Fig. 4.–Pelvis of Apteryx australis. Lateral view. a, Acetabulum; il, ilium; is, ischium; p, pectineal process of pubis; p1, pubis. (From Wiedersheim, after Marsh.)
7. The Posterior Limbs, or Legs, are composed of the Femur or thigh, the Tibia and Fibula, making the shank or "drumstick," and the bones of the Foot. The thigh, however, being hidden by the plumage, the shank of a Bird might easily be taken for the thigh, and the metatarsus (the cannon-bone of some) for the shank. The tibia and fibula commonly unite to some extent, and the former, as it now exists in adult Birds, is strictly a "tibio-tarsus," since with it is fused the proximal portion of the originally existing tarsal elements. Similarly the distal tarsal elements unite with the metatarsus, which is therefore properly a "tarso-metatarsus," though often called merely "tarsus" by ornithologists. This arises from a fusion of the second, third, and fourth metatarsal bones, which in the adult (except among the Sphenisci and to some extent in Psittaci) do not lie in the same plane; the middle one having its upper end thrust backward and its lower end forward in the course of growth to maturity. The fifth metatarsal practically disappears, while the first remains more or less separate, and lies behind the distal portion of the other metatarsals.
Of the toes the fifth is not traceable in Birds; the first is often aborted, but the second only in Struthio, and to a less extent in Ceyx and Alcyone, and the fourth (nearly) in Cholornis. The hallux, or hind toe, has two phalanges, the second digit three, the third four, and the fourth five; Cypselus and Panyptila (Swifts), however, are exceptions, and possess only three in each of the anterior toes, while the Caprimulginae (true Nightjars) and Pteroclidae (Sand-Grouse) have only four joints on the outer. In Owls the fourth digit is reversible at will, the same being true to a less extent of the Musophagidae (Plantain-eaters) and Leptosoma (akin to the Roller); when this condition is permanent, as in the Cuculidae, Psittaci and Pici the foot is termed zygodactylous. In Trogones the second toe is reversed (heterodactylous). Colius can turn the first toe forward and the fourth backward, while certain Swifts, and to a less degree some Nightjars, have the whole number permanently pointing to the front (pamprodactylous). Membranes more or less connecting the anterior digits produce a webbed or swimming foot, even the hallux being united with the rest in the Steganopodes. The hind-toe is often elevated, or higher than its fellows, when it is commonly reduced and sometimes lacks a nail. The Ostrich has little or no claw on the outer toe, while that of the third toe is toothed or serrated in a considerable number of Birds, but this is a character of very slight importance.
The covering of the metatarsus is usually "scutellated," but when the scutellae, or scales, which may be oblong or polygonal, are smaller than usual–and generally hexagonal–it is called reticulated. In some cases the surface becomes nearly or quite smooth ("ocreated" or "booted"), or more or less granulated.
8. The structure of the Skull is a study in itself and affords considerable help in Taxonomy (Classification). It must suffice here to refer for the names of the parts to the subjoined figure.
Fig. 5.–Skull of a Wild Duck (Anas boscas), from the side. ag, Angular; als, alisphenoid; ar, articular; bt, basitemporal; d, dentary; en, external nostrils; e.o, exoccipital; eth, ethmoid; fr, frontal; j, jugal; lc, lacrymal; mx, maxilla; mx.p, maxillopalatine process; n, nasal; p, parietal; pg, pterygoid; pl, palatine; ps, presphenoid; px, premaxilla; q, quadrate; q.j, quadratojugal; s.ag, supra-angular; s.o, supraoccipital; sq, squamosal: ty, tympanic cavity; v, vomer; II, foramen for optic nerve; V, for trigeminal. (From Wiedersheim, after Parker.)
The Bill, or Beak, is composed of an upper jaw or maxilla, and an under jaw or mandible. From the figure it will be seen that "maxilla" is not strictly the whole upper portion, though the term is thus used for convenience, as is the plural "mandibles" for the two jaws when mentioned simultaneously. The "rhamphotheca," or horny sheath, may be simple (undivided), or compound, that is, made of several distinct pieces. In the Anseres the covering is soft with a horny (corneous) tip or "nail"; in the Limicolae it varies extremely, producing a hard pickaxe, as in the Oystercatcher, or a delicate sensory organ as in the Snipe and Woodcock. The rhamphotheca at times has extraordinary outgrowths, as in the Hornbills, Sheathbills, and elsewhere. In the Accipitres, or Diurnal Birds of Prey, and most Psittaci, the base is soft and becomes a "cere," while the similar formation in the Columbae is due to a swelling of the operculum or covering of the nostrils. This operculum, moreover, may be leathery (coriaceous), as in the Charadriidae, Trochilidae and so forth, or rolled up, as in Rhinochetus; it may even result in a short soft tube, as in Caprimulgus, or in the hard double tube which gives the name of Tubinares to the Petrels. "Impervious" nostrils are those with a septum, or division, between the nasal cavities, "pervious" those with none. The narrow slit-like or entirely closed nostrils of the Steganopodes should also be mentioned.
The form of the bill varies from the "spoon" of Platalea and Eurynorhynchus (spatulate) to the "arch" of Numenius, the scissors of Rhynchops, the "wedge" of Picus, the big rounded feature of the Psittaci, and so forth; but for details the characters of the several Families must be consulted, as also for helmets, shields, horns, knobs, and peculiarities due to the elongation, distorting or crossing of the mandibles. These, too, are often notched, serrated, lobed or "festooned," or emarginate (slightly indented); the curious transverse serrations or lamellae of the beak in Anseres, and the somewhat similar sifting apparatus in Phoenicopterus, Prion and Anastomus being especially remarkable. Teeth were probably lost by Birds before Tertiary times, but were possessed at least by Archaeopteryx, Hesperornis and Ichthyornis. The so-called "egg-tooth" of embryos is merely a calcareous protuberance on the upper surface of the bill, which is cast after being used to crack the shell.
9. The organs of deglutition and digestion begin with the tongue, which is subject to much variation of structure, according to the different groups of Birds, and is of course correlative with their habits. It has little connexion with taste, though often of assistance in obtaining nutriment. To this follows the gullet (oesophagus), which in many cases has an enlargement forming the crop (ingluvies), wherein the food may be temporarily retained before passing into the stomach, the last-named always having an antechamber (proventriculus) where digestion is largely accomplished, in front of the gizzard (ventriculus). This has frequently strong muscular walls, and its action is often assisted by the mechanical process of comminution performed by stones, grit or sand, swallowed for that purpose. The stomach is succeeded by the intestines, which in most cases have a pair of blind-sacs (caeca) attached to them, often acting as aids to digestion, though these are not always functional, and are absent in many Birds, while in others they attain a very large size, their condition being in consequence of some importance as a systematic character.