THE SKULL.
Three distinct sets of elements may enter into the composition of the
skull. These are (1) the cranium proper, composed of true endoskeletal
elements originally formed in cartilage, to which are usually added
exoskeletal osseous elements, formed in the manner already described
p. 542, and known in the higher types as membrane bones. (2) The
visceral arches formed primitively as cartilaginous bars, but in the
higher types largely supplemented or even replaced by exoskeletal
elements. (3) The labial cartilages.
These parts present themselves in the most various forms, and their
study constitutes one of the most important departments of vertebrate
morphology, and one which has always been a favourite subject of study
with anatomists. At the end of the last century and during the first
half of the present century the morphology of the skull was handled
from the point of view of the adult anatomy by Goethe, Oken, Cuvier,
Owen, and many other anatomists, while Dugés and, nearer to our own
time, Rathke, laid the foundation of an embryological study of its
morphology. A new era in the study of the skull was inaugurated by
Huxley in his Croonian lecture in 1858, and in his lectures on
Comparative Anatomy subsequently delivered before the Royal College of
Surgeons. In these lectures Huxley disproved the then widely accepted
view that the skull was composed of four vertebræ; and laid the
foundation of a more satisfactory method of dealing with the
homologies of its constituent parts. Since then the knowledge of the
development of the skull has made great progress. In this country a
number of very interesting memoirs have been published on the subject
by Parker, which together constitute a most striking contribution to
our knowledge of the ontogeny of the skull in a series of types; and
in Germany Gegenbaur's monograph on the cephalic skeleton of
Elasmobranchii has greatly promoted a scientific appreciation of the
nature of the skull.
In the present chapter only the most important features in the
development of the skull will be touched on.
It will be convenient to describe, in the first instance, the
development of the cartilaginous elements of the skull.
[FIG. 323. HEAD OF EMBRYO DOGFISH, SECOND STAGE; BASAL VIEW OF
CRANIUM FROM ABOVE, THE CONTENTS HAVING BEEN REMOVED. (From Parker.)
_ol._ olfactory sacs; _au._ auditory capsule; _nc._ notochord; _py._
pituitary body; _pa.ch._ parachordal cartilage; _tr._ trabecula;
_inf._ infundibulum; _C.tr._ cornua trabeculæ; _pn._ prenasal
element; _sp._ spiracular cleft; _br._ external branchiæ; _Cl._ 2,
4. visceral clefts.]
The Cranium. The brain is at first enveloped in a continuous layer of
mesoblast known as the membranous cranium, into the base of which the
anterior part of the notochord is prolonged for some distance. The
primitive cartilaginous cranium is formed by a differentiation within
the membranous cranium, and is always composed of the following parts
(fig. 323):
(1) A pair of cartilaginous plates on each side of the cephalic
section of the notochord, known as the parachordals (_pa.ch_). These
plates together with the notochord (_nc_) enclosed between them form a
floor for the hind- and mid-brain. The continuous plate, formed by
them and the notochord, is known as the basilar plate.
(2) A pair of bars forming the floor for the fore-brain, known as the
trabeculæ (_tr_). These bars are continued forward from the
parachordals. They meet behind and embrace the front end of the
notochord; and after separating for some distance bend in again in
such a way as to enclose a space--the pituitary space. In front of
this space they remain in contact and generally unite. They extend
forwards into the nasal region (_pn_).
(3) The cartilaginous capsules of the sense organs. Of these the
auditory (_au_) and olfactory capsules (_ol_) unite more or less
intimately with the cranial walls; while the optic capsules, forming
the usually cartilaginous sclerotics, remain distinct.
The parachordals and notochord. The first of these sets of elements,
viz. the parachordals and notochord, forming together the basilar
plate, is always an unsegmented continuation of the axial tissue of
the vertebral column. It forms the floor for that section of the brain
which belongs to the primitive postoral part of the head (_vide_ p.
314), and its extension is roughly that of the basioccipital of the
adult skull. Its mode of development is almost identical with that of
the vertebral column, except that the notochord, even in many forms
where it persists in the vertebral column, disappears in the basilar
plate; though in a certain number of cases remnants of it are found in
the adult state.
It will be convenient to say a few words here with reference to the
notochord in the head. It always extends along the floor of the mid-
and hind-brains, but ends immediately behind the infundibulum. The
limits of its anterior extension are clearly shewn in fig. 43. The
front end of the notochord often becomes more or less ventrally flexed
in correspondence with the cranial flexure; its anterior end being in
some instances (Elasmobranchii) almost bent backwards (fig. 324).
Kölliker has shewn that in the Rabbit[203], and I believe that a more
or less similar phenomenon may also be observed in Birds, the anterior
end of the notochord is united to the hypoblast of the throat in
immediate contiguity with the opening of the pituitary body; but it is
not clear whether this is to be looked upon as the remnant of a
primitive attachment of the notochord to the hypoblast, or as a
secondary attachment.
[203] "Embryologische Mittheilungen." _Festschrift d. Naturfor.
Gesell._, Halle, 1879.
[FIG. 324. LONGITUDINAL SECTION THROUGH THE BRAIN OF A YOUNG
PRISTIURUS EMBRYO.
_cer._ commencement of the cerebral hemisphere; _pn._ pineal gland;
_In._ infundibulum; _pt._ ingrowth from mouth to form the pituitary
body; _mb._ mid-brain; _cb._ cerebellum; _ch._ notochord; _al._
alimentary tract; _Iaa._ artery of mandibular arch.]
Before the parachordals are formed the anterior end of the notochord
has usually undergone a partial atrophy; and its front end often
becomes somewhat dorsally flexed. Within the basilar plate it often
exhibits two or more dilatations, which have been regarded by Parker
and Kölliker as indicative of a segmentation of this plate; but they
hardly appear to me to be capable of this interpretation.
In Elasmobranchs where, as shewn above, a very primitive type of
development of the vertebral column is retained, we find that the
basilar plate is at first formed of (1) the notochord invested by its
cartilaginous sheath, and (2) of lateral masses of cartilage, the
parachordals, homologous with the arch tissue of the vertebral column.
This development probably indicates that the basilar plate contains in
itself the same elements as those from which the neural arches and the
centra of the vertebral column are formed; but that it never passes
beyond the unsegmented stage at first characteristic of the vertebral
column. The hinder end of each parachordal forms a condyle
articulating with the first vertebra; so that in the cartilaginous
skull there are always two occipital condyles. The basilar plate
always grows up behind (fig. 326, _so_), and gives rise to a complete
cartilaginous ring enveloping the medulla oblongata, in the same
manner that the neural arches envelope the spinal cord. This ring
forms an occipital cartilaginous ring; in front of it the basilar
plate becomes laterally continuous with the periotic cartilaginous
capsules, and the occipital ring above usually spreads forward to form
a roof for the part of the brain between these capsules. In the higher
Vertebrates the periotic cartilages may be developed continuously with
the basilar plate (fig. 325).
The trabeculæ. The trabeculæ, so far as their mere anatomical
relations are concerned, play the same part in forming the floor for
the front cerebral vesicle as the parachordals for the mid- and
hind-brains. They differ however from the parachordals in one
important feature, viz. that, except at their hinder end (fig. 323),
they do not embrace between them the notochord.
The notochord constitutes, as we have seen, the primitive axial
skeleton of the body, and its absence in the greater part of the
region of the trabeculæ would probably seem to indicate, as pointed
out by Gegenbaur, that these parts, in spite of their similarity to
the parachordals, have not the same morphological significance.
[FIG. 325. VIEW FROM ABOVE OF THE INVESTING MASS AND OF THE
TRABECULÆ OF A CHICK ON THE FOURTH DAY OF INCUBATION. (After
Parker.)
In order to shew this, the whole of the upper portion of the head
has been sliced away. The cartilaginous portions of the skull are
marked with the dark horizontal shading.
_cv_ 1. cerebral vesicle (sliced off); _e._ eye; _nc._ notochord;
_iv._ investing mass; 9. foramen for the exit of the ninth nerve;
_cl._ cochlea; _hsc._ horizontal semicircular canal; _q._ quadrate;
5. notch for the passage of the fifth nerve; _lg._ expanded anterior
end of the investing mass; _pts._ pituitary space; _tr._ trabeculæ.
The reference line _tr._ has been accidentally made to end a little
short of the cartilage.]
The nature of the trabeculæ has been much disputed by morphologists.
The view that they cannot be regarded as the anterior section of the
vertebral axis is supported by the consideration that the forward
limit of the primitive skeletal axis, as marked by the notochord,
coincides exactly with the distinction we have found it necessary to
recognise, on entirely independent grounds, between the fore-brain,
and the remainder of the nervous axis. But while this distinction
between the parachordals and the trabeculæ must I think be admitted, I
see no reason against supposing that the trabeculæ may be plates
developed to support the floor of the fore-brain, for the same
physiological reasons that the parachordals have become formed at the
sides of the notochord to support the floor of the hind-brain. By some
anatomists the trabeculæ have been held to be a pair of branchial
bars; but this view has now been generally given up. They have also
been regarded as equivalent to a complete pair of neural arches
enveloping the front end of the brain. The primitive extension of the
base of the fore-brain through the pituitary space is an argument, not
without force, which has been appealed to in support of this view.
In the majority of the lower forms the trabeculæ arise quite
independently of the parachordals, though the two sets of elements
soon unite; while in Birds (fig. 325) and Mammals the parachordals and
trabeculæ are formed as a continuous whole. The junction between the
trabeculæ and parachordals becomes marked by a cartilaginous ridge
known as the posterior clinoid.
[FIG. 326. SIDE VIEW OF THE CARTILAGINOUS CRANIUM OF A FOWL ON THE
SEVENTH DAY OF INCUBATION. (After Parker.)
_pn._ prenasal cartilage; _aln._ alinasal cartilage; _ale._
aliethmoid; immediately below this is the aliseptal cartilage.
_eth._ ethmoid; _pp._ pars plana; _ps._ presphenoid or interorbital;
_pa._ palatine; _pg._ pterygoid; _z._ optic nerve; _as._
alisphenoid; _q._ quadrate; _st._ stapes; _fr._ fenestra rotunda;
_hso._ horizontal semicircular canal; _psc._ posterior vertical
semicircular canal: both the anterior and the posterior semicircular
canals are seen shining through the cartilage. _so._ supraoccipital;
_eo._ exoccipital; _oc._ occipital condyle; _nc._ notochord; _mk._
Meckel's cartilage; _ch._ ceratohyal; _bh._ basihyal; _cbr._ and
_ebr._ ceratobranchial; _bbr._ basibranchial.]
The trabeculæ are usually somewhat lyre-shaped, meeting in front and
behind, and leaving a large pituitary space between their middle parts
(figs. 323 and 325). Into this space there primitively projects the
whole base of the fore-brain, but the space itself gradually becomes
narrowed, till it usually contains only the pituitary body. The
carotid arteries always pass through it in the embryo; but in the
higher forms it ceases to be perforated in the adult. The trabeculæ
soon unite together both in front and behind and form a complete plate
underneath the fore-brain, and extending into the nasal region[204]. A
special vertical growth of this plate in the region of the orbit forms
the interorbital plate of Teleostei, Lacertilia and Aves (fig. 326,
_ps_), on the upper surface of which the front part of the brain
rests. The trabecular floor of the brain does not long remain simple.
Its sides grow vertically upwards, forming a lateral wall for the
brain, in which in the higher types two regions may be distinguished,
viz. an alisphenoidal region (fig. 326, _as_) behind, growing out from
what is known as the basisphenoidal region of the primitive trabeculæ,
and an orbitosphenoidal region in front growing out from the
presphenoidal region of the trabeculæ. These plates form at first a
continuous lateral wall of the cranium. At the front end of the brain
they are continued inwards, and more or less completely separate the
true cranial cavity from the nasal region in front. The region of the
cartilage forming the anterior boundary of the cranial cavity is known
as the lateral ethmoid region, and it is always perforated for the
passage of the olfactory nerves.
[204] In Man (Kölliker) the trabeculæ form from the first a
continuous plate in front of the pituitary space, and the latter
very early acquires a cartilaginous floor.
The cartilaginous walls which grow up from the trabecular floor of the
cranium generally extend upwards so as to form a roof, though almost
always an imperfect roof, for the cranial cavity. In the higher types,
in Mammals more especially, this roof can hardly be said to be formed
at all. The region of the trabeculæ in front of the brain is the
ethmoid region. The basal part of this region forms an internasal
plate, from which an internasal septum may grow up (fig. 326). To its
sides the olfactory capsules are attached, and there are usually
lateral outgrowths in front forming the trabecular cornua, while from
the posterior part of the ethmoidal plate, forming the anterior
boundary of the cranial cavity, there often grows out a prefrontal or
lateral ethmoidal process.
These and other processes growing out from the trabeculæ have
occasionally been regarded as rudimentary præoral branchial arches. I
have already stated it as my view that the existence of branchial
arches in this region is highly improbable, and I may add that the
development of these structures as outgrowths of the skull is in
itself to my mind a nearly conclusive argument against their being
branchial arches, in that true branchial arches hardly ever or perhaps
never arise in this way.
The sense capsules. The most important of these is the auditory
capsule, which, as we have seen, fuses intimately with the lateral
walls of the skull. In front there is usually a cleft separating it
from the alisphenoid region of the skull, through which the third
division of the fifth nerve passes out. This cleft becomes narrowed to
a small foramen (fig. 327, _V_). The sclerotic cartilage is always
free, but profoundly modifies the region of the cranium near which it
is placed. The nasal investment forms in Elasmobranchs (fig. 327,
_Na_) a capsule open below, and continuous with the ethmoid region of
the trabeculæ. In most types however it becomes more closely united
with the ethmoid region and the accessory parts belonging to it.
[FIG. 327. SKULL OF ADULT DOGFISH, SIDE VIEW. (From Parker.)
_O.C._ occipital condyle; _Au._ periotic capsule; _Pt.O._ pterotic
ridge; _Sp.O._ sphenotic process; _S.Or._ supraorbital ridge; _Na._
nasal capsule; _P.N._ prenasal cartilage; _II._ optic foramen; _V._
trigeminal foramen; _Pl.Pt., Qu._ pterygo-quadrate arcade; _M.Pt._
metapterygoid ligament (including a small cartilage); _Pl.Tr._
ethmo-palatine or palato-trabecular ligament; _Mck._ lower jaw;
_Sp._ spiracle; _H.M._ hyomandibular; _C.Hy._ ceratohyal; _m.h.l._
mandibulohyoid ligament; _Ph.Br._ pharyngobranchial; _E.Br._
epibranchial; _C.br._ ceratobranchial; _H.Br._ hypobranchial;
_B.Br._ basibranchial; _Ex.Br._ extra-branchial; _l_^1, [^2], [^3],
[^4], [^5]. labial cartilages; the dotted lines within _Mck._
indicate the basihyal.]
The cartilaginous cranium, the development of which has been thus
briefly traced, persists in the adult without even the addition of
membrane bones in the Cyclostomata, Elasmobranchii (fig. 327) and
Holocephali. In the Selachioid Ganoids it is also found in the adult,
but is covered over by membrane bones. In all other types it is
invariably present in the embryo, but becomes in the adult more or
less replaced by osseous tissue.
_Branchial skeleton._
The most primitive type of branchial skeleton in any existing form
would appear to be that of the Petromyzonidæ, which is developed in a
superficial subdermal tissue, and consists of a series of bars united
by transverse pieces, so as to form a basketwork. It is known as an
extra-branchial system, and an early stage of its development in the
Lamprey is shewn in fig. 47. In the higher forms this system is
replaced by a series of bars, known as the branchial bars, so situated
as to afford support to the successive branchial pouches. Outside
these bars there may be present in some primitive forms
(Elasmobranchii) cartilaginous elements, which are supposed to be
remnants of the extra-branchial system (fig. 327, _Ex.Br_); while a
series of membrane bones is also usually added to them, which will be
dealt with in a separate section. The branchial bars are developed as
simple cartilaginous rods in the deeper parts of the mesoblast which
constitutes the primitive branchial arches.
The position of the branchial bars in relation to the somatopleure and
splanchnopleure can be determined from their relation to the so-called
head cavities. These cavities atrophy before the formation of the
cartilaginous branchial bars, but it will be observed (fig. 328), that
the artery of each arch (_aa_) is placed on the inner side of the head
cavity (_pp_). The cartilaginous bar arises at a later period on the
inner side of the artery, and therefore on the inner side of the
section of the body cavity primitively present in the arches.
An anterior arch, known as the mandibular arch, placed in front of the
hyomandibular cleft, and a second arch, known as the hyoid arch,
placed in front of the hyobranchial cleft, are developed in all types.
The succeeding arches are known as the true branchial arches, and are
only fully developed in the Ichthyopsida.
In some Sharks (Notidani) seven branchial arches may be present (not
including the hyoid and mandibular). In other Ichthyopsida five are
usually present, in the embryo at any rate, while in the Amniota there
are usually two or three post-hyoid membranous arches, in the interior
of which a cartilaginous bar is usually formed. The general form of
these bars at an early stage of development is shewn in the dogfish
(Scyllium) in fig. 329.
The simple condition of these bars in the embryo renders it highly
probable that forms existed at one time with a simple branchial
skeleton of this kind: at the present day however such forms no longer
exist. The first arch has in all cases changed its function and has
become converted into a supporting skeleton for the mouth; the hyoid
arch, though retaining in some forms its branchial function, has in
most acquired additional functions and has undergone in consequence
various peculiar modifications. The true branchial arches retain their
branchial functions in Pisces and some Amphibia, but are secondarily
modified and largely aborted in the abranchiate forms. Since the
changes undergone by the true branchial bars are far less complicated
than those of the hyoid and mandibular bars it will be convenient to
treat of them in the first instance.
[FIG. 328. HORIZONTAL SECTION THROUGH THE PENULTIMATE VISCERAL ARCH
OF AN EMBRYO OF PRISTIURUS.
_ep._ epiblast; _vc._ pouch of hypoblast which will form the walls
of a visceral cleft; _pp._ segment of body-cavity in visceral arch;
_aa._ aortic arch.]
[FIG. 329. HEAD OF EMBRYO DOGFISH, 11 LINES LONG. (From Parker.)
_Tr._ trabecula; _Pl.Pt._ pterygo-quadrate; _M.Pt._ metapterygoid
region; _Mn._ mandibular cartilage; _Hy._ hyoid arch; _Br._ 1. first
branchial arch; _Sp._ mandibulohyoid cleft; _Cl_^1. hyobranchial
cleft; _Lch._ groove below the eye; _Na._ olfactory rudiment; _E._
eyeball; _Au._ auditory mass; _C_ 1, 2, 3. cerebral vesicles; _Hm._
hemispheres; _f.n.p._ nasofrontal process.]
These bars are, as already mentioned, most numerous in certain very
primitive forms (seven in Notidanus), while as we ascend the series
there is a gradual tendency for the posterior of them to disappear.
This tendency is the result of a gradual atrophy of the posterior
branchial pouches, which commenced at a stage in the evolution of the
Chordata long prior to the appearance of cartilaginous or osseous
branchial bars, and reaches its climax in the Amniota.
In a fully developed branchial bar the primitively simple rod of
cartilage becomes divided into a series of segments, usually four,
articulated so as to be more or less mobile: and either remaining
cartilaginous or becoming partially or wholly ossified. Each bar (fig.
327) forms a somewhat curved structure, embracing the pharynx. The
dorsal and somewhat horizontally placed segment is known as the
pharyngobranchial (_Ph.Br_), the next two as the epibranchial (_E.Br_)
and ceratobranchial (_C.Br_), and the ventral segment as the
hypobranchial (_H.Br_). There is also typically present a basal
unpaired segment, uniting the bars of the two sides, known as the
basibranchial (_B.Br_). The arches often bear cartilaginous rays which
support the gill lamellæ.
In Teleostei dental plates are usually developed as an exoskeletal
covering on parts of the branchial arches.
In the Amphibia four or three branchial arches are present in the
embryo. These parts are more or less completely retained in the
Perennibranchiata and Caducibranchiata, but in the Myctodera and Anura
they become largely reduced, and entirely connected with the hyoid.
In the Anura they never reach any considerable development, and are
soon reduced to a plate (fig. 330)--the coalesced basihyal and
basibranchial plate--the posterior processes of which represent the
remnants of the branchial arches.
According to Parker the posterior process of this plate in the adult
is a remnant of the fourth branchial bar; the next one is the third
branchial bar, while the anterior lamina behind the hyoid is stated by
him (though this is somewhat doubtful) to be a remnant of the first
two bars.
[FIG. 330. YOUNG FROG, WITH TAIL JUST ABSORBED; SIDE VIEW OF SKULL.
(From Parker.)
_Au._ auditory capsule; in front of it is the cranial side wall;
_A.N._ external nostril; _St._ stapes; _Mck._ Meckelian cartilage;
_B.Hy._ basihyobranchial plate; _St.Hy._ stylohyal or ceratohyal;
_Br._ 1. first branchial arch.
Bones: _E.O._ exoccipital; _Pr.O._ prootic; _Pa._ parietal; _Fr._
frontal; _Na._ nasal; _Pmx._ premaxillary; _Mx._ maxillary; _Pt._
pterygoid; _Sq._ squamosal; _Qu.Ju._ quadratojugal; _Art._
articular; _D._ dentary.]
In the Amniota, the branchial arches become still more degenerated, in
correlation with the total disappearance of a branchial respiration at
all periods of life. Their remnants become more or less important
parts of the hyoid bone, and are solely employed in support of the
tongue. Their basal portions are best preserved, forming parts of the
body of the hyoid. The posterior (thyroid) cornua of the hyoid are
remnants of the true arches. Of these there are two in the Chelonia
and Lacertilia, and one in the Aves and Mammalia. In Aves the cornu
formed from the first branchial arch (fig. 331, _cbr_) is always
larger than that of the true hyoid arch (_ch_).
Mandibular and Hyoid arches. The adaptations of both the mandibular
and hyoid bars, to functions entirely distinct from those which they
primitively served, are most remarkable; and the adaptations of the
two bars are in many cases so intimately bound together, that it is
not possible to treat them separately.
[FIG. 331. VIEW FROM BELOW OF THE BRANCHIAL SKELETON OF THE SKULL OF
A FOWL ON THE FOURTH DAY OF INCUBATION. (After Parker.)
_cv_ 1. cerebral vesicles; _e._ eye; _fn._ frontonasal process; _n._
nasal pit; _tr._ trabeculæ; _pts._ pituitary space; _mr._ superior
maxillary process; _pg._ pterygoid; _pa._ palatine; _q._ quadrate;
_mk._ Meckel's cartilage; _ch._ ceratohyal; _bh._ basihyal; _cbr._
ceratobranchial; _ebr._ proximal portion of the cartilage in the
third visceral (first branchial) arch; _bbr._ basibranchial; 1. first
visceral cleft; 2. second visceral cleft; 3. third visceral arch.]
The most important change of function is undoubtedly that of the
mandibular arch, which becomes entirely converted into a skeleton for
the jaws. It may be noted as a peculiarity of the mandibular arch that
it is never provided with an unpaired basal element.
The simplest forms of metamorphosis are those undergone by
Elasmobranchii, of which the Dogfish (Scyllium) and Skate (Raja) have
been studied (Parker, No. 456). In some of these forms, _e.g._ the
Skate, part of the mandibular bar is still related to the
hyomandibular cleft (the spiracle).
Elasmobranchii. In Scyllium the hyoid and mandibular arches are at
first very similar to those which follow. Soon however each of them
sends an anteriorly directed dorsal process (fig. 329). The regions
which may be distinguished owing to the growth of these processes have
received names from ossifications in them which are found in other
types. The anterior process of the mandibular arch is known as the
pterygo-quadrate bar (_Pl.Pt_); the dorsal end of the primitive bar
from which it starts (_M.Pt_) is known as the metapterygoid process;
while the ventral end of the bar forms the Meckelian cartilage. The
upper end of the hyoid arch is known as the hyomandibular.
In a somewhat later stage changes take place which cause these parts
practically to assume the adult form (fig. 327). The mandibular arch
becomes segmented at its bend into (1) a pterygo-quadrate bar
(_Pl.Pt_) which grows forwards in front of the mouth and forms an
upper jaw, and (2) a Meckelian cartilage (_Mck_) which is placed
behind the mouth, and forms a lower jaw. The two jaws are articulated
together, and the cartilages of the two sides composing them meet each
other distally.
At the articulation of the Meckelian cartilage with the quadrate part
of the pterygo-quadrate is situated a ligament (_M.Pt_), which takes
the place of the metapterygoid process of the previous stage, and
passes up on the anterior side of the spiracle, to be attached to the
cranium in the front part of the auditory region. This ligament, which
is supplemented by a second ligament, the ethmo-palatine ligament,
passing from the pterygo-quadrate bar to the antorbital region of the
skull, is not the most important support of the jaw. The main support
is, on the contrary, given by the hyoid arch; the hyomandibular
segment of which (_H.M_) as well as the adjoining segment (ceratohyoid
_C.Hy_) are firmly attached by ligament to the mandibular arch. The
hyomandibular is articulated with the cranium beneath the pterotic
ridge (_Pt.O_).
In the type just described, the hyoid and mandibular arches undergo
less modification than in almost any other case. The hyoid arch has
altered its form, but retains its respiratory function. It has however
acquired the secondary function of supporting the mandibular arch. The
mandibular arch is divided into two elements, which form respectively
the upper and lower jaws. It is not directly articulated with the
skull, and its mode of support by the hyoid arch has been called by
Huxley (No. 445) hyostylic.
[FIG. 333. HEAD OF EMBRYO SKATE, 1-1/3 IN. LONG. (From Parker.)
_Tr._ trabecula; _Pl.Pt._ pterygo-quadrate bar; _Mn._ mandibular
bar; _M.Pt._ metapterygoid cartilage; _H.M._ hyomandibular; _Hy._
remainder of hyoid arch; _Br._ 1. first branchial arch; _Sp._
mandibulohyoid cleft or spiracle; _Pn._ pineal gland; _Au._ auditory
vesicle; _C._ 1, _C._ 2, and _C._ 3. vesicles of the brain.]
The development of the hyoid and mandibular arches in the Skate is
characterised by a few important features (fig. 333). The anterior
element of the hyoid arch, which forms the hyomandibular (_H.M_),
becomes entirely separate from the posterior part of the arch, and
only serves to support the jaws. The posterior part of the arch (_Hy_)
carries on the respiratory functions of the hyoid, and is closely
connected with the first branchial arch. The upper or metapterygoid
element of the mandibular arch (_M.Pt_) has a considerable
development, and, becoming separated from the remainder of the arch,
forms a mass of cartilage with one or two branchial rays, in the front
wall of the spiracle, and constitutes a section of the mandibular arch
still retaining traces of its primitive function in supporting the
wall of a branchial pouch.
Although the development of other Elasmobranch types is not known, it
is necessary to call attention to the mode of support of the
mandibular arch in certain forms, notably Notidanus, Hexanchus and
Cestracion, where the pterygo-quadrate region of the mandibular arch
is directly articulated to the cranium between the optic and
trigeminal foramina. In the two former genera the metapterygoid region
of the arch is moreover continuous with the pterygo-quadrate, and
articulates with the postorbital process of the auditory region of the
skull. In spite of these attachments the mandibular arch continues to
be partially supported by the hyomandibular. The skulls in which the
mandibular arch has this double form of support have been called by
Huxley amphistylic.
[FIG. 334. CRANIAL SKELETON OF A SALMON FRY, SECOND WEEK AFTER
HATCHING; MEMBRANE BONES, EYEBALLS, AND NASAL SACS REMOVED. (From
Parker.)
_T.Cr._ tegmen cranii; _S.Or._ supraorbital band; _Fo._ superior
fontanelle; _Au._ auditory capsule; _Pa.ch._ parachordal cartilage;
_Ch._ notochord; _Tr._ trabecula; above the trabecula, the
interorbital septum is seen, passing into the cranial wall above and
reaching the supraorbital band; _H._ optic foramen; _V._ trigeminal
foramen; _l_^1, _l_^2. labial cartilages; _Pl.Pt._ palatopterygoid
bar; _M. Pt._ metapterygoid tract; _Qu._ quadrate region; _Mck._
Meckelian cartilage; _H.M._ hyomandibular cartilage; _Sy._
symplectic tract; _I.Hy._ interhyal; _C.Hy._ ceratohyal; _H.Hy._
hypohyal; _G.Hy._ glossohyal; _Br._1. first branchial arch.]
Considering the in many respects primitive characters of the forms
with amphistylic skulls it seems not improbable that they preserve the
original mode of support of the mandibular arch; from which
differentiations in two directions have taken place, viz.
differentiations in the direction of a complete support of the
mandibular arch by the hyoid, which is characteristic of most
Elasmobranchii and, as will be shewn below, of Ganoidei and Teleostei;
and differentiations towards a direct articulation or attachment of
the mandibular arch to the cranium, without the intervention of the
hyoid. The latter mode of attachment is called by Huxley autostylic.
It is found in Holocephala, Dipnoi, Amphibia and the Amniota.
Teleostei. In addition to that of Elasmobranchii, the skull of the
Salmon is the only hyostylic skull in which, by the admirable
investigation of Parker (No. 451), the ontogeny of the hyoid and
mandibular bars has been satisfactorily worked out. Apart from the
presence of a series of membrane bones, the development of these bars
agrees on the whole with the types already described.
[FIG. 335. YOUNG SALMON OF THE FIRST SUMMER, ABOUT 2 INCHES LONG;
SIDE VIEW OF SKULL, EXCLUDING BRANCHIAL ARCHES. (From Parker.)
The palato-mandibular and hyoid tracts are detached from their
proper situations, a line indicating the position where the
hyomandibular is articulated beneath the pterotic ridge.
_ol._ olfactory fossa; _c.tr._ trabecular cornu; _ul_^a. _ul_^b.
upper labial cartilages; _p.s._ presphenoid tract; _t.cr._ tegmen
cranii; _s.o.b._ supraorbital band; _fo._ superior fontanelle;
_n.c._ notochord; _b.o._ basilar cartilage; _tr._ trabecula; _p.c._
condyle for palatine cartilage; 5. trigeminal foramen; 7_a._ facial
foramen; 8. foramen for glossopharyngeal and vagus nerves; _mk._
Meckelian cartilage; _op.c._ opercular condyle.
Bones: _e.o._ exoccipital; _s.o._ supraoccipital; _e.p._ epiotic;
_pt.o._ pterotic; _sp.o._ sphenotic; _op._ opisthotic; _pro._
prootic; _b.s._ basisphenoid; _al.s._ alisphenoid; _o.s._
orbitosphenoid; _l.e._ ectethmoid or lateral ethmoid; _pa._
palatine; _pg._ pterygoid; _m.pg._ mesopterygoid; _mt.pg._
metapterygoid; _qu._ quadrate; _ar._ articular; _h.m._
hyomandibular; _sy._ symplectic; _i.h._ interhyal; _ep.h._
epiceratohyal; _c.h._ ceratohyal; _h.h._ hypohyal; _g.h._ glosso- or
basihyal.]
The hyoid arch, though largely ossified, undergoes a process of
development very similar to that in Raja. It is formed as a simple
cartilaginous bar, which soon becomes segmented longitudinally into an
anterior and a posterior part (fig. 334). The former constitutes the
hyomandibular (_H.M_), while the latter, becoming more and more
separated from the hyomandibular, constitutes the hyoid arch proper;
owing to the disappearance of the hyobranchial cleft, it loses its
primitive function, and serves on the one hand to support the
operculum covering the gills, and on the other to support the tongue.
It becomes segmented into a series of parts which are ossified (fig.
335) as the epiceratohyal (_ep.h_) above, then a large ceratohyal
(_ch_), followed by a hypohyal (_hh_), while the median ventral
element forms the basi- or glossohyal (_g.h_).
The hyomandibular itself is articulated with the skull below the
pterotic process (fig. 334, _H.M_). Its upper element ossifies as the
hyomandibular (fig. 335, _h.m._), while its lower part (fig. 334,
_Sy_), which is firmly connected with the mandibular arch, ossifies as
the symplectic (fig. 335, _sy_). A connecting element between the two
parts of the hyoid bar forms an interhyal (_ih_).
There are more important differences in the development of the
mandibular arch in Elasmobranchii and the Salmon than in that of the
hyoid arch, in that, instead of the whole arcade of the upper jaw
being formed from the mandibular arch, a fresh element, in the form of
an independently developed bar of cartilage, completes the upper
arcade in front; but even with this bar the two halves of the upper
branch of the arch do not meet anteriorly, but are separated by the
ends of the trabeculæ.
The anterior bar of the upper arcade is known as the palatine; but it
appears to me as yet uncertain how far it is to be regarded as an
element, primitively belonging to the upper arcade of the mandibular
arch, which has become secondarily independent in its development; or
as an entirely distinct structure which has no counterpart in the
Elasmobranch upper jaw. The latter view is adopted by Parker and
Bridge, and a cartilage attached to the hinder wall of the nasal
capsule of many Elasmobranchii is identified by them with the palatine
rod of the Teleostei.
The arch itself is at first very similar to the succeeding arches; its
dorsal extremity soon however becomes broadened, and provided with an
anteriorly directed process. This part (fig. 334, _M.Pt_ and _Qu_) is
then segmented from the lower region, and forms what may be called the
pterygo-quadrate cartilage, though not completely homologous with the
similarly named cartilage in Elasmobranchs; while the lower region
forms the Meckelian cartilage (_Mck_), which has already grown
inwards, so as to meet its fellow ventrally below the mouth. The whole
arch becomes at the same time widely separated from the axial parts of
the skull.
Nearly simultaneously with the first differentiation of the mandibular
arch, a bar of cartilage--the palatine bar already spoken of--is
formed on each side, below the eye, in front of the mouth. The dilated
anterior extremity of this bar soon comes in contact with an anterior
process of the trabeculæ, known as the ethmo-palatine process.
In a later stage the pterygoid end of the pterygo-quadrate cartilage
unites with the distal end of the palatine bar (fig. 334, _Pl.Pt_),
and there is then formed a continuous cartilaginous arcade for the
upper jaw, which is strikingly similar to the cartilaginous upper jaw
of Elasmobranchii.
A large dorsal process of the primitive pterygo-quadrate now forms a
large metapterygoid tract (_M.Pt_); while the whole arch becomes
firmly bound to the hyomandibular (_H.M_).
In the later stages the parts formed in cartilage become ossified
(fig. 335). The palatine is first ossified, the pterygoid region of
the pterygo-quadrate is next ossified as a dorsal mesopterygoid
(_m.pg_) and a ventral pterygoid proper (_pg_). The quadrate region,
articulating with the Meckelian cartilage, becomes ossified as a
distinct quadrate (_qu_), while the dorsal region becomes also
ossified as a metapterygoid (_mt.pg_).
In the Meckelian cartilage a superficial ossification of the ventral
edge and inner surface forms an articulare (_ar_); but the greater
part of the cartilage persists through life.
Some of the above ossifications, at any rate those of the palatine and
pterygoid, seem to be started by dental osseous plates adjoining the
cartilage. They will be spoken of further in the section dealing with
the membrane bones.
Amphibia. The development of the autostylic piscine skulls
has unfortunately not yet been studied; and the most primitive
autostylic types whose development we are acquainted with are
those of the Amphibia; on which a large amount of light has
been shed by the researches of Huxley and Parker.
The modifications of the hyoid arch are comparatively simple
and uniform. It forms a rod of cartilage, which soon articulates
in front with the quadrate element of the mandibular arch, and
is subsequently attached by ligaments both to the quadrate and
to the cranium. In those Amphibia in which external gills and
gill clefts are lost, it fuses with the basal element of the hyoid
(fig. 330), which, together with the basal portions of the following
arches, forms a continuous cartilaginous plate. On the completion
of these changes the paired parts of the hyoid arch have
the form of two elongated rods, known as the anterior cornua of
the hyoid, which attach the basihyal plate to the cranium behind
the auditory capsule.
It is still uncertain whether there is any distinct element corresponding
to the hyomandibular of fishes.
Parker holds that the columella auris of the Anura is the homologue of
the hyomandibular. The columella develops comparatively late and
independently of the remainder of the hyoid arch, but the similarity
between its relations to the nerves and those of the hyomandibular is
put forward by Parker as an argument in favour of his view. The early
ligamentous connection between the quadrate and the upper end of the
primitive hyoid is however an argument in favour of regarding the
upper end of the primitive hyoid as the hyomandibular element, not
separated from the remainder of the arch.
The history of the mandibular arch is more complicated than that of
the hyoid. The part of it which corresponds with the upper jaw of
Elasmobranchii exhibits most striking variations in development; so
striking indeed as to suggest that the secondary modifications it has
undergone are sufficiently considerable to render great caution
necessary in drawing morphological conclusions from the processes
which are in some instances observable. A more satisfactory judgment
on this point will be possible after the publication of a memoir with
which Parker is now engaged on the skulls of the different Anura.
The membrane bones applying themselves to the sides of the mandibular
arch are relatively far more important than in the lower types. This
is especially the case with the upper jaw where the maxillary and
premaxillary bones functionally replace the primitive cartilaginous
jaw; while membranous pterygoids and palatines apply themselves to,
and largely take the place of, the cartilaginous palatine and
pterygoid bars.
Two types worked out by Parker, viz. the Axolotl and the common Frog,
may be selected to illustrate the development of the mandibular arch.
[FIG. 336. YOUNG AXOLOTL, 2-1/4 INCHES LONG; UNDER VIEW OF SKULL,
DISSECTED, THE LOWER JAW AND GILL ARCHES HAVING BEEN REMOVED. (From
Parker.)
_nc._ notochord; _oc.c._ occipital condyle; _f.o._ fenestra ovalis;
_st._ stapes; _tr._ trabecular cartilage; _i.n._ internal nares;
_c.tr._ cornu trabeculæ; _pd._ pedicle of quadrate; _q._ quadrate;
_pg._ outline of pterygoid cartilage; 5´. orbito-nasal nerve; 7.
facial nerve.
Bones: _pa.s._ parasphenoid; _e.o._ exoccipital; _v._ vomer; _px._
premaxillary; _mx._ maxillary; _pa._ palatine; _pg._ pterygoid.]
In the Axolotl, which may be taken as the type for the Urodela, the
mandibular arch is constituted at a very early stage of (1) an
enlarged dorsal element, corresponding with the pterygo-quadrate of
the lower types, but usually known as the quadrate; and (2) a ventral
or Meckelian element. The Meckelian bar very early acquires its
investing bones, while the dorsal part of the quadrate becomes divided
into two characteristic processes, viz. an anterior dorsal process
which grows towards _and soon permanently fuses with_ the trabecular
crest, and a posterior process known as the otic process, which
applies itself to the outer side of the auditory region. The anterior
of these processes, as pointed out by Huxley, is probably homologous
with the anterior process of the pterygo-quadrate bar in Notidanus,
which articulates with the trabecular region of the cranium, while the
otic process is homologous with the metapterygoid process. Hardly any
trace is present of an anterior process to form a pterygoid bar, but
dentigerous plates forming a dermal palatopterygoid bar have already
appeared.
At a somewhat later stage a fresh process, called by Huxley the
pedicle, grows out from the quadrate, and articulates with the ventral
side of the auditory region (fig. 336, _pd_). Shortly afterwards a rod
of cartilage grows forward from the quadrate under the membranous
pterygoid (_pg_), which corresponds with the cartilaginous pterygoid
bar of other types (fig. 336), and an independent palatine bar,
arising even before the pterygoid process, is formed immediately
dorsal to the dentigerous palatine plate (_pa_), and is attached to
the trabecula. These two bars eventually meet, but never become firmly
united to the more important membrane bones placed superficially to
them.
The mandibular arch in the Frog stands, so far as development is
concerned, in striking contrast to the mandibular arch of the Axolotl,
in spite of the obvious similarity in the arrangement of the adult
parts in the two types.
[FIG. 337. EMBRYO FROG, JUST BEFORE HATCHING; SIDE VIEW OF HEAD,
WITH SKIN REMOVED. (From Parker.)
_Na._ olfactory sack; _E._ involution for eyeball; _Au._ auditory
sack; _Tr._ trabecula; _Mn._ mandibular; _Hy._ hyoid; _Br.I._ first
branchial arch; the gill-buds are seen on the first two branchial
arches; _l._ labial cartilages.]
In the earliest stage it forms a simple bar in the membranous
mandibular arch, parallel to and very similar to the hyoid bar behind
(fig. 337, _Mn_). In the next stage observed, that is to say in
Tadpoles of four, five, to six lines long, an astonishing
transformation has taken place. The mandibular arch (fig. 338) is
turned directly forwards parallel to the trabecula, to which it is
attached in front (_p.pg_) and behind (_pd_). The proximal part of the
arch thus forms a subocular bar, and the space between it and the
trabecula a subocular fenestra. In front of the anterior attachment it
is continued forwards for a short distance, and to the free end of
this projecting part is articulated a small Meckelian cartilage
directed upwards (_mk_). The Meckelian cartilage is at this stage
placed in front of the nasal sacks, in the lower lip of the suctorial
mouth. The greater part of the arch, parallel with the trabeculæ, is
equivalent to what has been called in the Axolotl the quadrate, while
its anterior attachment to the trabeculæ is the rudiment of the
palatopterygoid cartilage. The posterior attachment is known as the
pedicle.
[FIG. 338. TADPOLE OF COMMON TOAD, ONE-THIRD OF AN INCH LONG;
CRANIAL AND MANDIBULAR CARTILAGES SEEN FROM ABOVE; THE PARACHORDAL
CARTILAGES ARE NOT YET DEFINITE. (From Parker.)
_nc._ notochord; _ms._ muscular segments; _au._ auditory capsule;
_py._ region of pituitary body; _tr._ trabecula; _c.tr._ cornu
trabeculæ; _p.pg._ palatopterygoid bar; _pd._ pedicle; _q._ quadrate
condyle; _mk._ Meckelian piece of mandibular arch; _so.f._ subocular
fenestra; _u.l._ upper labial cartilage. The dotted circle within
the quadrate region indicates the position of the internal nostril.]
The condition of the mandibular arch during this and the next stage
(fig. 339) is very perplexing. Its structure appears adapted in some
way to support the suctorial mouth of the Tadpole.
Reasons have been offered in a previous part of this volume for
supposing that the suctorial mouth of the Tadpole is probably not
simply a structure secondarily acquired by this larva, but is an organ
inherited from an ancestor provided through life with a suctorial
mouth.
The question thus arises, is the peculiar modification of the
mandibular arch of the Tadpole an _inherited_ or an _acquired_
feature?
If the first alternative is accepted we should have to admit that the
mandibular arch became first of all modified in connection with the
suctorial mouth, before it was converted into the jaws of the
Gnathostomata; and that the peculiar history of this arch in the
Tadpole is a more or less true record of its phylogenetic development.
In favour of this view is the striking similarity which Huxley has
pointed out between the oral skeleton of the Lamprey and that of the
Tadpole; and certain peculiarities of the mandibular arch of Chimæra
and the Dipnoi can perhaps best be explained on the supposition that
the oral skeleton of these forms has arisen in a manner somewhat
similar to that in the Frog; though with reference to this point
further developmental data are much required.
On the other hand the above suppositions would necessitate our
admitting that a great abbreviation has occurred in the development of
the mandibular arch of the otherwise more primitive Urodela; and that
the simple mode of growth of the jaws in Elasmobranchii, from the
primitive mandibular arch, is phylogenetically a much abbreviated and
modified process, instead of being, as usually supposed, a true record
of ancestral history.
If the view is accepted that the characters of the mandibular arch of
the Tadpole are secondary, it will be necessary to admit that the
adaptation of the mandibular arch to the suctorial mouth took place
after the suctorial mouth had come to be merely a larval organ.
In view of our imperfect knowledge of the development of most Piscine
skulls I would refrain from expressing a decided opinion in favour of
either of these alternatives.
[FIG. 339. TADPOLE WITH TAIL BEGINNING TO SHRINK; SIDE VIEW OF SKULL
WITHOUT THE BRANCHIAL ARCHES. (From Parker.)
_n.c._ notochord; _au._ auditory capsule; between it and _eth._ the
low cranial side wall is seen; _eth._ ethmoidal region; _st._
stapes; 5. trigeminal foramen; 2. optic foramen; _ol._ olfactory
capsules, both seen owing to slight tilting of the skull; _c.tr._
cornu trabeculæ; _u.l._ upper labial, in outline; _su._ suspensorium
(quadrate); _pd._ its pedicle; _ot.pr._ its otic process; _or.p._
its orbitar process; _t.m._ temporal muscle, indicated by dotted
lines passing beneath the orbitar process; _pa.pg._ palatopterygoid
bar; _mk._ Meckelian cartilage; _l.l._ lower labial, in outline;
_c.h._ ceratohyal; _b.h._ basihyal. The upper outline of the head is
shewn by dotted lines.]
As the tail of the Tadpole gradually disappears, and the metamorphosis
into the Frog becomes accomplished, the mandibular arch undergoes
important changes (fig. 339): the palatopterygoid attachment (_pa.pg_)
of the quadrate subocular bar becomes gradually elongated; and, as it
is so, the front end of the subocular bar (_su_) rotates outwards and
backwards, and soon forms a very considerable angle with the
trabeculæ. The Meckelian cartilage (_mk_) at its free end becomes at
the same time considerably elongated. These processes of growth
continue till (fig. 330) the palatopterygoid bar (_Pt_) forms a
subocular bar, and is considerably longer than the original subocular
region of the quadrate; while the Meckelian cartilage (_Mck_) has
assumed its permanent position on the hinder border of the no longer
suctorial mouth, and has grown forwards so as nearly to meet its
fellow in the median line.
[FIG. 340. YOUNG FROG, NEAR END OF FIRST SUMMER; UPPER VIEW OF
SKULL, WITH LEFT MANDIBLE REMOVED, AND THE RIGHT EXTENDED OUTWARDS.
(From Parker.)
_b.o._ basioccipital tract; _s.o._ supraoccipital tract; _fo._
frontal fontanelle; _e.n._ external nostril; internal to it,
internasal plate; _a.t._ tympanic annulus.
Bones: _e.o._ exoccipital; _pr.o._ prootic, partly overlapped by
_p._ parietal; _f._ frontal; _eth._ rudiment of sphenethmoid; _na._
nasal; _pmx._ premaxillary; _mx._ maxillary; _pg._ pterygoid, partly
ensheathing the reduced cartilage; _q.j._ quadratojugal; _sq._
squamosal; _ar._ articular; _d._ dentary; _m.mk._ mento-Meckelian.]
The metapterygoid region of the quadrate gives rise to a posterior and
dorsal process (fig. 339, _ot.pr_), the end of which is constricted
off as the tympanic annulus (fig. 340, _a.t_); while the proximal part
of the process remains as the otic (metapterygoid) process,
articulating with the auditory cartilage.
The pedicle (_pd_) retains its original attachment to the skull.
The palatopterygoid soon becomes segmented into a transversely placed
palatine, and a longitudinally placed pterygoid (fig. 340). With the
exception of a few ossifications, which present no features of special
interest, the parts of the mandibular arch have now reached their
final condition, which is not very different from that in the Axolotl.
Sauropsida. In the Sauropsida the modifications of the hyoid and
mandibular arches are fairly uniform.
The lower part of the hyoid arch, including the basihyoid, unites with
the remnants of the arches behind to form the hyoid bone, to which it
contributes the anterior cornu and anterior part of the body.
The columella is believed by Huxley and Parker to represent, as in the
Anura, the independently developed dorsal (hyomandibular) element of
the hyoid, together with the stapes with which it has become
united[205].
[205] The strongest evidence in favour of Huxley's and Parker's
view of the nature of the columella is the fusion in the adult
Sphenodon of the upper end of the hyoid with the columella
(_vide_ Huxley, No. 445). From an examination of a specimen in
the Cambridge museum I do not feel satisfied that the fusion is
not secondary, but have not been able to examine the junction of
the hyoid and columella in section. For a different view to that
of Huxley _vide_ Peters, "Ueb. d. Gehörknochelchen u. ihr
Verhältniss zu. Zungenbeinbogen b. Sphenodon." _Berlin
Monatsberichte_, 1874.
The membranous mandibular arch gives off in the embryos of all the
Sauropsida an obvious bud to form the superior maxillary process, and
the formation of this bud appears to represent the growth forwards of
the pterygoid process in Elasmobranchii, which is indeed accompanied
by the formation of a similar bud; but the skeletal rod, which appears
in the axis of this bud, is as a rule independent of that in the true
arch (fig. 331, _pa_, _pg_). The former is the pterygo-palatine bar;
the latter the Meckelian and quadrate cartilages.
The pterygo-palatine bar is usually if not always ossified directly,
without the intervention of cartilage.
Born has recently shewn that Parker was mistaken in supposing that the
palatopterygoid bone is cartilaginous in Birds. In the Turtle a short
cartilaginous pterygoid process of the quadrate would seem to be
present (Parker, No. 458).
The quadrate and Meckelian cartilages are either from the first
separate, or very early become so.
The quadrate cartilage ossifies as the quadrate bone, and supplies the
permanent articulation for the lower jaw. Its upper end exhibits a
tendency to divide into two processes, corresponding with the pedicle
and otic processes of the Amphibia. The Meckelian cartilage becomes
soon covered by investing bones, and its proximal end ossifies as the
articulare. The remainder of the cartilage usually disappears.
Mammalia. The most extraordinary metamorphosis of the hyoid and
mandibular arches occurs in the Mammalia, and has been in part known
since the publication of the memoir of Reichert (No. 461).
[FIG. 341. EMBRYO PIG, TWO-THIRDS OF AN INCH LONG; ELEMENTS OF THE
SKULL SEEN SOMEWHAT DIAGRAMMATICALLY FROM BELOW. (From Parker.)
_pa. ch._ parachordal cartilage; _nc._ notochord; _au._ auditory
capsule; _py._ pituitary body; _tr._ trabeculæ; _c.tr._ trabecular
cornu; _pn._ prenasal cartilage; _e.n._ external nasal opening;
_ol._ nasal capsule; _p.pg._ palatopterygoid tract enclosed in the
maxillopalatine process; _mn._ mandibular arch; _hy._ hyoid arch;
_th.h._ first branchial arch; 7_a._ facial nerve; 8_a._
glossopharyngeal; 8_b._ vagus; 9. hypoglossal.]
Both the hyoid and mandibular arches develop at first more completely
than in any of the other types above Fishes; and are articulated to
each other above, while the pterygo-palatine bar is quite distinct.
The main features of the subsequent development are undisputed, with
the exception of that of the upper end of the hyoid, which is still
controverted. The following is Parker's (No. 452) account for the Pig,
which confirms in the main the view originally put forward by Huxley
(No. 445).
The mandibular and hyoid arches are at first very similar (fig. 341
_mn_ and _hy_), their dorsal ends being somewhat incurved, and
articulating together.
In a somewhat later stage (fig. 342) the upper end of the mandibular
bar (_mb_), without becoming segmented from the ventral part, becomes
distinctly swollen, and clearly corresponds to the quadrate region of
other types. The ventral part of the bar constitutes the Meckelian
cartilage (_mk_).
[FIG. 342. EMBRYO PIG, AN INCH AND A THIRD LONG; SIDE VIEW OF
MANDIBULAR AND HYOID ARCHES. THE MAIN HYOID ARCH IS SEEN AS
DISPLACED BACKWARDS AFTER SEGMENTATION FROM THE INCUS. (From
Parker.)
_tg._ tongue; _mk._ Meckelian cartilage; _ml._ body of malleus;
_mb._ manubrium or handle of the malleus; _t.ty._ tegmen tympani;
_i._ incus; _st._ stapes; _i.hy._ interhyal ligament; _st.h._
stylohyal cartilage; _h.h._ hypohyal; _b.h._ basibranchial; _th.h._
rudiment of first branchial arch; _7a._ facial nerve.]
The hyoid arch has in the meantime become segmented into two parts, an
upper part (_i_), which eventually becomes one of the small bones of
the ear--the incus--and a lower part which remains permanently as the
anterior cornu of the hyoid (_st.h_). The two parts continue to be
connected by a ligament.
The incus is articulated with the quadrate end of the mandibular arch,
and its rounded head comes in contact with the stapes (fig. 342, _st_)
which is segmented from the fenestra ovalis. The main arch of the
hyoid becomes divided into a hypohyal (_h.h_) below and a stylohyal
(_st.h_) above, and also becomes articulated with the basal element of
the arch behind (_bh_).
In the course of further development the Meckelian part of the
mandibular arch becomes enveloped in a superficial ossification
forming the dentary. Its upper end, adjoining the quadrate region,
becomes calcified and then absorbed, and its lower, with the exception
of the extreme point, is ossified and subsequently incorporated in the
dentary.
The quadrate region remains relatively stationary in growth as
compared with the adjacent parts of the skull, and finally ossifies to
form the malleus bone of the ear. The processus gracilis of the
malleus is the primitive continuation into Meckel's cartilage.
The malleus and incus are at first embedded in the connective tissue
adjoining the tympanic cavity (hyomandibular cleft, _vide_ p. 528);
and externally to them a bone known as the tympanic bone becomes
developed so that they become placed between the tympanic bone and the
periotic capsule. In late foetal life they become transported
completely within the tympanic cavity, though covered by a reflection
of the tympanic mucous membrane.
The dorsal end of the part of the hyoid separated from the incus
becomes ossified as the tympano-hyal, and is anchylosed with the
adjacent parts of the periotic capsule. The middle part of the bar
just outside the skull forms the stylohyal (styloid process in Man)
which is attached by ligament to the anterior cornu of the hyoid
(ceratohyal).
While the account of the formation of the malleus, incus, and stapes
just given is that usually accepted in this country, a somewhat
different view of the development of these parts has as a rule been
adopted in Germany. Reichert (No. 461) held that both the malleus and
the incus were derived from the mandibular bar; and this view has been
confirmed by Günther, Kölliker and other observers, and has recently
been adopted by Salensky (No. 462) after a careful research especially
directed towards this point. Reichert also held that the stapes was
derived from the hyoid bar; but, though his observations on this point
have been very widely accepted, they have not met with such universal
recognition as his views on the origin of the malleus and incus.
Salensky has recently arrived at a view, which is in accord with that
of Parker, in so far as the independence of the stapes of both the
hyoid and mandibular arches is concerned. Salensky however holds that
it is formed from a mass of mesoblast surrounding the artery of the
mandibular arch, and that the form of the stapes is due to its
perforation by the mandibular artery. A product of this artery
permanently perforates the stapes in a few Mammalia, though in the
majority it atrophies.
In view of the different accounts of the origin of the incus the exact
nature of this bone must still be considered as an open question, but
should Reichert's view be confirmed the identification of the incus
with the columella of the Amphibia and Sauropsida must be abandoned.
_Membrane bones and ossifications of the cranium._
The membrane bones of the skull may be divided into two classes, viz.
(1) those derived from dermal osseous plates, which as explained above
(p. 542) are primitively formed by the coalescence of the osseous
plates of scales; and (2) those formed by the coalescence of the
osseous plates of teeth lining the oral cavity. Some of the bones
sheathing the edge of the mouth have been formed partly by the one
process and partly by the other.
In the Fishes there are found all grades of transition between simple
dermal scutes, and true subdermal osseous plates forming an integral
part of the internal skeleton. Dermal scutes are best represented in
Acipenser and some Siluroid Fishes.
Where the membrane bones still retain the character of dermal plates,
those on the dorsal surface of the cranium are usually arranged in a
series of longitudinal rows, continuing in the region of the head the
rows of dermal scutes of the trunk; while the remaining cranial scutes
are connected with the visceral arches. The dermal bones on the dorsal
surface of the head are very different in number, size, and
arrangement in different types of Fishes; but owing to their linear
disposition it is usually possible to find a certain number both of
the paired and unpaired bones which have a similar situation in the
different forms. These usually receive the same names, but both from
general considerations as to their origin, as well as from a
comparison of different species, it appears to me probable that there
is no real homology between these bones in different species, but only
a kind of general correspondence[206].
[206] For some interesting remarks on the arrangement of these
bones in Fishes, _vide_ Bridge, "On the Osteology of Polyodon
folium." _Phil. Trans._, 1878.
It is not in fact till we get to the types above the Fishes that we
can find a series of homologous dorsal membrane bones covering the
roof of the skull. In these types three paired sets of such bones are
usually present, viz. from behind forwards the parietals, frontals and
nasals, the latter bounding the posterior surface of the external
nasal opening. Even in the higher types these bones are liable to vary
very greatly from the usual arrangement.
Besides these bones there is usually present in the higher forms a
lacrymal bone on the anterior margin of the orbit derived from one of
a series of periorbital membrane bones frequently found in Fishes.
Various supraorbital and postorbital bones, etc. are also frequently
found in Lacertilia, etc. which are not impossibly phylogenetically
independent of the membrane bones inherited from Fishes; and may have
been evolved as bony scutes in the subdermal tissue of the papillæ of
the sauropsidan scales.
The visceral arches of Fishes, especially of the Teleostei, are
usually provided with a series of membrane bones. In the true
branchial arches these take the form of dentigerous plates; but no
such plates are found in the Amphibia or Amniota.
The opercular flap attached to the hyoid arch is usually supported by
a series of membrane bones, which attain their highest development in
the Teleostei. One of these bones, the præopercular, is very constant
and is primitively attached along the outer edge of the hyomandibular.
It seems to be retained in Amphibia as a membrane bone, overlapping
the attachment of the quadrate and known as the squamosal; though it
is not impossible that this bone may be derived from a superficial
membrane bone, widely distributed in Teleostei and Ganoids, which is
known as the supra-temporal. In Dipnoi the bone which appears to be
clearly homologous with the squamosal would seem from its position to
belong to the series of dorsal plates, and therefore to be the
supra-temporal; but it is regarded by Huxley (No. 446) as the
præopercular[207].
[207] It is not impossible that the solution of the difficulty
about the præopercular is to be found by supposing that the
præopercular as it exists in Teleostei is derived from a dorsal
dermal plate, and that in the Dipnoi this plate retains more
nearly than in Teleostei its primitive position.
In the Amniota the squamosal forms an integral part of the osseous
roof of the skull; but in the Sauropsida it continues, as in Amphibia,
to be closely related to the quadrate.
A larger series of persistent membrane bones are related to the
mandibular, and its palato-quadrate process.
Overlying the palato-quadrate process are two rows of bones, one row
lying at the edge of the mouth, on the outer side of the
pterygo-palatine process, and the other set on the roof of the mouth
superficial to the pterygo-palatine process.
The outer row is formed of the præmaxilla, maxilla, jugal, and very
often quadratojugal. Of these bones the maxilla and præmaxilla, as is
more especially demonstrated by their ontogeny in the Urodela, are
partly derived from dentigerous plates and partly from membrane plates
outside the mouth; while the jugal, and quadratojugal when present,
are entirely extra-oral. In the Amphibia and Amniota the præmaxillæ
and maxillæ are the most important bones in the facial region, and are
quite independent of any cartilaginous substratum.
The second row of bones is clearly constituted in the Dipnoi and
Amphibia by the vomer in front, then the palatine, and finally the
pterygoid behind. Of these bones the vomer is never related to a
cartilaginous tract below, while the palatines and pterygoids usually
are so. The position and growth of the three bones in many Urodela
(Axolotl) are especially striking (Hertwig. No. 442). In the Axolotl
they form a continuous series, the vomer and palatine being covered by
teeth, but the pterygoid being without teeth. The vomer and palatine
originate from the united osseous plates of the bases of the teeth,
while the pterygoid is in the first instance continuous with the
palatine.
In Teleostei, Amia, etc., there are dentigerous plates forming a
palatine and pterygoid, which in position, at any rate, closely
correspond with the similarly named bones in Amphibia; and there is
also a dentigerous vomer which may fairly be considered as equivalent
to that in Amphibia.
In the Amniota the three bones found in Amphibia are always present,
but with a few exceptions amongst the Lacertilia and Ophidia, are no
longer dentigerous. The cartilaginous bars, which in the lower types
are placed below the palatine and pterygoid membrane bones, are
usually imperfectly or not at all developed.
On Meckel's cartilage important membrane bones are almost always
grafted. On the outside and distal part of the cartilage a dentary is
usually developed, which may envelope and replace the cartilage to a
larger or smaller extent. Its oral edge is usually dentigerous. The
splenial membrane bone is the most important bone on the inner side of
Meckel's cartilage, but other elements known as the coronoid and
angular may also be added. In Mammalia the dentary is the only element
present (_vide_ p. 590).
On the roof of the mouth a median bone, the parasphenoid, is very
widely present in the Amphibia and Fishes, except the Elasmobranchii
and Cyclostomata, and has no doubt the same phylogenetic origin as the
vomer and membranous palatines and pterygoids.
It is less important in the Sauropsida, and becomes indistinguishably
fused with the sphenoid in the adult, while in Mammalia it is no
longer found.
Ossification of the Cartilaginous Cranium. In certain Fishes the
cartilaginous cranium remains quite unossified, while completely
enveloped in dermal bones. Such for instance is its condition in the
Selachioid Ganoids. In most instances, however, the investment of the
cartilaginous cranium by membrane bones is accompanied by a more or
less complete ossification of the cartilage itself.
In the Dipnoi this occurs to the smallest extent, the only
ossifications occurring in the lateral parts of the occipital region,
and forming the exoccipitals.
In Teleostei and bony Ganoids, a considerably greater number of
ossifications occur in the cartilage.
In the region of the occipital cartilaginous ring there appears a
basioccipital and supraoccipital and two exoccipitals. The
basioccipital is the only bone on the floor of the skull ossifying
that part into which the notochord is primitively continued[208].
[208] The notochord appears also to enter into the posterior part
of the region which ossifies as the basisphenoid.
In the region of the periotic cartilage a large number of bones may
appear. In front there is the prootic, which often meets the
exoccipital behind; behind there is above and in close connection with
the supraoccipital the epiotic, and below in close connection with the
exoccipital the opisthotic. On the dorsal side of the cartilage there
is a projecting ridge composed mainly of a bone known as the pterotic,
sometimes erroneously called the squamosal, and continued in front by
the sphenotic. The pterotic, or the cartilaginous region corresponding
to it, always supplies the articular surface for the hyomandibular.
In the floor of the skull, in the region of the pituitary body, there
is formed a basisphenoid; while in the lateral parts of the wall of
this part of the cranium, there is a bone known as the alisphenoid.
In front, parts of the lateral walls of the cranium ossify as the
orbitosphenoids.
In view of the very imperfect ossification of the cartilaginous
cranium of the Dipnoi, and of the fact that there is certainly no
direct genetic connection between the Teleostei on the one hand, and
the Amphibia and Amniota on the other, it is very difficult to believe
that most of the ossifications of the cranium in the Amphibia and
Amniota have more than a general correspondence with those in the
Teleostei.
In the Amphibia the ossifications in the cartilage are comparatively
few. In the occipital region there is a lateral ossification on each
side of the exoccipital, the basioccipital region being unossified,
and the supraoccipital at the utmost indurated by a calcareous
deposit.
The periotic capsule is ossified by a prootic centre, which meets the
exoccipital behind.
The front part of the cartilaginous cranium is ossified by a complete
ring of bone--the sphenethmoid bone--which embraces part of the
ethmoid region, and of the orbitosphenoid and presphenoid regions.
In the Amphibia the cartilaginous cranium, with its centres of
ossification, is easily separable from the membranous investing bones.
In the Amniota the cartilaginous cranium, whose development in the
embryo has already been described, becomes in the adult much more
largely ossified, and the bones which replace the primitive cartilage
unite with the membrane bones to form a continuous bony cranium.
The centres of ossification become again much more numerous. In the
occipital segment analogous centres to those of Teleostei are again
found; and it is probable that the exoccipitals are homologous
throughout the series, the supraoccipital and basioccipital bones of
the higher types being merely identical in position with the similarly
named bones in Fishes.
In the periotic there are usually three centres of ossification, first
recognised by Huxley. These are the prootic, the epiotic and
opisthotic, the situations of which have already been defined. Of
these the prootic is the most constant.
In Reptiles, the prootic and opisthotic frequently remain distinct
even in the adult.
In Birds, the epiotic and opisthotic are early united with the supra-
and exoccipital; and at a later period the prootic is also
indistinguishably fused with the adjacent parts.
In Mammals the three ossifications fuse into a continuous whole--the
periotic bone--which may be partially united with the adjacent parts.
In the pituitary region of the base of the cranium a pair of osseous
centres or in the higher types a single centre (Parker[209]) gives
rise to the basisphenoid bone, and in front of this another basal or
pair of basal ossifications forms the presphenoid, while laterally to
these two centres there are formed centres of ossification in the
alisphenoid and orbitosphenoid regions, which may be extremely reduced
in various Sauropsida, leaving the side walls of the skull almost
entirely formed of membrane or cartilage.
[209] According to Kölliker there are two centres in Man in both
the basisphenoid and presphenoid.
In the ethmoid region there may arise a median ossification forming
the mesethmoid and lateral ossifications forming the lateral ethmoids
or prefrontals; which may assist in forming the front wall of the
brain-case, or be situated quite externally to the brain-case and be
only related to the olfactory capsules.
The labial cartilages. In most Fishes a series of skeletal structures,
known as the labial cartilages, are developed at the front and sides
of the mouth, and in connection with the olfactory capsules; and these
cartilages still persist in connection with the olfactory capsules,
though in a reduced form, in the higher types. They are more developed
in the Cyclostomata than in any other Vertebrate type.
The meaning of these cartilages is very obscure; but, from their being
in part employed to support the lips and horny teeth of the
Cyclostomata and the Tadpole, I should be inclined to regard them as
remnants of a primitive skeleton supporting the suctorial mouth, with
which, on the grounds already stated (p. 317), I believe the ancestors
of the present Vertebrata to have been provided.
BIBLIOGRAPHY.
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(440) C. Gegenbaur. _Untersuchungen z. vergleich. Anat. d.
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(441) Günther. _Beob. üb. die Entwick. d. Gehörorgans._ Leipzig, 1842.
(442) O. Hertwig. "Ueb. d. Zahnsystem d. Amphibien u. seine Bedeutung
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(443) T. H. Huxley. "On the theory of the vertebrate skull." _Proc.
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(444) T. H. Huxley. _The Elements of Comparative Anatomy._ London,
1869.
(445) T. H. Huxley. "On the Malleus and Incus." _Proc. Zool. Soc._,
1869.
(446) T. H. Huxley. "On Ceratodus Forsteri." _Proc. Zool. Soc._, 1876.
(447) T. H. Huxley. "The nature of the craniofacial apparatus of
Petromyzon." _Journ. of Anat. and Phys._, Vol. X. 1876.
(448) T. H. Huxley. _The Anatomy of Vertebrated Animals._ London,
1871.
(449) W. K. Parker. "On the structure and development of the skull of
the Common Fowl (Gallus Domesticus)." _Phil. Trans._, 1869.
(450) W. K. Parker. "On the structure and development of the skull of
the Common Frog (Rana temporaria)." _Phil. Trans._, 1871.
(451) W. K. Parker. "On the structure and development of the skull in
the Salmon (Salmo salar)." Bakerian Lecture, _Phil. Trans._, 1873.
(452) W. K. Parker. "On the structure and development of the skull in
the Pig (Sus scrofa)." _Phil. Trans._, 1874.
(453) W. K. Parker. "On the structure and development of the skull in
the Batrachia." Part II. _Phil. Trans._, 1876.
(454) W. K. Parker. "On the structure and development of the skull in
the Urodelous Amphibia." Part III. _Phil. Trans._, 1877.
(455) W. K. Parker. "On the structure and development of the skull in
the Common Snake (Tropidonotus natrix)." _Phil. Trans._, 1878.
(456) W. K. Parker. "On the structure and development of the skull in
Sharks and Skates." _Trans. Zoolog. Soc._, 1878. Vol. X. pt. IV.
(457) W. K. Parker. "On the structure and development of the skull in
the Lacertilia." Pt. I. Lacerta agilis, L. viridis and Zootoca
vivipara. _Phil. Trans._, 1879.
(458) W. K. Parker. "The development of the Green Turtle." _The
Zoology of the Voyage of H. M. S. Challenger._ Vol. I. pt. V.
(459) W. K. Parker. "The structure and development of the skull in the
Batrachia." Pt. III. _Phil. Trans._, 1880.
(460) W. K. Parker and G. T. Bettany. _The Morphology of the Skull._
London, 1877.
(460*) H. Rathke. _Entwick. d. Natter._ Königsberg, 1839.
(461) C. B. Reichert. "Ueber die Visceralbogen d. Wirbelthiere."
_Müller's Archiv_, 1837.
(462) W. Salensky. "Beiträge z. Entwick. d. knorpeligen
Gehörknöchelchen." _Morphol. Jahrbuch_, Vol. VI. 1880.
Vide also Kölliker (No. 298), especially for the human and mammalian
skull; Götte (No. 296).