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.

CHAPTER XIX.
GENERAL ACCOUNT OF THE SKELETON IN BIRDS.

In many birds the calamus or quill bears two vexilla or vanes, the second of which, called the aftershaft or hyporachis, is generally much the smaller, and is attached to the under surface of the main vexillum. In the Moas, Emeu and Cassowary the two vexilla in the adult bird are nearly equal in size; though in the nestling Emeu one is much longer than the other. The aftershaft is very small in most Passeres and gallinaceous birds, but is comparatively large in Parrots, Gulls, Herons and most birds of prey. It is absent or extremely small in the Ostrich, Apteryx, Rhea, Pigeons, Owls, Anseres, and others.

The quill feathers include two groups, the remiges or wing quills, and the rectrices or tail quills. In most birds the primary remiges, or those which are attached to the bones of the manus, are ten or eleven in number, and are set in grooves in the bones, being firmly attached to them. In the Ostrich however the primaries are little specialised in character and are as many as sixteen in number. They are also less definitely attached to the bones; as their ends do not lie in grooves in the bones, but project beyond them.

The secondary quills or those attached to the ulna vary much in number according to the length of the bone. The large dark quills in the wings of Cassowaries are the secondaries.

The wing of Penguins is very little differentiated. It is covered at the margin by overlapping scales which gradually merge into scale-like feathers at the proximal end. The wing of the Penguin has nothing comparable to the remiges of other birds.

In some birds, such as Herons (Ardea), there occur in places plumulae of a peculiar kind, which grow persistently and whose summits break off into fine powder as fast as they are formed. These feathers are known as powder-down feathers. They occur also in some Parrots and are then scattered indiscriminately all over the body.

Other exoskeletal structures besides feathers are commonly well developed. Thus the extremities of the jaws are sheathed in horny beaks whose form varies enormously according to the special mode of life.

In ducks and geese the beak with the exception of the anterior end is soft, and its edges are raised into lamellae, while in the Mergansers these lamellae become pointed processes supported by bony outgrowths. These lamellae act as strainers. In Parrots and Hawks, on the other hand, nearly the whole of the beak is hard.

The toes and tarso-metatarsus are usually featherless and are covered either with granular structures or with well-formed scales. The toes are nearly always provided with claws, and these vary in correlation with the character of the beak. Claws[105] also sometimes occur on the manus. Thus Archaeopteryx and some Ostriches and Rheas have claws on all three digits. Most Ostriches and Rheas, and many Anseres and birds of prey, have them on the first two digits, while the Secretary bird (Gypogeranus) and many fowls, ducks, and birds of prey, especially kestrels, have a claw only on the pollex. In the Cassowary, Emeu, Apteryx and some Ostriches and Rheas only the second digit is clawed.

Claws should not be confounded with spurs, which are conical horny structures developed on bony outgrowths of the radial side of the carpus, metacarpus, or metatarsus. They occur in a number of birds, but are most commonly developed in gallinaceous birds, by which they are used for fighting. A single spur occurs on the metacarpus in Megapodius, in Palamedea, in Parra jacana and in Hoplopterus spinosus, the Spur-winged plover. The Derbian Screamer, Chauna derbiana, has two metacarpal spurs, borne on the first and second metacarpals. The Spur-winged goose, Plectropterus gambensis, has a carpal spur borne on the radial carpal. Metatarsal spurs are quite common.

The male Solitaire (Pezophaps) has large bony excrescences on the wrist which may, like spurs, have been sheathed in horn and used for fighting.

Teeth do not occur in any living birds, but conical teeth imbedded in separate sockets are present in Archaeopteryx and Ichthyornis, while in Hesperornis similar teeth occur implanted in continuous grooves in the mandibles and maxillae, the premaxillae being toothless.

Except that teeth are partly dermal in origin, a dermal exoskeleton is quite unrepresented in birds.

ENDOSKELETON.

Perhaps the most striking feature of the endoskeleton of birds is its pneumaticity. In the embryo all the bones contain marrow, but as growth proceeds this becomes replaced by air to a variable extent in different forms. In all birds some part of the skeleton is pneumatic. Many small birds and Apteryx and Penguins among larger ones have air only in the skull; in Pigeons air is present in all the bones except the caudal vertebrae, the leg bones, and those of the antibrachium and manus; in Hornbills every bone contains air.

Vertebral Column.

The vertebral column of birds is readily divisible into a very mobile cervical region, and an extremely rigid post-cervical region. In most birds the vertebral centra are without terminal epiphyses, but these structures are found in Parrots. The cervical vertebrae are generally large and vary in number from eight or nine to twenty-three in Swans. Except in some extinct forms, such as Ichthyornis and Apatornis, in which they are biconcave, the centra are characterised by having saddle-shaped articulating surfaces, which in front are concave from side to side and slightly convex from above downwards, while posteriorly they are convex from side to side and concave from above downwards. The atlas is small and ring-like, and its centrum is fused with the axis forming the odontoid process. Cervical ribs are often well developed, and in some of the Ratitae they remain for a long time distinct from the vertebrae.

The thoracic vertebrae are distinguished from the cervical by the fact that their true ribs are united to the sternum by means of sternal ribs. This distinction, however, though convenient, is somewhat arbitrary, as it has been shown that in the fowl and gannet, two pairs of ribs which in the adult are free from the sternum, are connected with it in the embryo. When, as in the Swans, the thoracic vertebrae are not all fused together, they generally have saddle-shaped articulating surfaces, but sometimes, as in the Penguins, Auks and Plovers, the centra are convex in front and concave behind. The trunk vertebrae generally have well-marked neural spines, while in the Divers the anterior ones have peculiar bifurcating hypapophyses.

The trunk vertebrae are not readily divisible into thoracic and lumbar. There are two true sacral vertebrae, but as development proceeds a number of other vertebrae become fused with the true sacrals, the whole forming a large compound sacrum. These pseudosacral vertebrae generally include the lumbar, and some of the thoracic and caudal vertebrae. Sixteen to twenty vertebrae or even more may be included in the compound sacrum, and sometimes the whole of the trunk vertebrae are fused together. In Archaeopteryx however but five vertebrae take part in the formation of the sacrum.

In Archaeopteryx there are twenty long caudal vertebrae, of which the last sixteen carry a pair of feathers apiece, but in all other birds the tail is short and in the great majority of cases the posterior vertebrae are fused together, forming the pygostyle. In the Ratitae and Tinamidae a pygostyle is rarely or imperfectly developed. In Hesperornis there are twelve caudal vertebrae, six or seven of which are united by their centra only, forming an imperfect pygostyle.

The free caudal vertebrae are generally amphicoelous.

The Skull.

The skull of all birds from Archaeopteryx onwards is essentially similar, differing from the skull of reptiles mainly in the extent to which the cranium is arched, and its greater size in proportion to the jaws.

Most of the bones of the cranium are pneumatic, and all show a marked tendency to fuse together, and have their outlines obliterated by the disappearance of the sutures. The several bones remain longest distinguishable in the Ratitae and to a less extent in the Penguins. The orbits are very large and lie almost entirely in front of the cranium; they are separated by an interorbital septum which is sometimes, as in Chauna and Scythrops, very complete, sometimes, as in Hornbills and the Common Heron, very slightly developed. As a general rule the sclerotic is cartilaginous.

The anterior nares are almost always situated far back at the base of the beak near the orbits, but in Apteryx they are placed right at its extremity. In Phororhacos they are placed very high up on the enormous beak and are not separated by any bony partition.

The skull of Parrots has some peculiarities. In some Parrots the lachrymal sends back a process which meets the postorbital process of the frontal and completes the orbit. In most birds the upper beak is immovably fixed, but in some it is attached to the cranium, only by the nasals and by flexible processes of the premaxillae, so that by this means a kind of elastic joint is established and the beak is able to be moved on the cranium. In the Parrots and Opisthocomus there is a regular highly movable joint.

In Cassowaries the fronto-nasal region of the skull is produced into an enormous bony crest, and in Hornbills a somewhat similar structure occurs. Although true teeth do not occur in any known bird except Archaeopteryx, Hesperornis, and Ichthyornis, another extinct bird, Odontopteryx, has the margins of both jaws provided with forwardly-directed tooth-like serrations, formed of part of the actual jawbone: a living hawk, Harpagus, too, has a deeply notched bill, to which correspond serrations in the premaxillae.

A basi-pterygoid process of the basisphenoid abuts against the pterygoid in Ratitae and in Tinamous, plovers, fowls, pigeons, ducks and geese among Carinatae, recalling the arrangement met with in many reptiles. The squamosal is sometimes, as in the fowl, united with the postorbital process of the frontal. In the Carinatae the quadrate articulates with the cranium by a double convex surface, in the Ratitae by a single one. The premaxillae are always comparatively large bones, the maxillae on the contrary are small, but give rise to important inwardly-projecting maxillo-palatine processes.

The relations of the palatines, pterygoids, maxillae, and vomers vary considerably, and on them Huxley has based a classification of birds[106]. In the Ratitae and the Tinamous (Tinamidae), among Carinatae the vomers unite and form a large broad bone, separating the palatines and the pterygoids from the rostrum. Huxley uses the term Dromaeognathous to describe this condition. In all other Carinatae the vomers are narrow behind, and the palatines and pterygoids converge posteriorly and articulate largely with the rostrum. Three modifications of this condition are distinguished by Huxley, and termed Schizognathous, Ægithognathous, and Desmognathous.

In the Schizognathae the vomers coalesce and form a narrow elongated bone, pointed in front, separating the maxillo-palatine processes of the premaxillae. Waders, fowls, penguins, gulls, some falcons and eagles, American vultures, some herons and many owls have the Schizognathous arrangement. In pigeons and sandgrouse there is no vomer, but the other bones have the Schizognathous arrangement.

In the Ægithognathae the arrangement is the same as in the Schizognathae, except that the vomers are truncated in front. Passeres, swifts, woodpeckers, humming birds, rollers, hoopoes have this arrangement.

In the Desmognathae (fig. 60, A) the maxillo-palatine processes approach one another in the middle line, and either unite with the vomers, or unite with one another, hiding the vomers. Thus a more or less complete bony roof is formed across the palate. The vomers in Desmognathae are small or sometimes absent. Ducks, storks, most herons, most birds of prey and owls, pelicans, cormorants, parrots, and flamingoes are Desmognathous.

The mandible, as in other Sauropsids, consists of a cartilage bone, the articular, and a series of membrane bones, the dentary, splenial, coronoid, angular, and supra-angular, developed round the unossified Meckel's cartilage. The dentaries of the two rami are nearly always fused together, but in Ichthyornis and Archaeopteryx the two rami are but loosely united. There is often a fontanelle between the dentary and the posterior bones, while the angle is sometimes, as in the fowl, drawn out into a long curved process.

The hyoid apparatus (fig. 59, C) consists of a median portion, and a pair of cornua. The median portion is composed of three pieces placed end to end, and called respectively the os entoglossum, the basi-hyal, and the uro-hyal. The os entoglossum is shown by development to be formed by the union of paired structures and is probably homologous with the hyoid arch of fishes. The basi-hyal and the long cornua, each of which is composed of two or three pieces placed end to end, are homologous with the first branchial arch of fishes, while the uro-hyal is probably homologous with the second branchial arch of fishes. In Woodpeckers the cornua are enormously long, and curve over the skull, extending as far forwards as the anterior nares.

Ribs and Sternum.

Well-developed ribs are attached to the posterior cervical vertebrae as well as to the thoracic vertebrae. The ribs generally have uncinate processes and separate capitula and tubercula, but uncinate processes are absent in Chauna Palamedea and apparently in Archaeopteryx.

The sternum (fig. 63) is greatly developed in all birds. In the embryo[107] it is seen to be derived from the union of right and left plates of cartilage, formed by the fusion of the ventral ends of the ribs. In the Ratitae and a few Carinatae, such as Stringops, it is flat, but in the great majority of birds it is keeled, though the development of the keel varies greatly. It is large in the flightless Penguins, which use their wings for swimming. Traces of an interclavicle may occur in the embryo.