THE SKELETON OF THE WILD DUCK (_Anas boschas_).
I. EXOSKELETON.
The exoskeleton of the Duck and indeed of all birds is entirely
epidermal in origin. Its most important part consists of =feathers=,
but it includes also the following horny structures:--
(_a_) =scales=, which cover the toes and tarso-metatarsus;
(_b_) =claws=, which are attached to the distal phalanges of the toes
and of the pollex;
(_c_) the wide =beak=, which sheaths both upper and lower jaws, and
whose edges are raised into lamellae, which act as strainers.
FEATHERS.
A well developed feather, such as one of the large quill feathers of
the wing or tail, consists of the following parts: A main stem, the
=scapus=, which forms the axis running along the whole length of the
feather, and is divided into (1) a proximal hollow cylindrical
portion, the =calamus= or =quill=, and (2) a distal solid portion, the
=rachis= or =shaft=, which is square in section, flexible and grooved
along its ventral surface, and bears a number of lateral processes,
the =barbs=. The =calamus= which is partly imbedded in a pit in the
dermis, bears two holes: one, the =inferior umbilicus=, is at its
proximal end, and into it enters a vascular outgrowth from the dermis;
the other, the =superior umbilicus=, lies on the ventral surface at
the junction of the calamus and scapus.
The =barbs= are a series of narrow elastic plates, attached by their
bases to the rachis, and with their edges looking upwards and
downwards. The barbs are connected together by a number of smaller
processes, the =barbules=, which interlock with one another by means
of hooklets, and bear the same relation to the barbs that the barbs do
to the rachis. The barbs and barbules, together with the rachis,
constitute the =vexillum= or =vane= of the feather. Any feather having
the above type of structure is called a =penna= or a =contour
feather=, from the fact that it helps to produce the contour of the
body.
VARIETIES OF FEATHERS.
1. =Pennae.= There are two kinds of pennae or contour feathers.
(_a_) The =quills=. These form the large feathers of the wing and
tail. They are divided into two groups, the =remiges=, or wing quills,
and the =rectrices=, or tail quills.
The =remiges=[101] include three sets of feathers, the =primaries= or
=metacarpo-digitals=, which are attached to the bones of the manus,
the =secondaries= or =cubitals=, which are attached to the ulna, and
the =humerals=, which are attached to the humerus.
The =primaries= differ from all the other quill feathers in having the
posterior half of the vane much wider than the anterior half. They are
ten in number, and of these six, the =metacarpal= quills (fig. 57,
14), are attached to the second and third metacarpals, one, the
=ad-digital= (fig. 57, 15), to the phalanx of the third digit, two,
the =mid-digitals= (fig. 57, 16), to the first phalanx of the second
digit, and two, the =pre-digitals= (fig. 51, 17), to the second
phalanx of the second digit. One of the pre-digitals is very small,
and is called the =remicle= (fig. 57, 11).
[Illustration FIG. 56. THE WING OF A WILD DUCK (_Anas boschas_).
The upper figure shows the dorsal side of a right wing, the lower
figure the ventral side of a left wing. × 1/3. (Brit. Mus.)
1. scapulars.
2. tectrices marginales.
3. tectrices minores.
4. bastard wing.
5. tectrices majores.
6. metacarpo-digitals or primaries.
7. tectrices mediae.
8. cubitals or secondaries.
9. pennae humerales.
10. pennae axillares.]
In addition, a group of three quill feathers is attached to the first
digit, constituting the =bastard wing= or =ala spuria= (fig. 56, 4).
The =secondaries= or =cubitals= (fig. 56, 8) form a group of seventeen
feathers, attached to the ulna; they are shorter than the primaries,
and do not have the posterior half of the vane much wider than the
anterior half.
The =humerals= (figs. 56, 9 and 57, 12) form a group of eight small
feathers, of varying length, attached to the anterior half of the
humerus.
[Illustration FIG. 57. WINGS OF A WILD DUCK WITH THE COVERTS REMOVED
(_Anas boschas_). × 1/3.
A. Right wing seen from the dorsal side. B. Left wing disarticulated
and seen from the ventral side. (Brit. Mus.)
1. humerus.
2. radius.
3. ulna.
4. radial carpal.
5. ulnar carpal.
6. first phalanx of first digit.
7. second metacarpal.
8. third metacarpal.
9. first phalanx of second digit.
10. second phalanx of second digit.
11. remicle.
12. pennae humerales.
13. cubitals or secondaries.
14. metacarpal quills.
15. ad-digital.
16. mid-digitals.
17. pre-digital.]
(b) The =tectrices= or =coverts= are short feathers, which cover over
the quills of the rectrices and remiges, and clothe the body
generally. Their barbules are less developed than is the case with the
quill feathers, so that the barbs separate readily from one another,
especially at the base of the vane. The nomenclature of the various
patches of coverts on the wings is seen in fig. 56. A small patch of
backwardly-directed feathers surrounding the external auditory opening
are known as the =auriculars=.
2. The =filoplumes= are rudimentary feathers, consisting of a minute
stem and slightly developed vane. They are left in the skin after the
other feathers have been removed.
3. The =plumulae=, or down feathers, have the stem very slightly
developed, while the barbs are soft and free from one another. They
are distributed all over the body, not only among the contour
feathers, but also over the spaces (_apteria_) which bear no contour
feathers.
In the young bird the rudiments of the new feathers are formed at the
bases of the embryonic down feathers, and as they grow they push them
out from the skin. The embryonic down feathers however remain attached
to the apices of the new feathers till these have reached a length of
about an inch; they are then shed.
II. ENDOSKELETON.
As compared with that of the Turtle or Crocodile, the endoskeleton of
the Duck is characterised by:
1. The great lightness of the bones, many of which contain air
cavities.
2. The tendency to become ankylosed together shown by many of the
bones.
3. The modification of the anterior limbs and girdle for the purpose
of flight.
1. THE AXIAL SKELETON.
This, as in other vertebrates, is divisible into--
A. The vertebral column.
B. The skull.
C. The ribs and sternum.
A. THE VERTEBRAL COLUMN.
The vertebral column of the duck, like that of the great majority of
birds, presents a number of well-marked characteristics, contrasting
strongly with those of the generality of higher vertebrates. The centra
are always without epiphyses. The neck is exceedingly long, about as
long as all the rest of the vertebral column put together, and is
remarkable for its flexibility. The trunk portion of the vertebral
column on the other hand is characterised by extreme rigidity, and
the marked tendency shown by the component vertebrae to fuse together
into one almost continuous mass. The most rigid part of the vertebral
column is that to which the pelvis is united, as no less than seventeen
vertebrae take part in the union. The tail of the duck, like that of
all living birds, is very short, and the posterior caudal vertebrae are
united together, forming the =pygostyle=. The vertebral column may be
divided into cervical, thoracic, lumbar, sacral, and caudal regions,
but the boundaries between the several regions are ill-defined.
THE CERVICAL VERTEBRAE.
All the vertebrae anterior to the first one that bears a rib meeting
the sternum are regarded as cervical vertebrae. There are therefore
sixteen cervical vertebrae, the last two of which bear well developed
ribs. All are freely movable on one another.
As a typical cervical vertebrae, any one from the fifth to the ninth
may be taken. The vertebra is rather elongated, and is very lightly
and strongly made, its most characteristic feature being the shape of
the articulating surfaces of the centra, which are generally described
as saddle-shaped. The anterior surface is convex from above downward,
and concave from side to side, while the posterior and more prominent
surface is concave from above downwards and convex from side to side.
The neural arch is low, and is drawn out into a slight blade-like
=neural spine=. Its base is deeply notched on both sides posteriorly
for the exit of the spinal nerves. Above these notches it is drawn out
into two rather prominent diverging processes, which bear the
=postzygapophyses=,--two flattened surfaces which look downwards and
outwards. The =transverse processes= form irregular outgrowths from
the anterior two-thirds of the sides of the vertebra; each projects
for a short distance downwards and outwards, and is terminated
posteriorly by a short backwardly-projecting spine. The transverse
processes are shown by development to ossify from separate centres,
and are therefore to be regarded as cervical ribs, and each is
perforated at its base by a canal for the passage of the vertebral
artery. Above the anterior end of the vertebrarterial canal are a pair
of thickened outgrowths, which bear upwardly and inwardly directed
=prezygapophyses=. Each transverse process is perforated near its
middle by a prominent foramen through which passes a vein which is
connected with the jugular vein.
The third and fourth cervical vertebrae resemble the succeeding ones
in most respects, but have small =hypapophyses=, and the neural spines
are less blade-like. The posterior cervical vertebrae (tenth to
sixteenth) differ somewhat from the middle ones. They are shorter and
more massive, the neural arch is much shorter, being deeply notched in
the middle line in front and behind. The transverse processes arise
from the anterior half of the vertebra only, and in the eleventh
vertebra each is drawn out below into a pair of rather prominent
downwardly and inwardly directed processes. In the twelfth vertebra
these processes have almost coalesced, and in the thirteenth vertebra
they have coalesced completely, forming a prominent =hypapophysis=. In
the succeeding vertebrae this hypapophysis rapidly decreases in size.
The fifteenth and sixteenth cervical vertebrae resemble the succeeding
thoracic vertebrae, having short thick centra and prominent squarely
truncated neural spines; the sides of the neural arches are very
deeply notched. The fifteenth vertebra has a short transverse process,
perforated by a wide vertebrarterial foramen, but this foramen is
absent in the sixteenth. The transverse processes of the fifteenth
vertebra bear two facets for the articulation of the capitulum and
tuberculum of the rib. The sixteenth vertebra has its tubercular facet
on the transverse process, but the capitular facet is borne on the
centrum.
The second or =axis= vertebra is small, and has the centrum drawn out
into a comparatively very large hypapophysis. The posterior
articulating surface of the centrum is saddle-shaped, the anterior
nearly flat: above it the centrum is prolonged into the prominent
=odontoid process=, which is shown by development to be the detached
centrum of the atlas. The neural arch is deeply notched in the middle
line in front, and at the sides behind. It is drawn out posteriorly
into a wide massive outgrowth, which overhangs the third vertebra and
bears the downwardly-directed postzygapophyses. The prezygapophyses
are situated at the sides of the anterior end of the neural arch, and
look directly outwards. The transverse processes are very slightly
developed, and are pierced by the vertebrarterial canals.
The =atlas= vertebra is a very slight ring-like structure, thickened
ventrally and bearing in front a prominent concave cavity for
articulation with the occipital condyle of the skull. Posteriorly it
bears a more or less flattened surface for articulation with the
centrum of the axis. It surrounds a large cavity partially divided
into a larger dorsal portion, which is the neural canal, and a smaller
ventral portion which lodges the odontoid process. The sides of the
atlas are pierced by the vertebrarterial canals, above which there are
two slight backwardly-projecting outgrowths bearing the
postzygapophyses on their inner faces.
THE THORACIC VERTEBRAE.
The thoracic region includes all the vertebrae bearing free ribs,
except the first two, viz. those whose ribs do not reach the sternum.
There are seven thoracic vertebrae. The first four have centra with
saddle-shaped articulating surfaces, but are more or less firmly
united together by their neural spines; the last two are completely
ankylosed by their centra to the lumbar vertebrae.
Each of the first five vertebrae has a prominent, vertical, abruptly
terminated neural spine, and straight transverse processes. The
zygapophyses and articulating surfaces at the ends of the centra are
well developed. The third, fourth, fifth, and sixth vertebrae have
very prominent hypapophyses. The articular facets for the ribs are
well marked, those for the tubercula lying at the free ends of the
transverse processes, and those for the capitula at the sides of the
anterior ends of the centra. The sixth and seventh thoracic vertebrae
are firmly fused by their centra and neural arches to one another and
to the lumbar vertebrae behind, and by their transverse processes to
the ilia. The sixth has its centrum terminated in front by a
saddle-shaped articulating surface, and bears a pair of prominent
prezygapophyses. Its transverse processes and centrum bear facets for
the tubercula and capitula of the ribs respectively. In the seventh
vertebra the tubercular facet is wanting.
THE SACRUM.
[Illustration FIG. 58. A, DORSAL AND B, VENTRAL VIEW OF THE PELVIS AND
SACRUM OF A DUCK (_Anas boschas_).
1. ilium.
2. ischium.
3. pubis.
4. pectineal process.
5. lumbar vertebrae.
6. true sacral vertebrae.]
The =sacrum= generally consists of seventeen vertebrae fused with one
another and with the ilia. Their number may be reckoned from the
number of foramina for the exit of spinal nerves. The two most
anterior of these vertebrae bear ribs and have been already described
with the other thoracic vertebrae. Their neural spines and those of
the four succeeding vertebrae are fused together, forming a continuous
crest of bone completely united laterally with the ilia. The
transverse processes of all these six vertebrae are well developed,
but those of the posterior two (fig. 58, B, 5) are much the stoutest.
The next three vertebrae have broad centra, but their transverse
processes are very slightly developed and have no ventral elements.
These seven vertebrae belong to the =lumbar= series. The remaining
eight vertebrae have well-developed transverse processes, which in the
case of the first three or four are divisible into dorsal and ventral
elements. All the dorsal elements are united to form a pair of
flattened plates, partially separated by a series of foramina from a
median plate formed by the united neural arches. Laterally they are
continuous with the ischia. The first two of this series of vertebrae
are shown by their relation to the nerves to be the true =sacrals=
(fig. 58, B, 6), the remaining six belonging to the =caudal= series.
Behind them come the six free caudal vertebrae, succeeded by a
terminal piece, the =pygostyle=, formed of a number of vertebrae fused
together; this bears the rectrices or tail quills.
[Illustration FIG. 59. SKULL OF A DUCK (_Anas boschas_). × 1.
A. Dorsal view of the cranium. B. Palatal view of the mandible. C. The
Hyoid.
For numbers see Fig. 60.]
B. THE SKULL.
[Illustration FIG. 60. A. Ventral view of the cranium of a Duck (_Anas
boschas_). B. Cranium and mandible seen from the left side. × 1.
1. maxillae.
2. premaxillae.
3. anterior nares.
4. nasal process of premaxillae (fig. 59).
5. nasal.
6. frontal (fig. 59).
7. lachrymal.
8. postfrontal process.
9. parietal (fig. 59).
10. jugal.
11. quadratojugal.
12. quadrate.
13. condyle of mandible.
14. posterior articular process.
15. dentary at symphysis. }
16. basi-hyal. } (fig.
17. uro-hyal. } 59).
18. basibranchial. }
19. vomer.
20. palatine.
21. pterygoid.
22. anterior palatine foramen.
23. basitemporal.
24. foramen leading into tympanic cavity.
25. bristle inserted into posterior opening of carotid canal.
26. bristle inserted into posterior opening of Eustachian canal.
27. bristle emerging through anterior opening of carotid canal.
Close by is seen the bristle emerging through the anterior
opening of the Eustachian canal.
28. fenestral recess.
29. maxillo-palatine.
30. lambdoidal crest.
31. rostrum.
I. II. IV. V. IX. X. nerve foramina.]
The skull of the duck, like that of birds in general, is characterised
(1) by its lightness, (2) by the contrast between the bones of the
cranium proper and those forming the rest of the skull, for the bones
forming the cranium proper are closely fused together, the sutures
between them being nearly all completely obliterated in the adult,
while the bones forming the face are loosely connected with the
cranium proper; (3) by the prolongation of the face into a long
toothless beak; (4) by the size of the orbits, and their position
entirely in front of the cranium, so that they are separated from one
another only by a thin interorbital septum.
For purposes of description the skull may be divided into
(1) The cranial portion.
(2) The facial portion.
(3) The mandible.
(4) The hyoid.
(1) THE CRANIAL PORTION.
This is a rounded box expanded dorsally and posteriorly, but tapering
antero-ventrally. In the young skull the divisional lines between the
several bones can be easily seen, but in the adult they are quite
obliterated.
(_a_) The _dorsal surface_ is rounded, expanded in front and behind,
but encroached upon in the middle by the cavities of the orbits. There
is a prominent divisional line in front, separating it from the facial
part of the skull. It is formed mainly by the _frontal_ (fig. 59, A,
6) and _parietal_ bones, but the frontals diverge a little anteriorly
and enclose between them the ends of the _nasal processes_ (fig. 59,
A, 4) of the _premaxillae_. Just in front of the orbit the outer
margins of the frontals are either notched or pierced by a pair of
foramina.
(_b_) At the _posterior end_ of the cranium the most prominent feature
is the large, almost circular =foramen magnum=, through which the
spinal cord and brain communicate; this in young birds is seen to be
bounded by four distinct bones, dorsally by the =supra-occipital=,
ventrally by the =basi-occipital=, and laterally by the
=exoccipitals=.
The =basi-occipital= forms the main part of a prominent convex knob,
the =occipital condyle=, with which the atlas articulates. The
occipital condyle is slightly notched above, and the ventral surface
of the cranium is deeply pitted just in front of it; the exoccipitals
also contribute slightly to its formation. Slightly in front of and
ventral to the foramen magnum is a small foramen through which the
hypoglossal nerve leaves the cranial cavity.
The =supra-occipital= is separated from the parietal by a suture line
along which run a pair of prominent ridges, the =lambdoidal crests=
(fig. 60, B, 30). There are often a pair of prominent vacuities in the
supra-occipital dorsal to the foramen magnum. The =epi-otics= and
=opisthotics= become completely fused with the bones of the occipital
segment at a very early stage.
(_c_) The _ventral surface_ of the cranium is wide behind, where it is
formed by a broad transverse membrane bone, the _basitemporal_ (fig.
60, A, 23), the sides of which are fused with the auditory capsules.
Slightly in front of and an eighth of an inch external to the
hypoglossal foramen the cranial wall is pierced by a pair of foramina
through which the tenth or pneumogastric nerves leave (fig. 60, A, X).
At the sides of the basitemporal are a pair of depressions, the
=tympanic recesses=, in each of which are three holes. Straight lines
joining these holes would form an isosceles triangle with its apex
directed forwards. Of the two holes at the base of the triangle, the
one nearer the middle line and leading into the cranial cavity, is for
the exit of the ninth or glossopharyngeal nerve (fig. 60, A, IX), it
lies just in front of the pneumogastric foramen. The more external
leads into the tympanic cavity, while the more anterior at the apex of
the triangle is the =posterior opening of the carotid canal= (fig. 60,
A, 25), which traverses the base of the cranium, and during life
lodges the carotid artery.
The anterior end of the basitemporal is pierced near the middle line
by a pair of holes, the =anterior openings of the Eustachian canals=;
while just in front of these and a little further removed from the
middle line are the anterior openings of the =carotid canals=.
Bristles passed in through the posterior openings of the carotid
canals will emerge here (fig. 60, A, 27). In front of the basitemporal
the base of the cranium is formed by the =rostrum= (fig. 60, A, 31),
or thickened basal portion of the interorbital septum; this bears two
prominent surfaces with which the pterygoids articulate. In some kinds
of duck these surfaces are borne by well-marked basi-pterygoid
processes.
(_d_) _The side of the cranium._ At the base of the posterior end is
seen the deep =tympanic cavity=. The dorsal part of this is divided by
a vertical partition into two halves; of these the more anterior is
the larger, and forms a deep funnel-shaped cavity, the =posterior
opening of the Eustachian canal= (fig. 60, B, 26). A bristle passed
into this opening emerges through the anterior opening of the
Eustachian canal. The more posterior of the two is the =fenestral
recess= (fig. 60, B, 28), and is in its turn divided by a slender
horizontal bar into a dorsal hole, the =fenestra ovalis=, and a
ventral hole, the =fenestra rotunda=. During life the fenestra ovalis
lodges the proximal end of the =columellar= chain. Lying at the outer
side and slightly dorsal to the tympanic cavity is a deep depression,
the =lateral tympanic recess=, and immediately in front of this is the
articular surface for the quadrate. The tympanic cavity is bounded
below by the basitemporal, posteriorly by the exoccipital, and above
by the _squamosal_, a membrane bone, which roofs over a good deal of
the side of the cranium, and bears ventrally a prominent surface with
which the quadrate articulates. Just in front of this is a large round
hole, the =trigeminal foramen= (fig. 60, B, V), behind which the
squamosal is drawn out into a short process.
In front of the squamosal there is a prominent forwardly-projecting
=postfrontal process= (fig. 60, 8), which ossifies from a different
centre from that forming the squamosal, but in the adult is completely
fused with it.
The =orbit= forms a large more or less hemispherical cavity which
lodges the eyeball. It is separated from its fellow of the opposite
side by an imperfect partition, the =interorbital septum=. In the
young skull it is seen to be bounded above by the frontal, with which
the _lachrymal_ (fig. 60, 7) is fused anteriorly, forming a large
backwardly-projecting process; while behind it is bounded by
the =alisphenoid=. The interorbital septum is formed by the
ossification and coalescence of the =mesethmoid= in front, with the
=orbitosphenoid= behind, and the =rostrum= below. The boundary of the
orbit below is very imperfect, the zygomatic arch being incomplete.
The interorbital septum is pierced by the very prominent =optic
foramen= (fig. 60, B, 2), just behind which are the two much smaller
foramina for the exit of the oculomotor and pathetic (fig. 60, B, IV)
nerves, the more anterior being that for the oculomotor.
Above and slightly in front of the optic foramen is a median opening,
the =olfactory foramen.= This leads into the cranial cavity behind,
and in front is continued forwards as a groove between the
interorbital septum and the frontal.
(2) THE FACIAL PART OF THE SKULL.
This includes the olfactory capsule and associated bones, and the
upper jaw.
The bones associated with the olfactory capsules are the _nasals_ and
_vomer_. The _nasals_ (figs. 59 and 60, 5) lie on the dorsal surface
immediately in front of the cranium, and are separated from one
another by the nasal processes of the premaxillae. Each is completely
fused in the adult with the corresponding maxillae and premaxillae, the
three bones together forming the boundary of the =anterior nares.= The
_vomer_ (fig. 60, 19) is unpaired and forms a small median vertical
plate lying ventral to the anterior continuation of the interorbital
septum.
The bones of the upper jaw consist on each side of two slender arcades
which in front converge and are attached to the large beak, while
behind they diverge but are united by the =quadrate=.
The =inner arcade= is formed by the pterygoid and palatine. The
_pterygoid_ (fig. 60, 21) is a short flattened bone, which articulates
behind with the quadrate, and on its inner side with a large flattened
surface borne by the rostrum, in front it meets the palatine, or
sometimes ends freely with a long antero-dorsally directed point.
The _palatine_ (fig. 60, 20) is a slender irregular bone flattened
dorso-ventrally at its anterior end where it articulates with the
beak, and laterally behind. It gives off at its posterior end a
process, which is sometimes united with the vomer, sometimes projects
forwards, and meets its fellow dorsal to the vomer. In the large space
between it and the vomer is the opening of the =posterior nares=.
The _premaxillae_ (figs. 59 and 60, 2) are very large, and form nearly
a third of the big shovel-shaped beak. They constitute the inner, and
part of the front boundary of the anterior nares, and send back a pair
of _nasal processes_ which partially separate the nasals from one
another.
The =outer arcade= forms the slender =suborbital bar=, and consists
mainly of two rod-like bones, which in the adult are completely fused
together. The posterior of these is the _quadratojugal_ (figs. 59 and
60, 11) which articulates with the quadrate, the anterior is the small
and slender _jugal_ or _malar_ (figs. 59 and 60, 10). The extreme
anterior part of the bar is formed by the _maxillae_. The main part of
the maxillae however lies anterior to the suborbital bar, and extends
forwards along the side of the premaxillae forming all the lateral part
of the beak (figs. 59 and 60, 1); it also sends inwards a plate, the
=maxillo-palatine= (fig. 60, A, 29), which completely fuses with its
fellow in the middle line, and forms the posterior boundary of the
anterior palatine foramen. The term =desmognathous= describes the
condition of the skull in which the maxillo-palatines fuse with one
another in the middle line in this way.
The =quadrate= (fig. 60, 12), which unites the two arcades behind, is
a stout irregular four-cornered bone forming the =suspensorium=. It
articulates by its dorso-posterior corner with the squamosal, and by
its antero-internal corner with the pterygoid. The middle of its
ventral surface forms a hemispherical knob with which the mandible
articulates, while its dorso-anterior border is drawn out into a long
point which extends towards the interorbital septum.
(3) THE MANDIBLE.
The =mandible= or lower jaw consists of two =rami= which are flattened
and fused together in the middle line in front, while behind they
diverge from one another and articulate with the quadrates.
Each ramus is composed of five bones fused together, one being a
cartilage bone, and the other four membrane bones. The =articular= is
the only cartilage bone of the mandible, it bears the double condyle
(figs. 59 and 60, 13) or concave articular surface for the quadrate,
and is drawn out behind into a large hooked =posterior articular
process=. The articular is also drawn out into a prominent process on
each side of the articular surface for the quadrate, and is marked by
a deep pit opening posteriorly. The articular is continuous in front
with =Meckel's cartilage= which forms the original cartilaginous bar
of the lower jaw, and is ensheathed by the membrane bones. Of these
the _supra-angular_ forms the upper part of the mandible in front of
the articular, its dorsal surface is drawn out into a small =coronoid
process=, its outer surface also bearing a prominent process. The
_angular_ is a small bone which underlies the articular and
supra-angular on the inner side of the jaw. The _dentary_ (fig. 59,
15) forms the anterior half of each ramus, and is the largest bone of
the mandible; it is fused with its fellow at the symphysis in front,
and extends back below the supra-angular. The _splenial_ is a small
bone lying along the middle half of the inner side of each ramus of
the mandible.
(4) THE HYOID.
With the hyoid apparatus is included the =columella=. This forms a
minute rod of bone, one end of which is expanded and fits into the
fenestra ovalis, while the other end, terminated by a triradiate piece
of cartilage, is attached to the tympanic membrane. The structure is
as a whole homologous with the auditory ossicles of mammals and the
hyomandibular of fish.
The =hyoid= consists of a median unpaired portion, formed of two
pieces of bone, the =basi-hyal= (fig. 59, C, 16) in front, and the
=uro-hyal= (fig. 59, C, 17) behind, the two being placed end to end
and terminated anteriorly by an unpaired cartilaginous plate, the =os
entoglossum.= At the posterior end there come off a pair of long
=posterior cornua=, each of which consists of two pieces, a longer
=basibranchial= (fig. 59, C, 18), and a shorter =cerato-branchial=.
For the homology of these parts see p. 336.
THE RIBS AND STERNUM.
The last two cervical vertebrae bear long movable ribs which
articulate by distinct capitular and tubercular processes, but do not
meet the sternum. The thoracic ribs are eight in number, and each is
divisible into a =vertebral= and a =sternal= portion. The first five
thoracic ribs are flattened curved bars of bone, which articulate by a
prominent =capitulum= with the centrum of the corresponding vertebra,
and by a =tuberculum= with the transverse process. Projecting
backwards from each is a large hooked =uncinate process.= The last
three ribs which are without uncinate processes, become progressively
more slender, and in the eighth the tubercular processes are lost.
The sternal portions of the ribs are imperfectly ossified pieces,
short and comparatively thick in the case of the anterior ribs, longer
and more slender in the case of the posterior ribs.
THE STERNUM[1].
The =sternum= or breast bone is exceedingly large in the Duck, as in
all birds, and projects back far beyond the thorax over much of the
anterior part of the abdomen. It is an irregularly oblong plate of
bone, abruptly truncated behind, somewhat concave dorsally, and drawn
out ventrally into a prominent keel, the =carina=, which projects for
some distance forwards beyond the body of the sternum, and tapers off
gradually behind. The point where the carina joins the body of the
sternum is at the anterior end drawn out into a small process, the
=rostrum=[102]. Just dorsolateral to this are a pair of deep grooves,
the =coracoid grooves=, with which the coracoids articulate.
The sides of the sternum are drawn out in front into a pair of short
blunt =costal processes;= and just behind these are a series of seven
surfaces with which the ends of the sternal ribs articulate.
Immediately behind these surfaces the sides are produced into a pair
of long backwardly-projecting =xiphoid processes= which nearly meet
processes from the posterior end of the sternum.
2. THE APPENDICULAR SKELETON.
This consists of the skeleton of the anterior and posterior limbs and
of their respective girdles.
A. THE PECTORAL GIRDLE[103].
The pectoral girdle in almost all birds is strongly constructed and
firmly united to the sternum. It consists of three bones, a dorsal
element, the =scapula=, a posterior ventral element, the =coracoid=,
and an anterior ventral element, the _clavicle_.
The =scapula= forms a long curved flattened bone expanded at its
anterior end, where it meets the coracoid, and lying across the ribs
at its tapering posterior end. It helps to form the imperfect =glenoid
cavity=, with which the humerus articulates. The =coracoid=, a shorter
but stouter bone than the scapula, has its upper end or =head=
thickened and bears on its posterior border an irregular surface, with
part of which the scapula articulates, while the rest forms part of
the glenoid cavity. The inner border of the coracoid adjoining the
articular facet for the scapula is produced into a strong process
which helps to complete the =foramen triosseum=, a space lying between
the adjoining ends of the scapula and coracoid, through which the
tendon of the second pectoral muscle passes. The lower part of the
coracoid, which is much flattened and expanded, and abruptly truncated
posteriorly, articulates with the coracoid groove of the sternum. The
_clavicle_ is a thickened curved membrane bone, which is fused with
its fellow in the middle line below, the two forming the _furcula_ or
merrythought. Its dorsal end is drawn out into a process which
articulates with the coracoid.
THE ANTERIOR LIMB OR WING.
This consists of three parts, a proximal part, the upper arm or
=brachium=, a middle part, the fore-arm or =antibrachium=, and a
distal part, the =manus=. When extended for flight the parts lie
almost in the same straight line, but when at rest they are folded on
one another in the form of a Z, the brachium and manus pointing
backwards, and the antibrachium forwards. When extended for flight the
surfaces and borders of the wing correspond in position with those of
the primitive vertebrate limb[104], the pre-axial border being
directed forwards and the postaxial backwards, while the dorsal and
ventral surfaces look respectively upwards and downwards. But when the
wing is at rest, the humerus as it extends backwards becomes slightly
rotated, so that its dorsal surface looks more inwards than upwards,
while the dorsal surface of the antibrachium looks partially outwards
and upwards, and that of the manus mainly outwards.
The =brachium= or =upper arm= contains only a single bone, the
=humerus= (fig. 57, 1). This is a large nearly straight bone expanded
at both ends. The proximal end is specially expanded, forming two
=tuberosities=, and a large convex =head= articulating with the
glenoid cavity. The =pre-axial tuberosity= is the smaller of the two,
but is continued by a prominent =deltoid ridge=, which extends for a
very short distance down the shaft. The =postaxial tuberosity= is the
larger, and below it there is a very deep pit, the =pneumatic
foramen=, which leads into an air cavity in the shaft of the bone. The
shaft is long and straight, and at the distal end of the bone is the
=trochlea= with two convex surfaces, one pre-axial with which the
radius articulates, the other postaxial for the ulna.
The =fore-arm= or =antibrachium= consists of two bones, the =radius=
and =ulna=. These are of nearly equal length, and are separated from
one another by a considerable space except at their terminations.
The =radius= (fig. 57, 2), the pre-axial and smaller bone, is straight
and fairly stout; its proximal end articulates with the humerus by a
slightly cupped surface, while its distal end, which articulates with
the carpus, is convex and somewhat expanded.
The =ulna= (fig. 57, 3) is longer, stouter, and slightly curved. Its
proximal end is expanded, forming two surfaces which articulate with
the trochlea of the humerus; behind them it is drawn out into a short
blunt =olecranon process=. Its distal end is less expanded, and
articulates with the carpus and also with the radius.
The =Manus=. This includes the carpus or wrist, and the hand.
The =Carpus=. While in the embryo the carpus consists of five
distinct elements arranged in a proximal row of two and a distal row
of three, in the adult only the proximal bones can be clearly
distinguished, the distal ones having become completely ankylosed with
the metacarpals to form the =carpo-metacarpus=.
The two distinct carpal bones are the radial carpal and the ulnar
carpal. The radial carpal (fig. 57, 4) is a small somewhat cubical
bone, wedged in between the manus and the radius and ulna. The ulnar
carpal (fig. 57, 5) is a somewhat larger, more irregular bone, lying
adjacent to the end of the ulna. It is deeply notched to receive the
carpo-metacarpus.
=The hand.= In the adult bird the hand is in a much modified
condition; only the first three digits are represented, and the
metacarpals are all fused with one another and with the distal
carpalia to form the =carpo-metacarpus=.
The most prominent part of the carpo-metacarpus is formed by the
=second metacarpal= (fig. 57, 7), a stout, straight bone expanded at
both ends. The =third metacarpal= (fig. 57, 8) is a more slender
curved bone fused at both ends with the second metacarpal. The =first
metacarpal= forms simply a small projection on the radial side of the
proximal end of the second metacarpal.
The =phalanges=. The first digit or =pollex= includes two phalanges,
the distal one being very small and bearing a claw.
The second digit includes three phalanges, the proximal one being
somewhat flattened. The third digit has a single small phalanx.
THE PELVIC GIRDLE.
The bones constituting the pelvic girdle are not only as in other
higher vertebrates ankylosed together forming the innominate bones,
but are also ankylosed with a series of some seventeen sacral and
pseudosacral vertebrae. The =acetabulum= (fig. 61, 5) with which the
head of the femur articulates is incompletely ossified.
The =ilium= (figs. 58 and 61, 1) is the largest bone of the pelvis. It
forms a long flattened plate extending for a considerable distance
both in front of and behind the acetabulum, and is fused along its
whole length with the transverse processes and neural spines of the
sacral and pseudosacral vertebrae. It forms more than half the
acetabulum, above and behind which it is produced to form a process,
the =antitrochanter= (fig 61, 8), with which the great trochanter of
the femur articulates.
[Illustration FIG. 61. LATERAL VIEW OF THE PELVIS AND SACRUM OF A DUCK
(_Anas boschas_) × 2/3.
1. ilium.
2. ischium.
3. pubis.
4. pectineal process.
5. acetabulum.
6. ilio-sciatic foramen.
7. fused vertebrae.
8. antitrochanter.]
The =ischium= (figs. 58 and 61, 2) is a flattened bone which forms
about one-third of the acetabulum, and lies ventral to the posterior
part of the ilium. Its anterior portion is separated from the ilium by
the large oval =ilio-sciatic foramen= (fig. 61, 6), while behind this
the two bones are completely fused.
The =pubis= (figs. 58 and 61, 3) is a very long slender bar of bone
which forms only a very small part of the acetabulum and runs back
parallel to the ventral surface of the ischium with which it is
loosely connected at its posterior end. For the greater part of their
length the two bones are separated by the long narrow =obturator
foramen=. Behind the ischium the pubis is produced into a long curved
downwardly-projecting process, and in front of the acetabulum it bears
a short blunt =pectineal= or =pre-pubic process= (fig. 61, 4) probably
homologous with the pre-pubis of Orthopod Dinosaurs. The remainder of
the pubis is homologous with the post-pubis of Orthopod Dinosaurs.
THE POSTERIOR LIMB.
The leg of the bird is somewhat differently constructed from that of
other vertebrates owing to the fact that there is no free tarsus, the
proximal tarsals having fused with the tibia, and the distal with the
metatarsals.
The =thigh= consists of a single bone, the femur. The =femur= is a
comparatively short bone with a straight shaft and expanded ends. The
proximal end bears on its inner side a rounded =head=, which
articulates with the acetabulum. On its outer side is an irregular
outgrowth, the =great trochanter=, while between the two is the
surface which meets the antitrochanter of the ilium. The posterior end
also is expanded and marked by a wide groove which lodges the
=patella=. On each side of the groove is a strong =condylar ridge= for
articulation with the tibia. The external condyle is deeply grooved
behind for articulation with the fibula.
The =crus= or =shin= consists of two separate bones, (1) the
=tibio-tarsus=, formed by the fusion of the tibia with the proximal
row of tarsals, and (2) the =fibula=.
The =tibio-tarsus= is a thick straight bone nearly twice as long as
the femur. Both ends of the bone are considerably expanded. The
proximal end bears two slight depressions which articulate with the
condyles of the femur, and a third depression which partly lodges the
patella. The proximal end of the anterior or extensor surface is drawn
out into a very prominent =cnemial crest= which bends over towards the
postaxial side of the bone; a slight ridge is continued from it all
the way down the shaft. The proximal part of the shaft of the
tibio-tarsus bears a roughened ridge with which the fibula is closely
connected. The distal end is expanded and rotated outwardly and forms
a prominent pulley-like surface which articulates with the
tarso-metatarsus.
The =fibula= is reduced to the proximal portion only, which is
expanded and articulates with a depression behind the external condyle
of the femur. The fibula further extends about a third of the way down
the shaft of the tibio-tarsus. The =patella= or =knee-cap= is a
sesamoid bone due to an ossification in the tendon of the extensor
muscles of the leg.
The =ankle joint= lies between the proximal and distal tarsals which
as previously mentioned fuse respectively with the tibia and
metatarsus.
The =Pes=. The pes includes four digits, and consists of the
tarso-metatarsus and the phalanges. The proximal tarsals which are
fused with the tibia also really belong to the pes.
The =tarso-metatarsus= is a strong straight bone nearly as long as the
femur, and is formed by the fusion of the distal tarsals with the
second, third and fourth metatarsals. The proximal end of the bone is
expanded and bears two facets for articulation with the tibio-tarsus,
and near them on the posterior surface is a large roughened
projection. The lines of junction between the several metatarsals are
marked along the shaft by slight ridges. At the distal end of the bone
the three metatarsals diverge from one another and each bears a
prominent convex pulley-like surface. The =first metatarsal= is
reduced to the distal end, which tapers to a point proximally, and is
attached by ligaments near the distal end of the tarso-metatarsus.
The =digits=. Four digits are present, each consisting of a metatarsal
(already described) and a certain number of phalanges, the terminal
one being in each case clawed. The first digit or =hallux= has two
phalanges, the second three, the third four, and the fourth five.
FOOTNOTES:
[101] See R.S. Wray, _P.Z.S._, 1887, p. 343.
[102] Often called the manubrium, but not homologous with the
manubrium of the mammalian sternum.
[103] Cp. fig. 63.
[104] See p. 28.