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The morphology of the skull online

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roof free from the median parts.

583. An aponeurotic tract extending forwards from
the middle of the outer surface of the squamosal, has
become ossified, and meets and often coalesces with the
sphenotic process, forming a temporal bridge. The occi-
pital plane is much more vertical than in former stages,
and the auditory capsule has recovered fcom its great
obliquity, and is more erect than in the second stng^. A

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partial bony septum passes between the halves of the fore-
brain from the line of coalescence of the frontals, and there
is a transverse septum corresponding to the coronal or
parieto- frontal suture, to some extent separating the fore
from the midbrain. A sharp crest passes from the pro-
jecting ridge of the prootic (in front of the meatus intern us)
inwards and forwards to the sides of the posterior clinoid
wall of the basisphenoid. The arch of the anterior semi-
circular canal, which is now nearly vertical, is connected
by a ridge of bone with the fron to-parietal septum.

Pig. 73.

Fowl several years old ; under view of skull with lower jaw removed.

80. supraoccipital; 6.o. basioccipital; eu, orifice of Eustachian tubes ;
pf. sphenotic process ; pL pterygoid ; p, palatine ; v. vomer ; px. pre«»
maxillary ; mx. maxillary ; j. jug&l ; q.j, quacbato-jugal ; q, quadrate.

584. The basisphenoidal region, including the basi-
temporal plate, is of great thickness, and the thickening
of the cranial floor reaches back nearly to the occipital
condyle. The pituitary fossa is a deep well, elegantly
rounded, turning backwards below, beneath the posterior
clinoid wall ; and it is perforated by the internal carotid
arteries. One of these arteries enters the basis cranii outside

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VII.] THE a!dult fowl. 257

and in front of the exit of each vagus nerve, in a notcli
between the temporal wing of the exoccipital and the
outer and posterior angle of the basi temporal, and these
two bones coalesce together to form a bridge outside the
artery. When the outer table of the bone, together with
a portion of the diploe, is removed in the auditory region,
the whole membranous labyrinth is seen to be enclosed
in a bony case exactly corresponding to the membranous
structures within, the cochleae appearing in the floor of the
skull at a moderate distance from each other, like little
bony balls.

585. Besides the anterior tympanic recess which
reaches forwards above the narrow Eustachian tubes
(§ 541), there is at the summit of the tympanic cavity an
upper tympanic recess*, running into the diploe of the
occiput on each side. The base of the stapes and the
two fenestra? (ovalis and rotunda) lie at the bottom of a
common recess in the tympanic cavity.

586. The tympanic membrane is atto^^hed to a delicate
rim of bone some distance within the margin of the bony
tympanic hollow produced by the tympanic wing of the
exoccipital : this margin is extended outwards by a thick
fold of skin to form the proper meatus auditorius extemus.
The fine rim of attachment for the membrane can be
traced just outside the three principal recesses of the
tympanum, the anterior tympanic, the posterior tympanic,
and the fenestral recess. The bony regions which furnish
the attachment are the occipito-otic and basitemporal.
The membrane is fixed in front to a strong aponeurosis
passing down from the squamosal to the extremity of the
pretemporal wing of the basisphenoid (§ 541), behind the
head of the quadrate. This aponeurosis becomes ossified
in old birds, thus excluding the quadrate from direct
relation with the tympanic cavity in this type.

1 In the Crocodile these recesses meet above the occipital arch between
the outer and inner tables of bone.

B. M.

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In birds which have a second head on the otic process of
the quadrate, the aponeurotic band here spoken of is attached
to the quadrate and the hinder head ia inchided in the cavity ;
the band itself frequently becomes the seat of tympanic ossifi-
cations, of which the number varies from one to seven in
different types.

587. The columella (Fig. 74?) is a very delicate styloid
bone, having an ovoid stapedial plate in the fenestra
ovalis. Externally it is cartilaginous, where it abuts
against the middle of the tympanic membrane and distends
it. The stapedial plate (st) and the shaft of the columella
or mediostapedial ( form a continuous bone. Distally
there are three cartilaginous rays. Two outer ones, the


Adult Fowl, side view of colnmeUa auris.

St. stapedial (bonj) plate ; m.8t. mediostapedial (bone) ; the remaimng
parts are oartils^nons ; extrastapedial ; 8.8t suprastapedial ; /. fe-
nestra between these bars ; i.8t. interstapedial ; sth, stylobyal.

supra- and infrastapedial (,, pass at right angles
from the shaft. The upper is bifurcate, and is attached
by a membranous band to the posterior part of the tym-
panic roof. The third or median process (extrastapedial, is broader than the rest, spatulate, and decurved.
Extending from its outer edge to the extremity of the
suprastapedial is a bar of cartilage, a fenestra (/.) being
left in the angle between the extra- and suprastapedial.
This bar lies against the middle and anterior part of the
tympanic membrane towards the quadrate. The small
infrastapedial is directed downwards and a little forwards,

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and is sometimes expanded below. This lower region
was distinct in an early stage (§ 523), but has become
confluent with the infrastapedial. It represents the top of
the stylobyal {sUh,),

588. The mandible varies considerably in difierent
individuals ; in some specimens there is scarcely a trace
of the great fenestra seen on either side in the Tetraonidae
between the forks of the dentary and the hinder elements.
There is usually some remnant of the suture between the
posterior wedge of the dentary and the angular and sur-
augular ; but all the other sutures close.

589. The comua (minora) of the hyoid have coalesced
to form a glossohyal : but the bone is tipped with cartilage
anteriorly, and a chink-like foramen remains in the median
part of the bone. This elegant sagittiform tongue-piece

Fig. 75.

Fowl several years old; hyobranchial apparatus from aboye.
C.h, ceratohyal ; &r. 1, first basibranchial ; b,br. second basibranchial ;
cbr, ceratobranchial ; epibranchial.

(Fig. 75, c. h.) articulates by a synovial joint with the first
basibranchial (6r. 1), which itself has a similar articulation
with the long slender proximally-ossified second basi-
branchial (6. Jr.). The branchial arch articulates on either
side by a synovial joint with the posterior end of the first
basibranchial, but the proximal (ventral) piece or cerato-
branchial (c. 6r.) is united to the distal or epibranchial
{e. br,) by a mass of fibrous tissue : the ceratobranchial is
cartilaginous abovOi and the epibranchial at both ends.

590. Summary. A striking feature in this history
is the acceleration of the determining events ; the charac-
ters which according to the theory of evolution are due


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to very far-reaching inheritance, are assumed with great
rapidity, the skull being definitely Sauropsidan at the end
of the first week of incubation. Those details of structure,
however, by which the Fowl is differentiated from other
birds, are much later and slower in appearing, several of
them being only gradually acquired after the first year of
life. There is no fact in the early development of the
Fowl's skull which does not illustrate or receive illus-
tration from the previous histories. This book might
be indefinitely enlarged if these correspondences were all
pointed out: but the student who patiently meditates
will perceive them for himself in due time.

591. Commencing with the simplest stage, we find
the axial parts at the base of the skull developed con-
tinuously, as if the disjunction found in lower types were
overpassed and their later condition of union at once
attained. Conversely, distinct elements formed out of
simple bars in other forms are here separate at first,
without the necessity of any process of segmentation.
That which was an event of development before has now
become a primary condition. Again, the first appearance
of the hyoid and branchial arches in the Fowl corresponds
with the condition attained after metamorphosis in the

592. It is to be remarked how small a proportion of
the brain-case itself is preformed in cartilage. The same
elements are present as in previous cases, but their pro-
portionate size in the structure is much less, and their
total share in the completed skull is really small. Yet
unity of plan is manifest in the occurrence of the same
bony centres in cartilage, even if they form but insig-
nificant nodules. Together with these facts in relation
to the chondrified parts, we notice the increased size of
the membrane-bones, the perfection of their adaptation in
the adult organism, and their intricate interlocking with
cartilage-bones. If the parasphenoidal elements in the
Fowl are compared with the parasphenoid as previously

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described, their essential likeness as well as their special
diversities will be better understood.

593. In the first stage we see an elementary skull,
already advanced to a condition reached in lower forms
by coalescence and segmentation. In the second stage
the principal outlines of the skull are visible : the occipital
ring and the orbitonasal septum are in existence. There
is a cleft palate like that of Striithious birds, and the
angular processes of the mandibular cartilage indicate
the type to which the Fowl belongs. In the next period
numerous membrane-bones have appeared, and a few
centres in cartilage ; the nasal region is highly developed.
In the fourth stage, still before hatching, the skull has
attained many of its bones, the vomer being a notable
addition ; and the bony cranial floor has advanced in its
remarkable process of composition. It is not till after
hatching that some notable osseous centres appear, and
the coalescence of bones becomes very marked : this goes
on gi-adually until adult age, when most of the sutures are
obliterated. The proportionate length of the precranial
part of the skull increases, the bones become thick and
solid, and mostly smooth externally. Much of the nasal
labyrinth remains cartilaginous throughout life; some
cartilage remains in the columella, the mandible, the
palatopterygoid tracts, and the hyobranchial apparatus.
Although bony deposit appears in the palatine and ptery-
goid tissues before true cartilage is formed in them, yet
wherever in these tracts the ossification does not extend,
cartilage is subsequently found.

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594. Notwithstanding the great yarieties of outward form to be
seen in the skulb of Birds, there is extremely little modification in
the general morphological relations of parts. The palatal structures
t>resent the largest amount of change, yet some of the features o(
importance in that respect are only to be discerned after yery
careful study. In Struthious birds there is a mudi greater per-
sistence of sutures than in higher forms; the parieials are notaUy
laige. The rostrum is of greater size proportionally than in any
other group of Birds. The cranio-facial notch is absent. The body
of the basisphenoid intervenes (below) between the openings of the
Eustachian tubes ; in nearly all other birds the basitemporals meet
below these tubes so that they open together close to the middle
line. Only in some of the Struthionidse are the Eustachian tubes at
all enclosed in bone, and this occurs by the projection of a ridge of
the basitemporal to meet another from the *' pretympanic wing."
The mesethmoid ossifies the whole interorbital and nasal septuuL
The otic process of the quadrate is not divided into two heads ; its
iuiterior fork remains tipped with cartilage in the adult. In this
last particular, as well as in the structure of the palate, the genus
Tincunus agrees with all Struthious birds, and this compound group
may be spoken of as dromcBognathoiu {DromasMy the Emeu). The
vomer is very broad, and unites in front with the maxillopalatine
plates, while behind it receives the posterior extremities of the
palatines and the anterior ends of the pterygoids, which are thus
prevented from entering into any extensire articulation with the
rostrum. There are strong basipterygoid (bony) processes from the
basisphenoid, and these articulate with the pterygoids near their
outer or hinder ends. The basitemporals are but feebly developed.

595. All other types have the posterior ends of the palatines
and the anterior ends of the pterygoids articulating directiy with
the rostrum. The Fowl belongs to the schizognathous type (having
a cleft palate). The vomer tapers to a point anteribrly; while
[)Osteriorly it embraces the rostrum. The maxillopalatine plates

1 See Huxley " On the Classification of Birds," Proe. Zool, Soc. 1867,
and *'0n the AleotoromorphsB,'' ib, 1868; also Parker, Art. ** Birds,*'
Encycl, BriU, Vol. ni.

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vil] the skulls of birds. 263

leave a broader or narrower fissure in the palate between them-
sehes and the vomer. The restricted Gallinaceous group (or Alecto-
lomorphse) excludes the Pigeons and Tinamous. The other principal
Bchizognathous types are the Plovers, the Cranes and Rails, the
Gulls, Pigeons, and Auks, and the Penguins. In Pigeons and Sand-
grouse the vomer is absent. Some, as the Albatross and the Gull,
possess an os uncinatum or antorbital bone ; others, as the Hum-
ming-Birds and Kagu, have septo-maxiUaries. All except the Alec-
toromorphse have mesopterygoids.

596. A group named desmognathous is distinguished by the
following characters : vomer mostly small or absent ; masillopalatino
plates united across the middle line, either directly or by the inter-
mediation of ossifications in the nasal septum. Falcons and Geese
are examples of what may be termed dire^ desmognathism ; the
maxillopalatine plates meet below at the middle line as in the mam-
mal. In the Falcon the nasal septum is anchylosed to this hard
palate; in the Goose it remains free. Indirect desmognathism Ih
exemplified in Eagles, Vultures, and Owls; the maxillopalatine
plates are anchylosed to the nasal septum, but are separated from
one another by a chink. In Dicholophus crintatus the maxillopalatine
plates are united by harmony suture and not by coalescence. In
Megaloema asiatica they are closely articulated with, and separated
by, a median ossification beneath the nasal septum, the '* median
septo-maxillary.'' The most advanced condition may be called
double desmognathism, where the palatines as well as the maxillo-
palatine plates coalesce to form a continuous palate; this is seen
in Podargtis, and to a less extent in ihe large Hornbills. In
Ducks and Swans a pair of ossicles is found between the palatines,
stretching towards tiie maxillopalatine plate. In several Ardeidw
an additional bone, the postmaxillary, is formed behind the angle
of the maxillary (this occurs also in the Emeu). la various desmo-
gnatlious forms the palatines unite for a considerable distance behind
the posterior nares, and send down a vertical crest at their junction.
This is largest in the Pelican, where all the parts in front of the
very mobile cranio-facial hinge are anchylosed into one mass, and
the nasal labyrinth is in its most aborted state. In PAcenicopleru-^
the median palatine keel is exquisitely thin in front, but where the
palatines extend beneath the rostrum, a separate lamella is sen*}
down from each bone and the two are bound together by fibrous

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tissue, and between them the vomer is wedged: the ethmopalatine
spurs are enormously long. In Sula alba the basitemporals are
relatirely as small as in Dromceus; the small Eustachian tubes, un-
covered by bone, open at a little distance apart, in a wide shallow
fossa where the three parasphenoidal elements meet. There is a
postmaxillary ; the cranio-facial hinge almost rivals that of the Parrot
and Toucan ; the columella auris is very long and bent, with a long
attenuated cartilaginous infrastapedial process, terminated by a
bony fusiform stylohyal. The hinge for the mandible is very far
back, as also in the Cormorant.

597. In the (desmognathous) Aetomorphas^ in addition to the
h(X>king of the beak, there is another character ; the maxillopalatine
plates are generally united with an ossification beneath the nasal
septum, or " median septo-maxillary." The vomer is azygous ; the
palatines often have a mediopalatine where they unite, or a pair of
mesopterygoids. In some forms the supra-orbital process of the
lachrymal is very large ; in others (Hawks, &c) there is a distinct
supra-orbital at its extremity. In the Sparrow-Hawk distinct pterotic
and sphenotic centres are developed ; and the orbitosphenoids are
preceded by cartilage.

598. The desmognathous Parrots are very uniform, having the
most complete cranio-facial cleft, with a perfect hinge-joint between
the frontal and nasal regions. There is no vomer; the palatines
are vertically elongated posteriorly, while anteriorly they are hori-
zontally flattened, and movabiy united with the rostrum. The
lachrymal and postorbital (or sphenotic) bend towards one another,
and unite by the intervention of a large os uiicinatum or antorbital.
In some also the temporal fossa is bridged over by the union of the
zygomatic process of the squamosal with the os uncinatum. The
nasal septum is a thick wall of I one ; an annular ossicle is found in the
alinasal Q2LTi\\^^QQi MdopsittaciLS undulatus; in Palwornis torqtmia
this part is largely ossified and anchylosed to the upper jaw, and the
alinasal turbinal is partly calcified. The Hornbills and Toucans
have a median septo-maxillary in front of the azygous vomer. In
Podargus^ when the lower palatine floor is cut away, there are to be
Been three small ossicles, the vomer (anterior), and two medio-pala-
tines. In Kingfishers and Hoopoes there is no vomer.

59d. The cegitJiognathous type (including the Passerines) is

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distinguished by the union of the yomers with the alinasal wall and
turbinal ; and the embryo at least has a pair of " upper labial " car-
tilages, which the Tomers ossify either partially or entirely. In
most cases there are septo-maxillaries, attached to the angles of the
vomers. In the fledgling Rook the basitemporal plate is an almost
transTcrse band of bone ; the rostrum bears no basipterygoid pro-
cesses ; the cranio-facial notch is nearly complete, but the nasals and
nasal processes of the premaxillaries are set into the frontals as
thin spurs. The palatines develope cartilage at their hinder (outer)
angle, and this part ossifies separately and late, like a transpalatine.
The mesopterygoid spur of the pterygoid coalesces with the palatine.
The maxillopalatine processes are hooked and flattened. There is
a chain of seven small tympanic bones, the principal of which be-
comes a perfect ring in the Crow, and surrounds the siphonium.
In many ^githognathse the lachrymal cannot be seen at any stage ;
in many that have it, it soon anchyloses either with the nasal in
front or with the ethmoid behind.

600. There is incomplete aegithognathism in the Tamix group;
the large labials are imperfectly ossified by the vomers, and these
bones are only strongly attached to the nasal labyrinth by fibrous
tissue. The following forms of complete segithognathism may be
distinguished : (1) in PachyrhamphuSy Pipra^ &c., the labials are
often only imperfectly ossified by the vomers, and these centres are
distinct from those ossifying the alinasal cartilages ; but the union
of the two tracts is complete ; (2) in a great number of the Passerines,
and in the Swifts, the labials are small and completely ossified by the
vomers, but the bony deposit extends largely into the alinasal wall
and turbinals ; (3) a compound condition, namely, desmognathism in a
perfectly segithognathous type, the maxillopalatiues uniting with a
highly ossified alinasal wall and nasal septum {Gymnorhinay Para-
discea, Artamus); the transpalatine process is a long spike.

601. The SaurognathotM group (Woodpeckers and Wrynecks)
have their palatal structures arrested at a most simple and La-
certian stage, whilst in other respects they are metamorphosed
and specialised beyond any other kind of birds. The principal
characteristics are as follows : the retention and ossification of the
trabecular^ comua; the great number and distinctness of the vo-
merine series of bones (vomers, septo-maxillaries, median septo-

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maxillary); absence of a distinct mesopterygoid ; presence of a
mediopalatine ; no distinct transpalatiue ; abortive maxillopalatine
plates; a distinct palatomaxillary on one side only. The nasal
labyrinth is unusually simple; the inferior turbinal, which has three
coils in Rhea and Tinamiigy and two in most birds, in Gecinus is
two< winged, and in Yunx makes less than a single turn. In Gecimts,
which is very specialised, the columella has two suprastapedial spurs
and two infrastapedial bands, which have united with the stylohyal.
The small ceratohyals in the Woodpeckers early coalesce into one
arrowhead-sh£4)ed bone, and behind it is a very long highly-ossified
and elastic basihyal. Joined to this behind is a pair of cerato-
branchials, half its length ; the epibrauchials are four times the
length of the ceratobranchials, and, passing first down the sides of
the upper part of the neck, they curve gently upwards and forwards,
lying in a furrow on the top of the skull ; bending slightly from the
middle line, they end on the nasal roof.

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First Stage: Embryos 7^ to 8 lines long,

602. The earliest embryos of the Pig in which the
foundations of the skull are perceptible present strong
resemblances in the cephalic region to those of lower
vertebrates ; nevertheless a few special features of mor-
phological elevation are manifest. In a front view the
second cerebral vesicle is seen as the uppermost of the
neural bulbs, although it does not advance quite to the
front ; thus the forehead is sloping. The distinct cerebral
hemispheres occupy the middle of the lateral aspect of the
head, and are together rather broader than the second
vesicle. Immediately beneath the hemispheres are the
nasal sacs, reniform in shape ; they are bounded inferiorly
by the lateral angles of the nasofrontal process in which
the •trabecular cornua are contained. The eyeballs, not
very prominent, are seen on either side of the front view,
above and behind the nasal sacs. Between the latter
is a nasofrontal tract of moderate width, forming with
its horns the anterior and upper margin of the mouth.
This opening is at present quite inferior and transverse,
bounded laterally by maxillopalatine processes which grow
forwards and downwards from the continuous super-
ficial tissues of the sides of the head, and posteriorly by
the mandibular arch, which constitutes a transverse bar
l)elow, and has its moieties convex outwardly at the sides.
Behind the mouth the skin of the broadish throat is con-
tinuous and unmarked ventrally.

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603. In a side view (Fig. 76), the great proportionate
size of the second cerebral vesicle ((72), which occupies the
whole crown of the head, is particularly marked. The
third vesicle (03) is much smaller, and on a lower level.

Fig. 76.


Embryo Pig, two-thirds of an inch long; side view of head and neck.
C 1, 2, 3, cerebral vesicles; e.n, external nasal opening; e, eyeball;
au, auditory mass ; tg, tongue-rudiment? ww. mandibular arch;
maxillopalatine process; cl. 1, 2, 3, 4, visceral clefts; hy, hyoid arch,
covered by the end of the fore limb.

There is a considerable depression between this vesicle and
the spinal region, the cervical part of which is bent at a
right angle with the dorsal, giving the embryo a strange
humpbacked appearance. The olfactory sacs are seen at

Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 22 of 31)