standing, that the skulls of the vertebrata consist, like the columns
to which they are attached, of vertebral joints, composed each, in the
more typical forms of head, as they are in the trunk, of five parts or
elements,—the centrum or body, the two spinous processes which enclose
the spinal cord, and the two ribs. These cranial vertebræ, four in
number, correspond, it is said, to the four senses that have their seat
in the head: there is the nasal vertebra, the centrum of which is the
vomer, its spinal processes the nasal and ethmoid bones, and its ribs the
_upper_ jaws; there is the ocular vertebra, the centrum of which is the
anterior portion of the sphenoid bone, its spinal processes the frontals,
and its ribs the _under_ jaws; there is the lingual vertebra, the centrum
of which is the posterior sphenoid bone, its spinal processes the
parietals, and its ribs the hyoid and branchial bones,—portions of the
skeleton largely developed in fishes; and, lastly, there is the auditory
vertebra, the centrum of which is the base of the occipital bone, and its
spinal processes the occipital crest, and which in the osseous fishes
bears attached to it, as its ribs, the bones of the scapular ring. And
the cerebral segments thus constructed we find represented in typical
diagrams of the skull, as real vertebræ. Professor Owen, in his lately
published treatise on “The Nature of Limbs,”—work charged with valuable
fact, and instinct with philosophy,—figures in his draught of the
archetypal skeleton of the vertebrata, the four vertebræ of the head, in
a form as unequivocally such as any of the vertebræ of the neck or body.
Now, for certain purposes of generalization, I doubt not that the
conception may have its value. There are in all nature and in all
philosophy certain central ideas of general bearing, round which, at
distances less or more remote, the subordinate and particular ideas
arrange themselves,
“Cycle and epicycle, orb in orb.”
In the classifications of the naturalist, for instance, all _species_
range round some central _generic_ idea; all genera round some central
idea, to which we give the name of _order_; all orders round some central
idea of _class_; all classes round some central idea of _division_;
and all divisions round the interior central idea which constitutes
a _kingdom_. Sir Joshua Reynolds forms his theory of beauty on this
principle of central ideas. “Every species of the animal, as well as
of the vegetable creation,” he remarks, “may be said to have a fixed
or determinate form, towards which nature is continually inclining,
like various lines terminating in a centre; or it may be compared to
pendulums vibrating in different directions over one central point,
which they all cross, though only one of their number passes through
any other point.” He instances, in illustrating his theory, the Grecian
_beau ideal_ of the human nose, as seen in the statues of the Greek
deities. It formed a straight line; whereas all deformity of nose is of
a convex or concave character, and occasioned by either a rising above
or a sinking below this medial line of beauty. And it may be of use, as
it is unquestionably of interest, to conceive, after this manner, of a
certain type of skeleton, embodying, as it were, the central or primary
type of all vertebral skeletons, and consisting of a double range of
rings, united by the bodies of the vertebræ, as the two rings of a
figure 8 are united at their point of junction; the upper ring forming
the enclosure of the brain,—spinal, and cephalic; the lower that of
the viscera,—respiratory, circulatory, and digestive. Such is the idea
embodied in Professor Owen’s archetypal skeleton. It is a series of
vertebræ composing double rings,—their _brain_-rings comparatively small
in the vertebræ of the trunk, but of much greater size in the vertebræ
of the head. But it must not be forgotten, that central ideas, however
necessary to the classification of the naturalist, are not historic
facts. We may safely hold, with the philosophic painter, that the outline
of the typical human nose is a straight line; but it would be very
unsafe to hold, as a consequence, that the first men had all straight
noses. And when we find it urged by at least one eminent assertor of the
development hypothesis,—Professor Oken,—that light was the main agent in
developing the substance of nerve,—that the nerves, ranged in pairs, in
turn developed the vertebræ, each vertebra being but “the periphery or
envelope of a pair of nerves,”—and that the nerves of those four senses
of smell, sight, taste, and hearing, which, according to the Professor,
“make up the head,” originated the four cranial vertebræ which constitute
the skull,—it becomes us to test the central idea, thus converted into a
sort of historic myth, by the realities of actual history. What, then,
let us inquire, is the real history of the cerebral development of the
vertebrata, as recorded in the rocks of the earlier geologic periods?
[Illustration: Fig. 7.
_Osseous points of placoid cranium._[12]
(Mag. twelve diameters)]
Though the vertebrata existed in the ichthyic form throughout the vastly
extended Silurian period, we find in that system no remains of the
cranium: the Silurian fishes _seem_, as has been already said, (page
53,) to have been exclusively Placoid, and the purely cartilaginous box
formed by nature for the protection of the brain in this order has in
no case been preserved. Teeth, and, in at least one or two instances,
the minute jaws over which they were planted have been found, but no
portion of the skull. We know, however, that in the fishes of the same
order which now exist, the cranium consists of one undivided piece of a
cartilaginous substance, set thickly over its outer surface with minute
polygonal points of bone, (fig. 7,) composed internally of star-like
rays, that radiate from the centre of ossification, and that present, in
consequence, seen through a microscope, the appearance of the polygonal
cells of a coral of the genus Astrea. The pattern induced is that of
stars set within polygons. Along the sides or top of this unbroken
cranial box, that exhibits no mark of suture, we find the perforations
through which the nerves of smell, sight, taste, and hearing passed from
the brain outwards, and see that they have failed to originate distinct
vertebral envelopes for themselves;—they all lodge in one undivided
mansion-house, and have merely separate doors. We find, further, that
the homotypal _ribs_ of the entire cranium consist, not of four, but
simply of a single pair, attached to the occiput, and which serves both
to suspend the jaws, upper and nether, in their place under the middle of
the head, and to lend support to the hyoid and branchial framework; while
the scapular ring we find existing, as in the higher vertebrata, not as
a cerebral, but as a cervical or dorsal appendage. In the wide range
of the animal kingdom there are scarce any two pieces of organization
that less resemble one another in form than the vertebræ of the placoids
resemble their skulls; and the difference is not merely external, but
extends to even their internal construction. In both skull and vertebræ
we detect an union of bone and cartilage; but the bone of each vertebra
forms an internal continuous nucleus, round which the cartilage is
arranged, whereas in the skulls it is the cartilage that is internal, and
the bone is spread in granular points over it. If we dip the body of one
of the dorsal vertebræ of a herring into melted wax, and then withdraw
it, we will find it to represent in its crusted state the vertebral
centrum of a Placoid,—soft without, and osseous within; but in order
to represent the placoid skull, we would have first to mould it out of
one unbroken piece of wax, and then to cover it over with a priming of
bone-dust. And such is the effect of this arrangement, that, while the
skull of a Placoid, exposed to a red heat, falls into dust, from the
circumstance that the supporting framework on which the granular bone
was arranged perishes in the fire, the vertebral centrum, whose internal
framework is itself bone, and so _not_ perishable, comes out in a state
of beautiful entireness,—resembling in the thornback a squat sand-glass,
elegantly fenced round by the lateral pillars, (fig. 8, _b_;) and in
the dog-fish (_a_) a more elongated sand-glass, in which the lateral
pillars are wanting. Such are the heads and vertebral joints of the
existing Placoids; and such, reasoning from analogy, seem to have been
the character and construction of the heads and vertebral joints of the
Placoids of the Silurian period,—earliest-born of the Vertebrata.
[Illustration: Fig. 8.
a. _Osseous centrum of Spinax Acanthias._
b. _Osseous centrum of Raja clavata._
(Nat. size.)]
The most ancient brain-bearing craniums that have come down to us
in the fossil state, are those of the Ganoids of the Lower Old Red
Sandstone; and in these fishes the true skull appears to have been
as entirely a simple cartilaginous box, as that of the Placoids of
either the Silurian period or of the present time, or of those existing
Ganoids, the sturgeons. In the Lower Old Red genera _Cheiracanthus_ and
_Diplacanthus_, though the heads are frequently preserved as amorphous
masses of colored matter, we detect no trace of internal bone, save
perhaps in the gill-covers of the first-named genus, which were fringed
by from eighteen to twenty minute osseous rays. The cranium seems to have
been covered, as in the shark family, by skin, and the skin by minute
shagreen-like scales; and all of the interior cerebral framework which
appears underneath exists simply as faint impressions of an undivided
body, covered by what seem to be osseous points,—the bony molecules,
it is probable, which encrusted the cartilage. The jaws, in the better
specimens, are also preserved in the same doubtful style, and this state
of keeping is the common one in deposits in which every true bone,
however delicate, presents an outline as sharp as when it occupied its
place in the living animal. The dermal or skin-skeleton of both genera,
which consisted, as has been shown (pages 55, 56) of shagreen-like
osseous scales and slender spines, both brilliantly enamelled, is
preserved entire; where as the interior framework of the head exists as
mere point speckled impressions; and the inference appears unavoidable
that parts which so invariably differ in their state of keeping now, must
have essentially differed in their substance originally.
[Illustration: Fig. 9.
a. _Portion of caudal fin of Cheiracanthus._[13]
b. _Portion of caudal fin of Cheirolepis Cummingiæ._
(Mag. three diameters.)]
Now, in the _Cheiracanthus_ we detect the first faint indications of
a peculiar arrangement of the dermal skeleton, in relation to certain
parts of the skeleton within, which—greatly more developed in some of its
contemporaries—led to important results in the general structure of these
Ganoids, and furnishes the true key to the character of the early ganoid
head. In such of the existing Placoids as I have had an opportunity of
examining, the only portions of the dermal skeleton of bone which conform
in their arrangement to portions of the interior skeleton of cartilage,
are the teeth, which are always laid on a base of skin right over the
jaws: there is also an approximation to arrangement of a corresponding
kind, though a distant one, in those hook-armed tubercles of certain
species of rays which run along the vertebral column; but in the shagreen
by which the creatures are covered I have been able to detect no such
arrangement. Whether it occurs on the fins, the body, or the head, or in
the scale form, or in that of the prickle, it manifests the same careless
irregularity. And on the head and body of the _Cheiracanthus_, and on
all its fins save one, the shagreen-like scales, though laid down more
symmetrically in lines than true shagreen, manifested an equal absence
of arrangement in relation to the framework within. On that one fin,
however;—the caudal,—the scales, passing from their ordinary rhomboidal
to a more rectangular form, ranged themselves in right lines over the
internal rays, (fig. 9, _a_,) and imparted to these such strength as a
splint of wood or whalebone fastened over a fractured toe or finger
imparts to the injured digit,—a provision which was probably rendered
necessary in the case of this important organ of motion, from the
circumstance that it was the only fin which the creature possessed that
was not strengthened and protected anteriorly by a strong spine. In the
_Cheirolepis_,—a contemporary fish, characterized, like its cogeners the
_Cheiracanthus_ and _Diplacanthus_, by shagreen-like scales, but in which
the spines were wanting,—we find a farther development of the provision.
In all the fins the richly-enamelled dermal-covering was arranged in
lines over the rays, (fig. 9, _b_;) and the scale, which assumes in the
fins, like the scales on the tail of the _Cheiracanthus_, though somewhat
more irregularly, a rectangular shape, is so considerably elongated,
that it assumes for its normal character as a scale, that of the joint
of an external ray. A similar arrangement of external protection takes
place in this genus over the bones of the head; the cartilaginous jaws
receive their osseous dermal covering, and, with these, the hyoid bones,
the opercules, and the cranium. And it is in these dermal plates,
which covered an interior skull, of which, save in one genus,—the
_Dipterus_,—not a vestige remains in any of the Old Red fishes thus
protected, that we first trace what seem to be the homologues of the
cranial bones of the osseous fishes,—at least their homologues so far as
the _cuticular_ can represent the _internal_. They appear for the first
time, not as modified spinous processes, broadened, as in the carapace
of the Chelonians, into _osseous_ plates, but like those _corneous_
external plates of this order of reptiles, (known in one species as the
tortoise-shell of commerce,) the origin of which is purely cuticular,
and which evince so little correspondence in their divisions with the
sutures of the bones on which they rest, that they have been instanced,
in their relation to the joinings beneath, as admirable illustrations of
the _cross-banding_ of the mechanician.
In the heads of the osseous fishes, the cranium proper, though
consisting, like the skulls of birds, reptiles, and mammals, of several
bones, exists from snout to nape, and from mastoid to mastoid, as
one unbroken box; whereas all the other bones of the head, such as
the maxillaries and intermaxillaries, the lower jaws, the opercular
appendages, the branchial arches, and the branchiostegous rays,
are connected but by muscle and ligament, and fall apart under the
putrefactive influences, or in the process of boiling. This unbroken box,
which consists, in the cod, of twenty-five bones, is the _homologue_ of
that cranial box of the Placoids which consists of one entire piece, and
the _homotype_, according to Oken, of the bodies and spinal processes of
four vertebræ; while the looser bones which drop away represent their
_ribs_. The upper surface of the box,—that extending from the nasal
bone to the nape,—is the only part over which a dermal buckler could
be laid, as it is the only part with which the external skin comes in
contact; and so it is between this upper surface and the cranial bucklers
of the earlier Ganoids that we have to institute comparisons. For it
is a curious fact, that, with the exception of the Old Red genera
_Acanthodus_, _Cheiracanthus_, and _Diplacanthus_,[14] all the Ganoids of
the period in which Ganoids first appear _have_ dermal bucklers placed
right over their true skulls, and that these, though as united in their
parts as the bones proper to the cranium in quadrupeds and fishes, are
composed of several pieces, furnished each with its independent centre
of ossification. The Dipterians, the Cœlacanths, the Cephalaspians, and
at least one genus placed rather doubtfully among the Acanths,—the genus
_Cheirolepis_,—all possessed cranial bucklers extending from the nape to
the snout, in which the plates, various, in the several genera, in form
and position, were fast _soldered_ together, though in every instance the
lines of suture were distinctly marked.
[Illustration: Fig. 10.
UPPER SURFACE OF CRANIUM OF COD.[15]
A, _Occipital bone_. B, B, _Parietals_. C, C, C, _Superior frontal_. D,
D, _Anterior frontal_. I, _Nasal bone_. F, F, _Posterior frontals_. E, E,
_Mastoid bones_. 2, 2, _Eye orbits_. a, a, _Par-occipital bones_.]
On each side of this external cranium the various cerebral plates, like
the corresponding cerebral _ribs_ in the osseous fishes, were free, at
least not anchylosed together; and some of their number unequivocally
performed, in part at least, the functions of two of these cerebral
ribs, viz. the upper and under jaws, with the functions of the opercular
appendages attached to the latter. In the cod, as in most other osseous
fishes, the upper portion of the cranium consists of thirteen bones,
which represent, however, only seven bones in the human skull,—the nasal,
the frontal, the two parietal, the occipital, and one-half the two
temporal bones. And whereas in man, and in most of the mammals, there are
four of these placed in the medial line,—the four which, according to the
assertors of the vertebral theory, form the spinal crests of the four
cerebral vertebræ,—in the cod there are but three. The super-occipital
bone, A, (fig. 10,) pieces on to the superior frontal, C, C, C; and the
parietals, B, B, which in the human subject from the upper and middle
portions of the cranial vault, are thrust out laterally and posteriorly,
and take their places, in a subordinate capacity, on each side of the
super-occipital. This is not an invariable arrangement among fishes;—in
the carp genus, for instance, the parietals assume their proper medial
place between the occipital and frontal bones; but so very general is
the displacement, that Professor Owen regards it as characteristic of
the great ichthyic class, and as the first example in the vertebrata,
reckoning from the lower forms upwards, of a sort of natural dislocation
among the bones,—“a modification,” he remarks, “which, sometimes
accompanied by great change of place, has tended most to obscure the
essential nature of parts, and their true relations to the archetype.”
[Illustration: Fig. 11.
CRANIAL BUCKLER OF COCCOSTEUS DECIPIENS.
a, a, _Points of attachment to the cuirass which covered the upper part
of the creature’s body_.]
Of all the cerebral bucklers of the first ganoid period, that which
best bears comparison with the cranial front of the cod is the buckler
of the _Coccosteus_, (fig. 11.) The general proportions of this portion
of the ancient Cephalaspian head differ very considerably from those of
the corresponding part in the modern cycloid one; but in their larger
divisions, the modern and the ancient answer bone to bone. Three osseous
plates in the _Coccosteus_, A, C, I, the homologues, apparently, of the
occipital, frontal, and nasal bones, range along the medial line. The
apparent homologues of the parietals, B, B, occupy the same position
of lateral displacement as the parietals of the cod and of so many
other fishes. The posterior frontals, F, and the anterior frontals,
D, also occupy places relatively the same, though the latter, which
are of greater proportional size, encroach much further, laterally and
posteriorly, on the superior frontal C, C, C, and sweep entirely round
the upper half of the eye orbits, 2, 2. The apparent homologue of the
mastoid bone, E, which also occupies its proper place, joins posteriorly
to a little plate, a, imperfectly separated in most specimens from the
parietal, but which seems to represent the par-occipital bone; and it is
a curious circumstance, that as, in many of the osseous fishes, it is to
these bones that the forks of the scapular arch are attached, they unite
in the _Coccosteus_ in furnishing, in like manner, a point of attachment
to the cuirass which covered the upper part of the creature’s body. Of
the true internal skull of the _Coccosteus_ there remains not a vestige
Like that of the sturgeon, it must have been a perishable cartilaginous
box.
[Illustration: Fig. 12.
CRANIAL BUCKLER OF OSTEOLEPIS.]
In the _Osteolepis_,—an animal the whole of whose external head I have,
at an expense of some labor, and from the examination of many specimens,
been enabled to restore,—the cranial buckler (fig. 12) was divided in a
more arbitrary style; and we find that an element of uncertainty mingles
with our inferences regarding it, from the circumstance that some of its
lines of division, especially in the frontal half, were not real sutures,
but formed merely a kind of surface-tatooing, resorted to as if for
purposes of ornament. The cranial buckler of the _Asterolepis_ exhibited,
as I shall afterwards have occasion to show, a similar peculiarity;—both
had their pseudo-sutures, resembling those false joints introduced by the
architect into his rusticated basements, in order to impart the necessary
aspect of regularity to what is technically termed the coursing and
banding of the fabric. We can however, determine, notwithstanding the
induced obscurity that the buckler of the _Osteolepis_ was divided
transversely in the middle into two main parts or segments,—an occipital
part, C, and a frontal part, A; and that the occipital segment _seems_
to include also the parietal and mastoid plates, and the frontal segment
to comprise, with its own proper plates, not only the nasal plate, but
also the representative of the anterior part of the vomer. All, however,
is obscure. But in our uncertainty regarding the homologies of the
divisions of this dermal buckler, let us not forget the homology of the
buckler itself, as a whole, with the upper surface of the true cranium
in the osseous fishes. Though frequently crushed and broken, it exists
in all the finer specimens of my collection as a symmetrically arranged
collocation of enamelled plates, as firmly united into one piece,
though they all indicate their distinct centres of ossification, as the
corresponding surface of the cranium in the carp or cod. The lateral
curves in the frontal part immediately opposite the lozenge-shaped plate
in the centre, show the position of the eyes, which were placed in this
genus, as in some of the carnivorous turtles, immediately over the
mouth,—an arrangement common to almost all the Ganoids of the Lower Old
Red Sandstone. The nearly semicircular termination of the buckler formed
the creature’s snout; and in the _Osteolepis_, as in the _Glyptolepis_
and the _Diplopterus_, it was armed on the under side, like the vomer of
so many of the osseous fishes, with sharp teeth. Some of my specimens
indicate the nasal openings a little in advance of the eyes. The nape of
the creature was covered by three detached plates, (9, 9, 9, fig. 13,)
which rested upon anterior dorsal scales, and whose homologies, in the
osseous fishes, may possibly be found in those bones which, uniting
the shoulder-bones to the head, complete the scapular belt or ring. The
operculum we find represented by a single plate (8) which had attached to
it, as its sub-operculum, a plate (13) of nearly equal size, (see figs.
14 and 15.) Four small plates (2, 4, 5) formed the under curve of the
eyes, described in many of the osseous fishes by a chain of small bones
or ossicles; a considerably larger plate (6) occupied the place of the
preopercular bone; while the intermaxillaries had their representatives
in well-marked plates, (3, 3,) which, in the genera _Osteolepis_,
_Diplopterus_, and _Glyptolepis_, we find bristling so thickly with
teeth along their lower edges, as to remind us of the miniature saws
employed by the joiner in cutting out circular holes. These external
intermaxillaries did not, as in the perch or cod, meet in front of the
nasal bone and vomer, but joined on at the side, a little in advance of
the eyes, leaving the rounded termination of the cranial buckler, which,
like the intermaxillaries, was thickly fringed with teeth, to form, as
has been already said, the creature’s snout.
[Illustration: Fig. 13.
UPPER PART OF HEAD OF OSTEOLEPIS.]