evolution by which the skull passes
through one grade of structure after another, becoming
advanced and changed by almost imperceptible gradations
until the adult type is attained, in a certain number of
days and weeks. This evolution is continually going on
within our experience ; and we think little of its marvels.
And yet many find it inconceivable that the same process
of evolution can have taken place in past ages, so as
to produce from small beginnings the varied fauna of the
globe. The natural forces which in a few days make a
chick out of a little protoplasm and a few teaspoonfuls of
yolk, are pronounced incompetent to give rise to a slowly-
chan^ng gradually-developing series of creatures under
changed conditions of life. Yet to our minds the one is as

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great a marvel as the other; in fact, both are but the dif-
ferent phases of one history of organic creation.

812. The genetic connections of all vertebrates, past
and present, will not be made out for a long time to come;
this is scarcely the place for a full discussion of them ; at
a future time, after further work, we hope to show what
light is thrown upon them by our special study, and
how the geographical distribution of animals and past
physical geography may be illustrated therefrom. But to
indicate the kind of ideas about the past which arise in
the mind of the worker in the pursuit of his researches,
we append quotations from two of the monographs on
which this book has been based.

813. The following passages, written in 1870, are
from the Memoir on the Frog, p. 201 :

" The mind both of the reader and the writer will be
strengthened as well as refreshed by a wider view, and each
separate type will be seen in the light of many other type?.
Indeed thus alone will it be possible to obtain broad views
in vertebrate morphology, ^as a man conveniently placed in
some eminent station may possibly see, at one view, all the
successive parts of a gliding stream; but he that sits by the
water's side, not changing hu place, sees the same parts only
because they succeed, and those that pass make way for them
that follow to come under his eye.*

<' 1 must confess to having subjected myself to this mol&*
like burrowing into so limited a territory that I may obtain
fresh material for ratiocination — *that way of attaining the
knowledge of things, by comparing one thing with another,
considering their mutual relations, connexions, dependencies,
and so arguing out what was more doubtful and obscure, from
what was more known and evident'

" To have worked out one single species in this way may seem
to be but like the forming of a single track in a primaeval
forest ; yet when well cleared, so perfect is the xmity of each
subkingdom, by such a narrow path the worker is * regularly
led on through the labyrinths of Nature, when still new dis-
coveries are successively made, every further inquiry ending in

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a further prospect^ and every new scene of things entertaining
the mind with fresh delight'

" Leaving for a while the suggestive morphology of the
Frog, it may be worth while for the palaeontologist to reflect
upon the empty spaces in the great vertebrate circle which are
darkly but really revealed by what is seen in both the earliest
and the latest stages of the Frog.

" Territories vacant, but larger far than those now occupied
by family after family, and order after order, have been sug-
gested to me by my long attention to the growth of the skull
in this Amphibian.

" Empty spaces of almost indefinite extent seem, to my
mind, to stretch themselves below the Myxinoid prototypes of
the Batrachia, and above and beyond the Frogs and Toads, in
the direction of the MammaUa.

** This last space is wholly undefined, and no light has yet
penetrated its deep abyss, in which lie buried the fundamental
Mammalian types. The lowest Mammals known to us, the
Platypus and the Echidna, may be fundamental to the Eden-
tata; they are not, they cannot be, to the Marsupials, the
Insectivora, and the Rodentia.

" Between the Monotremes and the Batrachia we certainly
have the Sauropsida — Reptiles and Birds ; but I am bold to
say that no Sauropsidan lies in a direct line between, forms
any part of a phylum which should connect together the nobler
Amphibian forms and the lowest MammaL On the Mammalian
side of this empty space we must suppose a form which should
be general to the whole class; I need not say that no such
form is extant. The extraordinary and unlooked-for morpho-
logical elevation of the adult Anuran, an elevation in very
important features attained by no Reptile or Bird, and which
brings it almost into contact at certain points with the Mam-
malian margin, is very suggestive. Such a discovery sheds a
certain but feeble light, useful though faint.

" The fact that the higher Batrachia go on metamorphosing
until several of their structures are so perfect as to require but
the gentlest modification to make them fit for the Mammal,
does not require one to suppose that the Toad and the Frog lie
in the direct route from the Ichthyic to the Mammalian types.
That such power of variation, sudb aptitude for transformation
exists in these essential but metamorphic Fish, suggests the
probability that some of the very earliest of the Amphibia,

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filial j>erhaps to forms far lower than the Lamprey, did not
stop at the last metamorphic stage of an Anuran, but chaDged
still further, and thus laid the foundation of the higher classes.

" We are all looking for further traces of the phylum which
shall complete the connexion between the cold-blooded, scaly
types of Sauropsida and the feathered, warm-blooded Birds;
even should this never be attained to, yet no one will doubt
that it has existed.

" An Amphibian, full of latent power of change, need not
have taken in its metamorphosis merely the path that leads
to the Reptile and the Bird; for the least deflection at first
may have sufficed to bring about all the differences which now,
in this late human period, we see between the Mammal and
the Bird. These warm-blooded groups are huge culminating
branches of the tree of vertebrate life ; yet it is not a wild
fancy to suppose that they may once have existed together in
the same common trunk.

" So much for the vacant space above the Myxinoids ; the
lower is much larger and n?ore pathless.

** The lowest existing Fish but one is the Myxinoid (Lam-
prey, Hag, Bdellostoma) ; between it and the lowest kn<»wn
Vertebrate, the Lancelet (Amphioxus), there is a gap the extent
of which has never been imagined ; and yet even the Lancelet
itself is not necessarily the actual boundary form.

" I have shown in my comparisons that the larval Lamprey
(Ammoccetes) is only a little lower than my third stage of the
Frog, whilst my fourth stage answers very closely to the adult

" Let us imagine three families of extinct Fishes below the
Lamprey : fii'st, a group arrested as to type at the Ammocoetine
stage; secondly, a group which may be morphologically repre-
sented by my second stage of the Batrachian embryo; and
thirdly, a group no higher than my first stage.

** These three families may have abounded in genera and
species, and have been as perfectly in harmony with their
surroundings as the highly-specialised and noble Ganoid Fishes.
How far these groups would tend to fill up the space between
the Amphioxus and the simplest of their species, I need not
say. Every anatomist will at once see that a creature no
higher in type than the unhatched embryo of the Frog is
yet an untold distance in advance of the Lancelet, which yet is
only the known lowest of the great Vertebrate subkingdom.

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814. The second quotation, written in 1868, is from
the Memoir on the Fowl, p. 803 :

" I would conclude by hinting at the importance of the
various isomorphisms displayed by the skull and face of this
one type in its stages of growth.

" I have described it upwards, but my long and really
anxious labour has been in the opposite direction ; the stages
were traced from that of the old bird downwards to that of the
chick of the fourth day of incubation.

" Whilst at work I seemed to myself to have been endea-
voiu-ing to decipher a palimpsesty and one not erased and writ-
ten upon again just once, but five or six times over.

" Having erased, as it were, the characters of the culmin-
ating type — those of the gaudy Indian bird — I seemed to be
amongst the sombre Grouse; and then, towards incubation, the
characters of the Sand-grouse and Hemipod stood out before
ma Rubbing these away, in my downward work the foinn of
the Tinamou looked me in the face; then the aberrant Ostrich
seemed to be described in large archaic characters; a little
while, and these faded into what could just be read off as per-
taining to the Sea-turtle; whilst underlying the whole, the
Fish in its simplest Myxinoid form could be traced in morpho-
logical hieroglyphics."

815. These words express the kind of thoughts which
have been developed by and have guided research into the
morphology of the skull. The result of this study is
to leave upon the mind a strong conviction that the pre-
sent Vertebrates are but the ultimate twigs of diverg-
ing branches of one great tree of life. Some branches
are small, others great; some nearer the main stock,
others more remote; some bear few twigs and appear
isolated, others are so crowded with forms that the
branches from which they spring can scarcely be dis-
cerned. But happily the growth of every form is capable
of revealing to us something of its own relationships, and
of the history of a time when the tree of life was confined
within narrower limits, when branches or branchlets now
extinct were in existence ; and by comparing these with
fossil remains we understand the meaning of "comprehen-

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sive types ", which represent in a condensed fashion whole
divergent groups of the present day. And we can ask the
acceptance of Evolution in past time, without necessarily
implying any particular view as to the causes of this Evo-
lution, except that they were slow and continuous.

816. We may be permitted to say in conclusion that
in our experience the study of animal morphology leads
to continually grander and more reverential views of crea-
tion and of a Creator. Each fresh advance shows us fur-
ther fields for conquest, and at the same time deepens the
conviction that, while results and secondary operations
may be discoverable by human intelligence, " no man can
find out the work that God maketh from the beginning to
the end." We live as in a twilight of knowledge, charged
with revelations of order and beauty ; we stedfastly look
for a perfect light which shall reveal perfect order and

Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 30 of 31)