CHAPTER XVIII.

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).

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.

(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.

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.

B. THE SKULL.

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.

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.