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into the trabecular cartilage on either side of the pituitary space,
but in front of this the cartilage is unossified and passes into the
extensive interorbital septum, which again is continued into the nasal
septum. The membranous floor of the pituitary space is supported
by the posterior broad end of the delicate styliform parasphenoid,
which underlies the posterior two-thirds of the interorbital septum.
The latter is largely fenestrate, and but slightly ossified. There is
no alisphenoid, but a membrane occupies the cranial wall from a
little behind the optic foramen to the front margin of the prootic
(which is notched for the trigeminal nerve). The parietal de-
scends slightly into this tract. The orbitosphenoid cartilage is
extensive, but very largely fenestrate; the cartilage extends for
some distance behind the optic foramen. There is an orbito-
sphenoid ossification below the fenestra. The nasal septum is
unossified: the nasal passage is floored on eit;}ier side by a large
septo-maxillary, which ascends as a small vertical plate a^^aiust the
septum.

505. The premaxillary as a rule is azygous, having a long
nai^al process. The nasals may be large as in Iguana, or very small
as in Monitors. There are two membraiie-bones in the antorbital
region, both of which may be perforated ; the upper or prefrontal is
the larger, and has an orbital plate; it narrows into an arcuate spur
between the orbits, uniting by suture with the frontal. There is one
supraorbital bone in Monitor, Iguana has two or three, the Scincoids
a considerable series. The frontals are simple superior bones,
united with the parietals by a straight coronal suture, whose ends
abut on the postorbitals (postfrontaJs). These membrane-bones
(peculiar to reptiles) are always large, and rest on the squamosal
behind, and sometimes on the jugal in front. As in Snakes, the
parietals are always densely anchylosed, except for the small parietal
foramen, so characteristic of these forms, which may be behind the



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VI.] THE SKULL OF CROCODILES. 217

coronal suture, in its course, or slightly in front of it Posteriorly
a large horn is sent out on each side from the parietal, passing
backwards to the ear-capsule and the parotic process of the ex-
occipital. The jugal is sometimes lai^ge, reaching to the postfrontal
and parietal. The quadrato-jugal is absent except in Hatteria.
The squamosal is connected with the posterior spur of the parietal,
and may be also united with the postfrontal. There is a supra-
temporal behind the squamosal The fossae which lie between
the external bones of a Lizard's skull may be distinguished as
supratemporal, between the parietal, postfrontal, and squamosal ;
post-temporal, between the parietal, supraoccipital, and auditory
prolongation; and lateral temporal, between the squamosal and
postfrontal above, the jugal and quadrate in front and behind, and
the quadrato-jugal ligament below.

506. In the Geckos the external lateral bones of the skull are
small or absent, so that the upper and lower lateral bars are not
constituted. In Sphenodon both arcades are complete, and the
quadrate is anchylosed to the squamosal, quadrato-jugal and ptery-
goid, and the front of the ear-capsule. There are two premaxilla-
ries. In Mosasaurtu the pterygoids articulate with one another
in the middle line behind the posterior nares. In Chamwleon the
skull has an interorbital septum, but no epipterygoid. The quadrate
bones in Amphishcena are directed downwards and forwards, and
there is no interorbital septum. In Chamseleon there is a backwardly
produced median crest from the occipital and parietal bones, joined
by a pair of lateral prolongations from the squamosals. The orbit
is closed by a process of the jugal, but the lower arcade is not
completed by a quadrato-jugal. The quadrate is articulated with
the squamosal and the auditory capsule, but the pterygoid does not
reach it.

The Skull of Crocodiles.

507. The hinder region of the skull is very complete, and there
are large alisphenoids. There is an interorbital septum ; and the
orbitosphenoids are little developed. There is no parietal foramen.
The bones anterior to the cranial cavity are greatly elongated, and
the palatal plates of the maxillaries and palatines are very large,
meeting in the middle line, and forming a secondary floor to the
nasal passages ; this palate is carried far backwards by the develop-
ment of a palatal plate on the pterygoids. The latter also extend



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218 MORPHOLOGY OF THE SKULL. [CHAP. VI.

upwards to the base of the cranium, being suturally united, and are
united externally with the upper and inner surface of the quadrates.
The vomers are usually concealed by the palatal plates. The large
quadrate is firmly fixed in the side of the skull, though not anchy-
loeed. The tympanic cavity is bounded by almost all the bones of
the hinder part of the skull, the epiotic and supraoccipital excepted.
Tne opisthotic is confluent with the exoccipital. The lateral bony
arches of the skull are very complete and strong, forming consider-
able fosssB resembling those of the Lacertilia. The Eustachian tubes
are very complex.



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( 219 )



CHAPTER VII.

THE SKULL OF THE COMMON FOWL.

First Stage : Embryo at the end of the fifth day of
incubation,

508. The head of the embryo chick has at this period
become well-defined, with notable bulbous protuberances
of the cranial vesicles, and very large ej"eballs. The
mesocephalic flexure is at its greatest intensity ; the head
is bent over so as nearly to touch the neck ; the mouth
faces the chest. The cerebral hemispheres are already
well-developed on the inferior aspect of the head (Fig. 54,
c.L) ; the midbrain is prominent in front of the cranial
bend; while the hindbrain is elongated and comparatively
small. The olfactory pits (n.) are little depressions below
and behind the forebrain, and just internal to the fore
part of the eyeballs. The auditory vesicles are some way
above and a little behind the eyeballs, at the sides of the
posterior region of the hindbrain.

509. The mouth (m.), beneath and behind the fore-
brain, has an oblong aperture, bounded in front by the
nasofrontal process (n./.), which separates the two olfactory
depressions; behind by the first visceral arch, and laterally
by the maxillopalatine processes (s.m.) growing forwards
from the sides of that arch, and lying underneath the
eyeballs. The nasal pits, as they deepen, acquire a break
in their rim, giving rise to a groove directed obliquely
downwards towards the cavity of the mouth. The naso-



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220 MORPHOLOGY OF THE SKULL. [CHAP.

frontal and maxillopalatine processes abutting on this
groove on either side, tend to deepen it : at a later period
they form a bridge over it, and thus enclose a channel
from the nasal pits to the mouth, opening on its root

Fig. 64.



Embryo Chick of the fourth day of incubation; head viewed from
below as an opaque object (Foster and Balfour). The neck has been
cut across between the third and fourth visceral folds.

c,h, cerebral hemispheres; f.b. first cerebral vesicle; op, eyeball;
n./. nasofrontal process ; m. mouth cavity ; «.m. superior maxillary or
maxillopalatine process from /. 1, the first visceral (mandibular) fold ;
/. 2, /.3, second and third visceral folds ; n. nasal pit.

In the section of the neck, al. alimentary canal ; wi.c. neural canal ;
ch. notochord; ao, below, dorsal aorta; v. vertebral vein; ao. above,
bulbus iurteriosus.

510. Three visceral folds (Fig. 54, /. 1, 2, 3) exist in
the side-walls of the throat, the hinder being the smaller;
and they are most prominent ventrally. Behind each fold
there is a cleft on either side, opening into the throat.

511. The primary parts of the cranio-facial skeleton,
from being constituted of stellate cells, are mostly passing
into hyaline cartilage. The cranial part of the notochord
(Fig. 55, w.c), slightly constricted in two places, extends
forwards nearly to the pituitary region (pts). It is
flanked on either side by the parachordals {paxh) : there



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VTI.] THE FOWL: FIRST STAGE. 221

is a prechordal bridge, and from this region the trabeculw
{tr.) bend downwards and diverge to surround the pituitary
space. A discontinuity of these primary elements has not
been discovered in any earlier stage. No separate carti-
laginous auditory capsule is formed in the chick ; but

Fi«. 55.



Embryo Chick, fifth day of incubation ; view of cranial structures from
above, the upper part of the head haying been sliced away horizon-
tally ; part of the first vesicle and the lower part of each eyeball are
left. The cartilaginous parts are those indicated by dark horizontal
shading.

c.v, 1, forebrain ; e, eyeball ; tr, trabecula (the line stops short of the
part indicated) ; pt,8. pituitary or intertrabecular space ; 5, notch for
trigeminal nerve ; l,g, process which becomes the lingula sphenoidalis ;
q. quadrate cartilage ; h^s.c. horizontal semicircular canal ; cL cochlea ;
pa,ch. parachordal cartilage ; n,c, notochord ; 9, foramen for hypoglossal
nerve.

each parachordal is from the first continuous in the middle
of its outer aspect with the chon drifted wall of the ear-sac,
in which the cochlea (cl.) and semicircular canals already
begin to be evident. A little way in front of the ear-mass
the parachordals are suddenly narrowed, forming a notch
tor the passage of the trigeminal nerve (5). In front



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222 MORPHOLOGY OP THE SKULL. [CHAP.

of this there is a slight angular expansion, on each side
of the front end of the notochord ; and at this point a
small transverse crest exists, marking out exactly the line
about which the mesocephalic flexure has taken place.

512. The space enclosed by the descending trabeculae
is longitudinally oval : below this region the paired rods
coalesce to form a broad intetmasal plate, lying behind the

Fig. 56.



Embryo Chick, fifth day of incubation; head viewed from below, with
skeletal parts seen through.

c.v.l, forebrain ; e. eyeball; n. nasal pit; f.n, nasofrontal process.;
tr. points to one of the trabecular horns, recurrent in the nasofrontal
process beneath the level of the main part of the trabeculs, which are
seen on either side of pts. the pituitary space ; mr. subocular bar, in which
pa. palatine and pg, pterygoid tracts are indicated ; q, quadrate ; mk.
meckelian cartilage ; 6.^. basihyal ; c.h, ceratohyal ; h,br, basibranchial ;
cM, ceratobranchial ; e,br, eplbranchial ; 1, 2, 3, visceral clefts.

forebrain. It is emarginate in the middle line, and
extended into a horn-shaped process on each side. These
cornua partially arch round the external nasal openings.
The inferior part of the internasal plate is curved some-
what backwards upon the remainder, so that the cornua
look directly backwards (see Fig. 56).



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VII.] THE fowl: first stage. 223

513. In the inner (palatine) region of the maxillo-
palatine process, the curved palatopterygoid bar (sub-
ocular in position) is beginning to be established, but it is
considerably later in solidifying than the mandibular arch
(Fig. oQ, pa,y pg). The latter is already segmented into
two pieces; (1) a smaller tuberous quadrate cartilage
{q,) lying behind the eyeball, and closely apposed to the
antero- external angle of the parachordal and otic cartilage
behind the exit of the trigeminal nerve ; and (2) a meckelian
rod {mk) lying in the first visceral fold, having a sinuous
articular surface for the quadrate and a slight angular
outgrowth behind. The meckelian bars nearly meet in
the middle line. The first visceral cleft (1), behind the
quadrate, is commencing its development into the complex
tympanic cavity.

514. The hyoid arch contains ventrally a pair of
small ill-differentiated rods (c. h,) and an azygous piece or
hasihyal (6. h). The second visceral cleft is fast closing,
and behind it the first branchial arch has a pair of well-
defined ceratobranchial rods (c. br.) nearly meeting in
the middle line, surmounted by very small distinct epi-
branchial pieces. A basibranchial (6. br,) is forming in the
middle line behind.

At this early stage the hyoid arch Las begun to lie within
the mandibular, just as the first branchial comes to lie within
the massive hyoid arch in the Osseous Fish. Already the third
visceral arch of the chick has outgi'own the second, — a state of
things universal in the class of Birds.

515. We here see that a grade of structure is reached
in this skull as soon as its rudiments can be made out,
which is only attained by a series of changes in the skulls
of inferior types. The basicranial elements are continuous
notwithstanding the mesocephalic flexure ; the intemasal
plate and comua are equally complete and continuous.
The otic masses are also coalesced from the first with the
special cranial structures. The separate development of
the upper piece of the mandibular arch is a feature of
high specialisation; the future palatopterygoid, so important



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224 MORPHOLOGY OF THE SKULL. [CHAP.

in the adult, is already distinguishable. The slightness of
the skeletal parts in the hinder arches is related to the
absence of any necessity for breathing in water.

Second Stage : Sixth and Seventh Days of Incubation.

516. The head, still disproportionately large, is now
five lines long, and the rudimentary cranium is well chon-
drified. The mesocephalic flexure has greatly diminished,
and the head is more squared and less bulbous (Fig. 57).
The forebrain occupies nearly the front half of the cranial
cavity; the midbrain lies bdiind; while the hindbrain,
considerably smaller, is entirely below the hinder part
of the midbrain. The mouth, instead of being totally
inferior, is now a space, the axis of which points more
forwards than downwards.

Fig. 67.



Embryo Chick, sixth day of incubation; head seen from below.
(After Huxley,)

la. cerebral hemispheres; a. eyeball; g, nasal pit; k, nasofrontal
process ; Z. maxillopalatine process ; 1, mandibular arch ; 2, hyoid arch ;
X, first yisceral cleft. The cavity of the mouth is seen between k and 1 ;
the darkest part indicates the opening into the throat.

517. The trabecular region of the cranium has be-
come longer than the parachordal. At the angle formed
by the bending of the cranial floor, marked by a slight
crest in the firat stage, there has arisen the high ridge.



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Vn.] THE FOWL: SECOND STAGE. 225

directed upwards and somewhat backwards, known an
the posterior clinoid wall, protecting the pituitary body
behind. The internal carotid arteries enter the cranial
cavity in front of this wall. The parachordal cartilages
are well consolidated, although the notochord is not
obliterated. Posteriorly the cartilage grows outwards
and upwards around the neural tube, forming exoccipital
regions, and tending to complete the occipital ring. An
external growth of the exoccipital behind the ear-mass
(tympanic wing) comes to overarch the tympanic cavity
{e.o. Fig. 58).

518. The trabecular rods at the sides of the pituitary
body appear slightly bulged out and more distinct from
the parachordals than in the first stage, though perfectly
continuous with them. In older embryos there is au
alisphenoid (lateral) tract of cartilage (a.s.) partially com-
pleting the base of the cranium, separated from the otic
mass by the trigeminal nerve below, but united with it out-
side the nerve. Externally this alisphenoid cartilage has
an oblique projecting ridge, partially bounding the orbit
behind, and overarching the quadrate. The trabecular
cartilage remains low for a little distance, as far as the
exit of the optic nerves. Underneath the fore part of the
basilar region the fibrous stroma is thickening to receive
the basitemporal ossification.

519. In front of the optic foramen and rising consi-
derably above its level, the brain has become elevated
upon a vertical cartilaginous intemasal and interorbital
plate, oblong in side view, thinner above, but thick below
where it is directly formed by the coalesced trabecular.
It extends continuously to the fore extremity of the head,
and its front margin is almost vertical; it is continued
below into a rounded knob, directed a little backwards;
and its upper edge is united on each side with the small
cartilaginous nasal roof.

520. Within a very short time in incubation the an-
terior part of this vertical plate grows forwards below, so
as to form the axis of a beak projecting downwards and

B.M. 15

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22g MORPHOLOGY OP THE SKULL. [CHAP.

forwards : the beaked part (prenasal cartilage) curving
below the level of the basicranial axis. Thus we have
an oblong interorbital plate (Fig. 58, p,8.)y with a little

Fig. 58.



a7i



Embryo Chick, seventh day of incubation ; side view of slraU.

o»c, occipital condyle ; nc. notochord ; e.o, exoccipital tract, with its
tympanic wing ; «.o. supraoccipital tract ; h.8.c. horizontal semicircular
canal, p.8.c. posterior semicircular canal, both seen through the
cartilage; a.8, alisphenoid cartilage, with its external ridge; 2, optic
foramen; p.8. presphenoidal region; below it, interorbital septum;
p.p. antorbital plate ; eth. ethmoidal region ; ale. aliethmoidal cartilage ;
al.n. alinasal cartilage; p.n. prenasal cartilage; pa. palatine, pg.
pterygoid tract; q. quadrate ; sf. stapes; f.r. fenestra rotunda; mk.
meckelian bar; c.h. ceratohyal ; b.h. basihyal; b.br. basibranchial ;
c.br. ceratobranchial ; e.br. epibranchial.

spine projecting upwards in the front wall of the cranial
cavity; and a triangular intemasal plate, with its apex
directed downwards and forwards. This intemasal plate
rapidly becomes differentiated into three regions, distin-
guished by the nature of the nasal outgrowths continuous
w ith its upper and anterior edge. In front there is the
alinasal valvular structure (al.n^, protecting the external
nostril; behind this the aliseptal roof in the proper septal
region, with soft coiling growths already projecting down-
wards and inwards from it; and finally the aliethmoidal
roof {al.e.) growing from the highest part of the vertical
plate, coiling to form an upper turbinal, and also develop-
iug a transverse vertical partition ip.p,) between the nasal
cavities and the orbits {antorbital plate).



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Vn.] THE FOWL: SECOND STAGE, 227

521. The arcuate rod in the maxillopalatine process
(pd-ypg-) is elongated to reach from the front of the
quadrate cartilage to the. middle of the nasal septum. In
section these rods and the thickenings in which they lie
appear as the walls of the cleft palate. The quadrate
(Fig. 58, q,) is now a very definite triradiate cartilage ; its
antero-intemal process passes under the trigeminal nerve,
but does not reach the cranial wall. The postero-extemal
or otic process forms a large hook extending along and
applied to the side of the ear-mass, and reaching to and
partially overarched by the exoccipital (tympanic) wing.
This otic process overarches the tympanic cavity, and the
fenestrae ovalis and rotunda (f. r,) are on the wall of the
capsule beneath and within it. The quadrate condyle for
the mandible, looking forwards and downwards, is grooved
so as to be double. The meckelian rods (mk.) are long
sinuous cartilages approximated anteriorly, extending as
far forwards as the beak ; thickest behind, where they
fork into two processes, the inner and the posterior angu-
lar processes.

522. A small piece of cartilage (stapedial or colu-
mellar), at first continuous with the ear*capsule, has
become detached from it below the otic process of the
quadrate, and consequently its thick base fills up an oval
fenestra (ovalis), behind and below which is another mem-
branous fenestra {rotunda), in the wall of the rudimentary
cochlea (Fig. 58, 8t.,f. r.).

523. The history of this part has been observed more
completely in the House-Martin (Chelidon urhica) where
the upper end has been observed at first continuous with
the auditory cartilage, but dilated at its free end into a
fan-shaped extrastapedial plate which is pointed above
and below, the upper process being the rudimentary supra-
stapedial, and the lower being continuous with a dehcate
ligamentous tract, which chondrifying afterwards becomes
the infrastapedial (see Fig. 74). From the lo\jrer end
of this ligament there passes backwards at a right angle
a small fiat oblong cartilage : this exactly corresponds to

15—2

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"228 MORPHOLOGY OF THE SKULL. [CHAP.

the stylohyal which Prof. Huxley described in the young
Crocodile (see his paper on the Representatives of the
Incus and Malleus, P.Z.S.). The dilated extrastapedial
part has the same shape as described by the same author
in Hatteria.

524. The basal hyoid and branchial cartilages are
the same in number as in the last stage, but are more
^olid, and the lateral moieties are directed backwards.

525. Comparing the skull at this stage with its pri-
mary condition, we see that a marvellous advance has
taken place, elevating it from an elementary grade to one
characterized by the notable interorbital septum. Already
the skull is definitely Sauropsidan, and in several points
Struthious. The hinder part of the cranial investment is
proceeding to form its occipital ring; the anterior has
developed the main outline, not only of the interorbital
framework, but also of the beak which distinguishes the
Bird. The prenasal cartilage is a very fit model on which
the premaxillaries of any type of bird might be formed ;
in no bird whatever, let the shape of its face be what it
may, is the type of the first model wholly lost, although in
every case the primordial structure very early undergoes
wasting. The palate is cleft as in Struthious Birds ; the
present articulation of the quadrate is like that which is
persistent in them. Likewise the temporary condition of
the branchial arches in this stage agrees with their form
in adult Struthionidae. The angular processes of the jaw
already indicate the type of the animal, for they have a
peculiar development in the whole Gallo- An serine series,
attaining their most extraordinary form in the Tetraonine
group. .

Third Stage : Embryos in the middle of the second week
of incubationy with heads from eight to nine lines
in length,

526. The skull has made great progress towards a
bird-like form. In this stage it is in some respects parallel



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yn.] THE FOWL: THIRD STAGE. 229

with the adult skull in certain Teleostean Fishes, where
there is a free development of the cartilaginous frame-
work, which is comparatively little ossified ; whilst the
parostoses are delicate, although numerous. The Lump-
fish (Cyclopterus) and the Salmon possess such skulls.

527. When the axial parts of the skull are dissected
out, they are seen to be very easily divisible into two main
regions : a large posterior rounded expansion, more than
hemispherical,and an elongated vertically-compressed plate,
lying between the orbits and between the olfactory organs,
and flanked anteriorly by moderate-sized appendicular
cartilages belonging to the nasal sacs. The posterior
expansion is remarkable for the curious way in which the
periotic masses are compressed and curved so as to be
moulded into the rounded form of the cranium (see
Fig. 60). Thus a considerable portion (the cochlear) of
the ear-mass is in the cranial floor, very median in position
and far forwards : but the region containing the canals is
much compressed and tilted backwards, rising high in the
side wall, where it becomes postero-extemal, to accommo-
date the large brain; so that all the semicircular canals
are visible in a posterior view.

528. The notochord has relatively retired, and is
separated from the pituitary body by a distinct bridge
of cartilage; this forms the somewhat elevated posterior
clinoid wall, and the cartilage shelves downwards on either
side of it. The cranial notochord is slightly constricted in
two places, thus presenting three subequal regions, the
anterior cylindricsJ, the other two fusiform; the hinder
of these is enclosed in a bony sheath, and at the posterior
extremity of the skull by a cartilaginous bridge above
and below. This is the only point where the parachordal
cartilages have as yet united ; but here they completely
invest the notochord, and form the single rounded occi-
pital condyle, the two mammillae which first appear
becoming one. Anteriorly indeed the parachordals have



Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 19 of 31)