_Skull._--When W.K. Parker wrote the account of the skull in the article
BIRDS for the 9th edition of the _Encyclopaedia Britannica_, he had
still to wrestle with the general problem of the composition and
evolution of the skull. That chapter of comparative anatomy (together
with other anatomical details, for which see the separate articles) is
now dealt with in the article SKULL; here only the most avine features
are alluded to, and since some of Parker's original illustrations have
been retained, the description has been shortened considerably.
One general feature of the adult bird's skull is the almost complete
disappearance of the sutures between the bones of the cranium proper,
whilst another is the great movability of the whole palatal and other
suspensorial apparatus. The occipital condyle (fig. 1) is a single
knob, being formed almost wholly by the basioccipital, while the
lateral occipitals (often perversely called exoccipitals) take but
little share in it. Part of the membranous roof between the
supra-occipital and parietal bones frequently remains unossified and
presents in the macerated skull a pair of fontanelles. The squamosals
form the posterior outer margin of the orbits and are frequently
continued into two lateral downward processes across the temporal
fossa. One of these, the _processus orbitatis posterior_, often
combines with an outgrowth of the alisphenoid, and may be, e.g. in
cockatoos, continued forwards to the lacrymal bone, so as to form a
complete infraorbital bridge. The posterior, so-called _processus
Zygomaticus_ is very variable; in many Galli it encloses a foramen by
distally joining the orbital process. The ethmoid frequently appears
on the dorsal surface between the frontals. There are three periotic
bones (pro-, epi-, opisth-otic). The prooetic encloses between it and
the lateral occipital the fenestra ovalis, into which fits the
columella of the ear. The epiotic is often small, ossifies
irregularly, and fuses with the supra-occipital. The opisthotic lies
between the epiotic and the lateral occipital with which it ultimately
fuses; in some birds, e.g. in _Larus_, it extends far enough to help
to bound the foramen magnum. The basisphenoids are ventrally overlaid,
and later on fused with, a pair of membrane bones, the basi-temporals,
homologous in part with the parasphenoid of lower vertebrates. They
contribute to the formation of the auditory meatus, and of the right
and left carotid canals which accompany the eustachian tubes. In many
birds the basisphenoids send out a pair of basipterygoid processes by
which they articulate with the pterygoids. Dorso-laterally the
basisphenoid is joined by the alisphenoid, which forms most of the
posterior wall of the orbit. The orbito-sphenoids diverge only
posteriorly, otherwise they are practically unpaired and form the
median interorbital septum, which is very large in correlation with
the extraordinary size of the eyeballs.,
[Illustration: FIG. 1.--End view of skull of a Chicken three weeks
old. Here the opisthotic bone appears in the occipital region, as in
the adult Chelonian. (After W.K. Parker.)
bo, Basi-occipital.
bt, Basi-temporal.
eo, Opisthotic.
f, Frontal.
fm, Foramen magnum.
fo, Fontanella.
oc, Occipital condyle.
op, Opisthotic.
p, Parietal.
pf, Post-frontal.
sc, Sinus canal in supra-occipital.
so, Supra-occipital.
sq, Squamosal.
8, Exit of vagus nerve.]
[Illustration: FIG. 2.--Ripe Chick's head, 11/4 in. long. (After W.K.
Parker.)
as, Alisphenoid. pa, Palatine.
bo, Basi-occipital. pf, Post-frontal.
bt, Basi-temporal. pg, Pterygoid.
dpx, Dentary process of pn, Prenasal cartilage.
premaxilla. ppx, Palatine process of pre-
eo, Opisthotic. maxillary.
eu, Eustachian tube. prp, Pterygoid process of sphenoid.
f, Frontal. qj, Quadratojugal.
fm, Foramen magnum. so, Supra-occipital.
j, Jugal. sq, Squamosal.
l, Lacrymal. ty, Tympanic cavity.
mx, Maxilla. v, Vomer.
mxp, Maxillo-palatine process. 8, Exit of vagus nerve.
oc, Occipital condyle. 9, Exit of hypoglossal nerve.]
[Illustration: FIG. 3.--Skull of an old Fowl, upper view. (After W.K.
Parker.)
eo, Lateral occipital. npx, Nasal process of premaxillary.
eth, Ethmoid. p, Parietal.
f, Frontal. pf, Post-frontal.
j, Jugal. px, Premaxilla.
l, Lacrymal. qj, Quadratojugal.
n, Nostril. so, Supra-occipital.
np, Upper process of nasal. sq, Squamosal.]
Prefrontal bones are absent; post-frontals are possibly indicated by a
frequently occurring separate centre of ossification in the
post-orbital process, to which the frontals always contribute. The
lacrymal is always present, and perforated by a glandular duct.
Attached to it or the neighbouring frontal is often a supraorbital;
infraorbitals occur also, attached to the jugal or downward process of
the lacrymal. The nasals were used by A.H. Garrod to distinguish the
birds as holorhinal (fig. 2) where the anterior margin of the nasal is
concave, and schizorhinal where this posterior border of the outer
nares is continued backwards into a slit which extends beyond the
frontal processes of the premaxilla. Many birds possess a more or less
well developed cross-joint in front of the frontals and lacrymals,
perhaps best developed in _Anseres_ and _Psittaci_. Owing to this
joint the whole upper beak can be moved up and down with extra
facility, according to the shoving forwards or backwards of the
palato-pterygo-quadrate apparatus which moves sledge-like upon the
cranial basis. The premaxilla is always unpaired, but each half has
three long processes directed backwards; one fuses with the maxillary
bone, another helps to form the anterior part of the palate, while the
third, together with its fellow, forms the "culmen" and extends
backwards to the frontals, or rather to the ethmoid which there crops
up on the surface. The maxillaries (fig. 3) have besides others, a
maxillo-palatine process directed inwards in a transverse horizontal
direction. The palatines are long, always fused anteriorly with the
premaxilla, and frequently with the maxillo-palatine processes;
posteriorly they slide upon the presphenoidal rostrum, and articulate
in most birds with the pterygoids; they form the greater part of the
palatal roof and border the choanae or inner nares. Between these,
resting vertically upon the rostrum, appears the vomer; very variable
in shape and size, often reduced to a mere trace, as in the Galli, or
even absent, broken up into a pair of tiny splints in Pici.
The taxonomic importance of the configurations of the palate was first
pointed out by J. de Cornay. T.H. Huxley, in 1868, divided the
carinate birds into Dromaeo-, Schizo-, Desmo-, and Aegithognathae, an
arrangement which for many years had a considerable influence upon
classification. However, subsequent additions and corrections have
detracted much from its value, especially when it became understood
that the above sub-orders are by no means natural groups.
_Dromaeognathae_ have a struthious palate, with a broad vomer meeting
in front the broad maxillo-palatal plates, while behind it reaches the
pterygoids. The only representatives are the Tinamous.
_Schizognathae_, e.g. fowls (fig. 4), pigeons, gulls, plovers, rails
and penguins, have the vomer pointed in front while the
maxillo-palatines are free, leaving a fissure between the vomer and
themselves. The schizognathous formation is doubtless the most
primitive, and its representatives form a tolerably natural assembly.
_Desmognathae_ (fig. 5) were supposed to have the maxillo-palatines
united across the middle line, either directly or by the
intermediation of ossifications in the nasal septum. This is a
hopeless assembly. Parker and Fuerbringer have demonstrated that
desmognathism has been produced in half a dozen ways, implying
numerous cases of convergence without any nearer relationship than
that they are all derived from some schizognathous group or other. The
_Aegithognathae_, meant to comprise the passeres, woodpeckers and
swifts, &c., are really schizognathous but with a vomer which is
broadly truncated in front.
[Illustration: FIG. 4.--Skull of adult Fowl. This skull is unusually
_schizognathous_, the vomer (v.) being very small, and the
maxillo-palatine process (mxp) much aborted.
bo, Basi-occipital. pg, Pterygoid.
bt, Basi-temporal. prp, Pterygoid process of sphenoid.
eo, Lateral occipital. px, Premaxilla.
eu, Eustachian tube. q, Quadrate.
ic, Internal carotid. qj, Quadratojugal.
j, Jugal. rbs, Rostrum of basisphenoid.
l, Lacrymal. so, Supra-occipital.
mx, Maxilla. v, Vomer.
mxp, Maxillo-palatine process. 8, Exit of vagus nerve.
oc, Occipital condyle. 9, Exit of hypoglossal nerve.
pa, Palatine.
pf, Post-frontal. (After W.K. Parker)]
[Illustration: FIG. 5.--Skull of a nestling Sparrow-hawk (_Accipiter
nisus_), palatal view. The circular space on each side of the
basi-temporal (bt.) is the opening of the anterior tympanic recess.
The basi-pterygoids (bpg) are mere knobs, and the common eustachian
opening is seen between them. The maxillo-palatine plates (mxp) are
dotted to show their spongy character.
bt, Basi-temporal. pg, Pterygoid.
bpg, Basi-pterygoid. px, Premaxilla.
eo, Lateral occipital. pto, Prootic.
f, Frontal. q, Quadrate.
fm, Foramen magnum. qj, Quadratojugal.
j, Jugal. sn, Nasal septum
l, Lacrymal. so, Supra-occipital
mpg, Mesopterygoid process ty, Tymapanic cavity.
of W.K. Parker. v, Vomer
mx, Maxillary. 8, Exit of vagus nerve.
mxp, Maxillo-palatine process. 9. Exit of hypoglossal nerve.
op, Opisthotic.
pa, Palatine. (After W.K. Parker.)]
The remainder of the appendicular skeleton (fig. 6) of the head
requires little description. The maxillaries are connected with the
distal anterior corner of the quadrate by the thin, splint-like jugal
and quadratojugal. The quadrate is invariably a conspicuous bone and
movably articulating with the cranium and by a special process with
the pterygoid. The mandible is composed of several bones as in
reptiles. The os articulare bears on its inner side the inner
mandibular process which serves for the insertion of part of the
digastric muscle or opener of the mouth; another portion of this
muscle is attached to the os angulare, which frequently forms a
posterior mandibular process. The greater part of the under-jaw is
formed by the right and left dentaries, which in all recent birds are
fused together in front. Supra-angular and coronoid splint-bones serve
for the insertion of part of the temporal or masseter muscle.
Additional splints rest on the inner side of the jaw. Like the
crocodiles, birds possess a _siphonium_, i.e. a membranous, or
ossified, tube which rises from a pneumatic foramen in the os
articulare, on the median side of the articulation, and passes upwards
between the quadrate and lateral occipital bone, opening into the
cavity of the middle ear.
[Illustration: FIG. 6.--Skull of adult Fowl. Here the temporal fossa
is bridged over by the junction of the post-frontal and squamosal
processes (pf., sq.). The processes of the mandible (iap, pap) are
characteristic of this type, and of the anseres.
a, Angular of mandible. pa, Palatine.
ar, Articular. pap, Posterior angular process of
bt, Basi-temporal. mandible.
d, Dentary. pe, Ethmoid.
eo, Lateral occipital. pf, Post-frontal.
eth, Ethmoid. pg, Pterygoid.
f, Frontal. ps, Pre-sphenoid.
iap, Interangular process px, Premaxilla.
of mandible q, Quadrate.
ios, Interorbital septum. qj, Quadratojugal.
j, Jugal. sa, Supra-angular or coronoid.
l, Lacrymal. so, Supra-occipital.
mx, Maxillar. sq, Squamosal.
n, Nasal. ty, Tympanic cavity.
os, Orbito-sphenoid. v, Vomer.
p, Parietal. 1, Exit of olfactory nerve.]
The _Hyoid apparatus_ is, in its detail, subject to many variations in
accord with the very diverse uses to which the tongue of birds is
put. It consists of (1) the basihyal variously called copula, or
corpus linguae, or unpaired middle portion. (2) The urohyal likewise
unpaired, rested ventrally on the larynx. (3) The os entoglossum
originally paired, but coalescing into an arrow-headed piece, attached
to the anterior end of the basihyal and lodged in the tongue proper.
It is homologous with the distal ends of the ceratohyals or ventral
elements of the hyoidean or second visceral arch. The dorsal or
hyomandibular portion of this same arch is transformed into the
auditory chain, ending in the fenestra ovalis. (4) A pair of
thyrohyals, homologous with the posterior hyoid horns of mammals, i.e.
third visceral or first branchial arch. As the most developed pair in
birds they are commonly, although wrongly, called the hyoid horns.
They articulate upon facets of the hinder outer corners of the
basihyal.
[Illustration: FIG. 7.--Oshyoides of adult Fowl.
c.h, Ceratohyals (confluent).
b.h, The so-called basihyal, answering to the first basibranchial of
a fish.
b.br, Basibranchial, or urohyal, answering to the rest of the
basibranchial series.
c.br, e.br, together form the thyrohyal, answering to the first
cerato- and epibranchials.]
The vertebrae are stereospondylous, the centrum or body and the arch
being completely fused into one mass, leaving not even a neuro-central
suture. The arch alone sends out processes, viz. the spinous process,
the anterior and posterior oblique (commonly called pre- and
post-zygapophyses), and the transverse processes. The latter
articulate with the tuberculum of the corresponding rib, while the
capitulum articulates by a knob on the side of the anterior end of the
centrum. In the cervical region the ribs are much reduced, fused with
their vertebrae and enclosing the transverse canal or foramen. When
the vertebrae are free their centra articulate with each other by
complicated joints, exhibiting four types. (1) Amphicoelous; each end
of the centrum is concave; this, the lowest condition, is embryonic,
but was retained in _Archaeopteryx_ and in the thoracic vertebrae of
_Ichthyornis_. (2) Procoelous, concave in front; only in the atlas,
for the reception of the occipital condyle. (3) Opisthocoelous, or
concave behind, only occasionally found in the thoracic region, e.g.
Sphenisci. (4) Heterocoelous (fig. 8) or saddle-shaped; the anterior
surface is concave in a transverse, but convex in a vertical
direction, which on posterior surface shows the conditions reversed.
This is the most perfect arrangement attained by the vertebral column,
and is typical of, and restricted to, birds. The intervertebral joints
are further complicated by the interposition of a cartilaginous or
fibrous pad or ring. This pad varies much; it is morphologically the
homologue of the pair of basiventral elements which by their lateral
extension give origin to the corresponding ribs. Later those pads fuse
with the anterior end of the centrum of the vertebra to which they
belong; where the vertebral column is rendered inflexible, the disks
are ossified with the centra and all trace of them is lost. Sometimes
the pad is reduced to a ventral semi-ring or meniscus; it retains its
largest almost original shape and size in the second vertebra, the
axis or epistropheus, where it forms a separately ossifying piece
which connects, and cooessifies with, the odontoid process (the centrum
of the atlas) and the centrum of the second vertebra. Sometimes the
ventral portions of these pads form paired or unpaired little
ossifications, then generally described as intercentra; such are not
uncommon on the tail. The atlas is composed of three pieces; a pair of
lateral elements (the right and left dorsal arch pieces) joining above
the spinal cord, and a ventral piece equivalent to the first
basiventral elements, i.e. serially homologous with the intervertebral
pads. In the adults the atlas forms a more or less solid ring. A
remnant of the _chorda dorsalis_ and its sheath persists as the
_ligamentum suspensorium_ between the central portions of the
successive vertebrae.
[Illustration: FIG. 8.--A cervical vertebra from the middle of the
neck of a Fowl; natural size. a, Side view; b, upper view; c, lower
view; pr.z, pre-zygapophyses; pt.z, post-zygapophyses.]
In birds we distinguish between the following regions of the _axial
skeleton_. (1) Cervical vertebrae, or those between the skull and the
first vertebra which is connected with the sternum by a pair of
complete ribs. The last 1 to 5 of these vertebrae have movable ribs
which do not reach the sternum, and are called cervico-dorsals. (2)
Dorsals, those which begin with the first thoracic rib, and end at the
last that is not fused with the ilium. The term "lumbar" vertebrae is
inapplicable to birds. (3) Pelvic, all those which are fused with the
iliac portion of the pelvis, generally a considerable number. (4)
Caudal, those which are not connected with the pelvis. It is to be
noted that often no absolute line of demarcation can be drawn in
regard to these regions, their definitions being rather convenient
than morphological.
[Illustration: FIG. 9.--The "sacrum" of a young Fowl; natural size,
seen from below. d.l, Dorso-lumbar, s, sacral, c, caudal vertebrae.]
In comparison with all other vertebrates the number of neck-vertebrae
of the birds is considerably increased; the lowest number, 14 to 15,
is that of most Passeres and many other Coraciomorphae; the largest
numbers, 20 or 21, are found in the ostrich, 23 in _Cygnus olor_ and
25 in the black swan. Dorsal vertebrae frequently have a ventral
outgrowth of the centrum; these hypapophyses may be simple vertical
blades, _|_-shaped, or paired knobs; they serve for the attachment of
the thoracic origin of the longus collianticus muscle, reaching their
greatest development in Sphenisci and Colymbidae. In many birds some
of the thoracic vertebrae are more or less cooessified, in most pigeons
for instance the 15th to 17th; in most Galli the last cervical and the
next three or four thoracics are coalesced, &c. The pelvic vertebrae
include of course the sacrum. There are only two or three vertebrae
which are equivalent to those of the reptiles; these true sacrals are
situated in a level just behind the acetabulum; as a rule between
these two primary sacral vertebrae issues the last of the spinal
nerves which contributes to the composition of the sciadic plexus.
These true sacrals alone are connected with the ilium by processes
which are really equivalent to modified ribs; but the pelvis of birds
extends considerably farther forwards and backwards, gradually coming
into contact with other vertebrae, which in various ways send out
connecting transverse processes or buttresses, and thus become pre-
and post-sacral vertebrae (fig. 9). The most anterior part of the
ilium often overlaps one or more short lumbar ribs and fuses with
them, or even a long, complete thoracic rib. Similarly during the
growth of the bird the posterior end of the ilium connects itself with
the transverse processes of vertebrae which were originally free, thus
transforming them from caudals into secondary post-sacrals.
Individual, specific and generic variations are frequent.
[Illustration: FIG. 10.--A side view of the Chick's sternum.]
The last six or seven caudal vertebrae coalesce into the pygostyle, an
upright blade which carries the rectrices. Such a pygostyle is absent
in _Archaeopteryx, Hesperornis, Tinami_ and _Ratitae_, but it occurs
individually in old specimens of the ostrich and the kiwi. In
_Ichthyornis_ it is very small. In all the _Neornithes_ the total
number of caudal vertebrae, inclusive of those which coalesce, is
reduced to at least 13.
_Sternum_ (figs. 10 and 11).--Characteristic features of the sternum
are the following. There is a well-marked _processus lateralis
anterior_ (the right and left together equivalent to the mammalian
manubrium), which is the product of two or three ribs, the dorsal
parts of which reduced ribs remain as cervico-dorsal ribs. Then
follows the rib-bearing portion and then the _processus lateralis
posterior_; this also is the product of ribs, consequently the right
and left processes together are equivalent to the xiphoid process or
xiphisternum of the mammals. The lateral process in most birds sends
out an outgrowth, directed out and upwards, overlapping some of the
ribs, the _processus obliquus_. The median and posterior extension of
the body of the sternum is a direct outgrowth of the latter, therefore
called meta-sternum. The anterior margin of the sternum, between the
right and left anterior lateral processes receives in sockets the feet
of the coracoids. Between them arises a median crest, which varies
much in extent and composition, and is of considerable taxonomic
value. It is represented either by a _spina interna_ or by a _spina
externa_, or by both, or they join to form a _spina communis_ which is
often very large and sometimes ends in a bifurcation. Eventually, when
the right and left feet of the coracoids overlap each other, the
anterior sternal spine contains a foramen. The keel, or _carina
sterni_, is formed as a direct cartilaginous outgrowth of the body of
the sternum, ossifying from a special centre. This keel is much
reduced in the New Zealand parrot, _Stringops_, less in various
flightless rails, in the dodo and solitaire. It is absent in the
Ratitae, which from this feature have received their name, but
considerable traces of a cartilaginous keel occur in the embryo of the
ostrich, showing undeniably that the absence of a keel in the recent
bird is not a primitive, fundamental feature. The keel has been lost,
and is being lost, at various epochs and by various groups of birds.
The swimming _Hesperornis_ (see ODONTORNITHES) was also devoid of such
a structure. In many birds the spaces between the meta-sternum and the
posterior processes and again the spaces between this and the oblique
process are filled up by proceeding ossification and either remain as
notches, or as fenestrae, or they are completely abolished so that the
breastbone is turned into one solid more or less oblong plate.
[Illustration: FIG. 11.--Sternum of a Chick (_Gallus domesticus_)
three days old, lower view. The cartilage is shaded and dotted, and
the bony centers are light and striated.]
_Shoulder Girdle._--Scapula, coracoid and clavicle, meet to form the
foramen triosseum, through which passes the tendon of the
_supracoracoideus_, or _subclavius_ muscle to the _tuberculum
superius_ of the humerus. The coracoid is one of the most
characteristic bones of the bird's skeleton. Its upper end forms the
acrocoracoid process, against the inner surface of which leans the
proximal portion of the clavicle. From the inner side of the neck of
the coracoid arises the precoracoidal process, the remnant of the
precoracoid. Only in the ostrich this element is almost typically
complete, although soon fused at either end with the coracoid. Near
the base of the precoracoidal process is a small foramen for the
passage of the _nervus supracoracoideus_. In most birds the feet of
the coracoids do not touch each other; in some groups they meet, in
others one overlaps the other, the right lying ventrally upon the
left. The scapula is sabre-shaped, and extends backwards over the
ribs, lying almost parallel to the vertebral column. This is a
peculiar character of all birds. The clavicles, when united, as usual,
form the furcula; mostly the distal median portion is drawn out into a
hypocleidium of various shape. Often it reaches the keel of the
sternum, with subsequent syndosmosis or even synostosis, e.g. in the
gannet. In birds of various groups the clavicles are more or less
degenerated, the reduction beginning at the distal end. This condition
occurs in the Ratitae as well as in the well-flying _Platyrcecinae_
amongst parrots.
[Illustration: FIG. 12.--Bones of Fowl's right wing, adult, nat. size.
h, Humerus.
r, Radius.
u, Ulnar.
r', u', Radial and ulnar carpal bones; with the three digits I.,
II., III.]
The _fore-limb_ or _wing_ (fig. 12); highly specialized for flight,
which, initiated and made possible mainly by the strong development of
quill-feathers, has turned the wing into a unique organ. The humerus
with its crests, ridges and processes, presents so many modifications
characteristic of the various groups of birds, that its configuration
alone is not only of considerable taxonomic value but that almost any
genus, excepting, of course, those of Passeres, can be "spotted" by a
close examination and comparison of this bone. When the wing is folded
the long glenoid surface of the head of the humerus is bordered above
by the _tuberculum externum_ or _superius_, in the middle and below by
the _tuberculum medium_ or _inferius_ for the insertion of the
_coraco-brachialis posterior_ muscle. From the outer tuberculum
extends the large _crista superior_ (insertion of _pectoralis major_
and of _deltoideus major_ muscles). The ventral portion of the neck is
formed by the strong _crista inferior_, on the median side of which is
the deep _fossa subtrochanterica_ by which air sacs enter the humerus.
On the outer side of the humerus between the head and the _crista
inferior_ is a groove lodging one of the coraco-humeral ligaments. The
distal end of the humerus ends in a trochlea, with a larger knob for
the ulna and a smaller oval knob for the radius. Above this knob is
often present an ectepicondylar process whence arise the tendons of
the ulnar and radial flexors. The radius is the straighter and more
slender of the two forearm bones. Its proximal end forms a shallow cup
for articulation with the outer condyle of the humerus; the distal end
bears a knob which fits into the radial carpal. The ulna is curved and
rather stout; it articulates with both carpal bones; the cubital
quills often cause rugosities on its dorsal surface. Of wrist-bones
only two remain in the adult bird; the original distal carpals
coalesce with the proximal end of the metacarpals. These are reduced,
in all birds, to three, but traces of the fourth have been observed in
embryos. The first metacarpal is short and fuses throughout its length
with the second. This and the third are much longer and fuse together
at their upper and distal ends, leaving as a rule a space between the
shafts. The pollex and the third finger are as a rule reduced to one
phalanx each, while the index still has two. The first and second
fingers frequently carry a little claw. The greatest reduction of the
hand-skeleton is met with in _Dromaeus_ and in _Apteryx_, which retain
only the index finger. It is of importance for our understanding of
the position of the Ratitae in the system, that the wing-skeleton of
the ostrich and rhea is an exact repetition of that of typical flying
birds; the bones are much more slender, and the muscles are
considerably reduced in strength also to a lesser extent in numbers,
but the total length of the wing of an ostrich or a rhea is actually
and comparatively enormous. Starting with the kiwi and cassowary,
people have got into the habit of confounding flightless with wingless
conditions. It is absolutely certain that the wings of the Ratitae
bear the strongest testimony that they are the descendants of typical
flying birds.
The _pelvis_ (fig. 13), consisting of the sacrum (already described)
and the pelvic arch, namely ilium, ischium and pubis, it follows that
only birds and mammals possess a pelvis proper, whilst such is
entirely absent in the Amphibia and in reptiles with the exception of
some of the Dinosaurs. The ventral inner margin of the preacetabular
portion of the ilium is attached to the pre-sacral vertebrae, whilst
the inner and dorsal margin of the postacetabular portion is attached
to the primary sacral and the postsacral vertebrae. In rare cases the
right and left preacetabular blades fuse with each other above the
spinous processes. In front of the acetabulum a thick process of the
ilium descends to meet the pubis, and a similar process behind meets
the ischium. The acetabulum is completely surrounded by these three
bones, but its cup always retains an open foramen; from its posterior
rim arises the strong antitrochanter. The ischium and postacetabular
ilium originally enclose the ischiadic notch or _incisura ischiadica_.
This primitive condition occurs only in the Odontornithes (q.v.),
Ratitae and Tinami; in all others this notch becomes converted into a
_foramen ischiadicum_, through which pass the big stems of the
ischiadic nerves and most of the blood-vessels of the hind-limb. The
pubis consists of a short anterior portion (_spina pubica_ or
pectineal process, homologous with the prepubic process of Dinosaurs)
and the long and slender pubis proper (equivalent to the _processus
lateralis pubis_ of most reptiles). The shaft of the pubis runs
parallel with that of the ischium, with which it is connected by a
short ligamentous or bony bridge; this cuts off from the long
_incisura pubo-ischiadica_ a proximal portion, the _foramen
obturatum_, for the passage of the obturator nerve. Only in the
ostrich the distal ends of the pubes meet, forming a dagger-shaped
symphysis, which is curved forwards. The pectineal process is
variable; it may grow entirely from the pubis, or both pubis and ilium
partake of its formation, or lastly its pubic portion may be lost and
the process is entirely formed by the ilium. It is largest in the
Galli and some of the Cuculi, in others it is hardly indicated. It
served originally for the origin of the ambiens muscle (see _Muscular
System_ below); shifting or disappearance of this muscle, of course,
influences the process.
[Illustration: FIG. 13.--Pelvis and caudal vertebrae of adult Fowl,
side view, natural size. Il. Ilium; Is, ischium; Pb, pubis; d.l,
dorso-lumbar vertebrae; Cd, caudal vertebrae; Am, acetabulum.]
_The Hind Limb._--The femur often possesses a well visible pneumatic
foramen on the median side of the proximal end of its shaft. The inner
condyle, the intercondylar sulcus, and a portion only of its outer
condyle, articulate with corresponding facets of the tibia. The outer
condyle articulates mainly with the fibula. There is a patella,
intercalated in the tendon of the _femori-tibialis_ or _extensor
cruris_ muscle. In _Colymbus_ the patella is reduced to a small
ossicle, its function being taken by the greatly developed pyramidal
_processus tibialis anterior_; in _Podiceps_ and _Hesperornis_ the
patella itself is large and pyramidal. The distal half of the fibula
is very slender and normally does not reach the ankle-joint; it is
attached to the peroneal ridge of the tibia. On the anterior side of
the tibia, is the intercondylar sulcus, which is crossed by an oblique
bridge of tendon or bone, acting as a pulley for the tendon of the
_extensor digitorum communis_ muscle. The condyles of the tibia are in
reality not parts of this bone, but are the three proximal tarsalia
which fuse together and with the distal end of the tibia. The distal
tarsalia likewise fuse together, and then on to the upper ends of the
metatarsals; the _tarsale centrale_ remains sometimes as a separate
osseous nodule, buried in the inter-articular pad. Consequently the
ankle-joint of birds is absolutely cruro-tarsal and tarso-metatarsal,
i.e. intertarsal, an arrangement absolutely diagnostic of birds if it
did not also occur in some of the Dinosaurs. Of the metatarsals the
fifth occurs as an embryonic vestige near the joint; the first is
reduced to its distal portion, and is, with the hallux, shoved on to
the inner and posterior side of the foot, at least in the majority of
birds. The three middle metatarsals become fused together into a
cannon bone; the upper part of the third middle metatarsal projects
behind and forms the so-called hypotarsus, which in various ways,
characteristic of the different groups of birds (with one or more
sulci, grooved or perforated), acts as guiding pulley to the tendons
of the flexor muscles of the toes. Normally the four toes have two,
three, four and five phalanges respectively, but in _Cypselus_ the
number is reduced to three in the front toes. Reduction of the number
of toes (the fifth shows no traces whatever, not even in
_Archaeopteryx_) begins with the hallux, which is completely or partly
absent in many birds; the second toe is absent in _Struthio_ only. The
short feet of the penguins are quite plantigrade, in adaptation to
which habit the metatarsals lie in one plane and are incompletely
co-ossified, thus presenting a pseudo-primitive condition.
LITERATURE.--Only a mere fraction of the enormous literature dealing
with the skeleton of birds can here be mentioned.
M.E. Alix _Essai sur l'appareil locomoteur des oiseaux_ (Paris, 1874);
E. Blanchard, "Recherches sur les caracteres osteologiques des oiseaux
appliquees a la classification," _Ann. Sci. Nat. Ser._ iv., t. xi.; W.
Dames, "Ueber Brustbein Schulter- und Beckenguertel der Archaeopteryx,"
_Math. Naturw. Mitsh._, Berlin, vii., 1897, pp. 476-492; T.C. Eyton,
_Osteologia avium_ (London, 1858-1881), with many plates; C.
Gegenbaur, _Untersuch. z. vergl. Anat. d. Wirbelthiere, I. Carpus und
Tarsus, II. Schultergurtel_ (Leipzig, 1864-1865); P. Harting,
_L'Appareil episternal des oiseaux_ (Utrecht, 1864); T.H. Huxley, "On
the Classification of Birds and on the Taxonomic Value of the
Modifications of certain of the Cranial Bones..." _P.Z.S._, 1867; G.
Jaeger, "Das Wirbelkorpergelenk der Voegel," _Sitzb. K. Ak. Wiss._,
Wien, xxxiii., 1858; A. Johnson, "On the Development of the Pelvic
Girdle and Skeleton of the Hind-limb in the Chick," _Q.J.M.S._,
xxiii., 1883, pp. 399-411; K.F. Kessler, "Osteologie der Vogelfuesse,"
_Bull. Soc. Imp. Nat._, Moscow, xiv., 1841; B. Lindsay, "On the Avian
Sternum," _P.Z.S._, 1885; E. Mehnert, "Entwickelung des Ospelvis der
Voegel," _Morph. Jahrb._, xiii., 1877; A.B. Meyer, _Abbildungen van
Voegel-Skeletten_ (Dresden, 1879); St G. Mivart, "On the Axial Skeleton
of the Ostrich, Struthionidae, Pelecanidae," _Trans. Zool. Soc._
viii., 1874; x., 1877; E.S. Morse, "On the Carpus and Tarsus of
Birds," _Ann. Lyc. N.H._, New York, x., 1874; J.S. Parker,
"Observations on the Anatomy and Development of Apteryx," _Phil.
Trans._, 1890, pp. 1-110, 17 pls.; W. K. Parker, numerous papers in
_Trans. L.S., R.S._ and _Z.S._, e.g. "Osteology of Gallinaceous
Birds," _T.Z.S._, v., 1863; "Rhinochetus," _ibid._ vi.; "Skull of
Aegithognathous Birds," _ibid._, x., 1878; "Skull in the Ostrich
Tribe," _Phil. Trans._ vol. 156, 1866; "Skull of Common Fowl," _ibid._
vol. 159, 1870; "Skull of Picidae," _T. Linn. Soc._, 1875; "Monograph
on the Structure and Development of the Shoulder-girdle and Sternum,"
_Ray Soc._ London, 1868; W.P. Pycraft, "On the Morphology and
Phylogeny of the Palaeognathae (_Ratitae_ and _Crypturi_) and
Neognathae," _Trans. Zool. Soc._ xv., 1900, pp. 149-290, pis. 42-45;
id. "Some points in the morphology of the Palate of the Neognathae,"
_T. Linn. Soc._ 28, pp. 343-357, pls. 31-32; P. Suschkin, "Zur
Morphologie des Vogelskelets. I. Schaedel von Tinnunculus," _Mem. Soc.,
Moscow_, xvi., 1900, pp. 1-63, pls.