The movements of the third eyelid are governed by two special muscles, the quadratus and pyramidalis, and its gland, the Harderian gland, is large. The stapes or columella auris is well-developed, provided with processes at its outer end, and it has an epihyal element added to it. The posterior and the horizontal semicircular canals unite where they cross, and the cochlea contains no otoliths, but has a cuticular membrane, the homologue of the membrana Corti of the Mammal. The tympanic cavity is continued into the lower jaw by a membranous or bony canal, the siphonium. A nasal gland, lying on the nasal bone or extending to the frontals above the orbit, pours its secretion into the nose.
All living birds are edentulous. But in some embryo Parrots there are dental papillae attached to the periosteum of the jaws, supplied by vessels and nerves, and capped by cells of the rete mucosum. These cells become cornified, and contain air. Three such papillae are found on the maxillae, and ten in shallow alveoli on the mandibles of Melopsittacus. They are however hidden in every instance beneath the horny Covering of the bill. In the Lamellirostres (= Chenomorphae) the edges of the bill are raised into oblique lamellae, and in the Mergansers the edges of the bill are produced into pointed processes, supported by corresponding bony processes. In Odontopteryx from the London Clay there are similar bony processes, large and small intermingled. Hesperornis and Ichthyornis from the Cretaceous strata possess numerous true teeth, composed of dentine and enamel with large pulp cavities. In the former the praemaxillae are edentulous, and the teeth lodged in grooves of the maxillae and mandibles. They have fangs composed of osteodentine.
The succession is lateral as in Mosa-saurus and the Crocodile among Reptilia. The teeth of Ichthyornis are implanted in sockets, and the succession is vertical as in Deinosauria. Archaeopteryx from the Solenhofen slates also has teeth which probably resemble those of Hesperornis. The tongue is well-developed, especially in Accipitrine birds and in the Parrots where it is thick and fleshy. Its anterior extremity may be forked (Humming Birds, Trochilidae), or terminate in a brush of delicate papillae (Trichoglossinae among Parrots): and it is of great length and extensile in Woodpeckers (Picidae) and Humming Birds. It has a horny surface and is often covered by recurved papillae. There are numerous glands in and around the tongue as well as palatal glands, and an especially large gland opening at the angle of the mouth1 In the oesophagus the circular muscle layer is external to the longitudinal, thus reversing the relations usual in Vertebrata. The oesophagus itself is of uniform calibre in omnivorous, frugivorous and insectivorous birds. In birds which swallow large masses of food at once, e.g. fish-eating birds, its calibre is large: and in birds of prey, and especially grain-eating birds, it is provided with a dilatation or crop in which the food is stored.
In grain-eating birds such as Gallinae, Pigeons, etc, the walls of the crop are generally said to be glandular, and the secretion of the glands to act upon the food (but see p. 53), whilst in other birds the walls are non-glandular. The stomach is divisible in all birds into a glandular stomach or proventriculus, and a muscular stomach and gizzard, connected by a narrow tube or lower oesophagus. The proventriculus varies much in size: its glands are simple or compound and are disposed in various ways. The gizzard is saccular, or more or less angular in outline. Its walls show two tendinous spots between which radiate muscle fibres. The degree to which these fibres are developed varies much with the food, least in flesh- and fruit-eating birds, most of all in grain-eating birds and Lamellirostres. The character of the secretion poured out by the glands varies in the same direction: and it either forms a tenacious coat, or a thick horny coat, which is continually worn away by the action of the food and of the stones swallowed with it to assist in trituration, and as continually secreted anew. The two apertures, one leading into the gizzard the other from it, are close together.
The pyloric aperture is often guarded by a raised fold to prevent stones, etc. passing on into the intestine: and, in a few birds, there is a small so-called 'pyloric stomach' intervening between the gizzard and the pylorus, e. g. in the Heron. The duodenum is, as a rule, wide and long. The length of the small intestine varies much, as does also the mode in which it is coiled, a point which has been utilised for classificatory purposes. The large intestine is usually straight and short: in the Ostrich alone it is of great length and disposed in coils. It opens into the anterior end of the cloaca, which is large in Ratitae and Accipitrine birds, but in others usually small. The urinary and genital ducts ordinarily open separately on its dorsal wall, but there may be a more or less distinct urogenital section of the cloaca. The liver ordinarily consists of two lobes into the fissure between which the apex of the heart is received. There are always two, and sometimes three bile-ducts: they open separately into the duodenal region of the intestine, and on one of them a gall-bladder is usually developed. It is absent in many Pigeons, some Parrots, etc. A large and compact pancreas with two or three ducts is always to be found in the concavity of the duodenal loop.
A ductus vitello-intestinalis, the duct of the yolk-sac of embryonic, and of early life subsequent to the hatching, is often to be found on the small intestine, especially in aquatic birds and waders. In Ratitae remains of the yolk may be found in it for a long time or during life. Two caeca, rarely absent, e. g. some Parrots, are appended to the intestine at the junction of the ilium with the large intestine. The Herons have but one. Their size varies much, and they are largest in grain-eating birds, Lamellirostres and Ratitae. In the Ostrich they are very large and contain a spiral valve. In all young birds, and in some adults, a Bursa Fabricii opens on the dorsal aspect of the cloaca close to its external aperture. This bursa is produced originally as a solid growth which subsequently acquires a cavity. It usually atrophies away: its function is unknown, but its walls contain a rich vascular supply, and numerous follicles derived from the epithelium of the intestine and surrounded by adenoid (reticular) tissue (see pp. 54-5).
1 The edible bird's nest produced by a Swift is composed of a substance closely akin to mucin, secreted by two glands lying one on either side of the tongue. These glands are said to enlarge at the nesting season. See Green, Nature, xxxiv. 1886: Journal of Physiol, vi. 1885.