Fig. 325. - A, Hind-limb of the Loon (Colymbus glacialis) - after Owen : i Innominate bone ; f Thigh-bone or femur ; t Tibia, with the proximal portion of the tarsus an-chylosed to its lower end; r Fibula; m Tarso-metatarsus, consisting of the distal portion of the tarsus anchylosed with the metatarsus ; p p Phalanges of the toes. B, Tail of the Golden Eagle; s Ploughshare-bone, carrying the great tail-feathers.
The tarso-metatarsus is followed inferiorly by the digits of the foot. In most birds the foot consists of three toes directed forwards and one backwards - four toes in all. In no wild bird are there more than four toes, but often there are only three, and in the Ostrich the number is reduced to two. In all birds which have three anterior and one posterior toe, it is the posterior thumb or hallux (that is to say, the innermost digit of the hind-limb) which is directed backwards; and it invariably consists of two phalanges only, its metatarsal being incomplete and united as a rule to the tarso-metatarsus by ligament only. The most internal of the three anterior toes (the "index") consists of three phalanges; the next ("middle") has four phalanges; and the outermost toe ("annularis") is made up of five phalanges (fig. 325, A). This increase in an arithmetical ratio of the phalanges of the toes, in proceeding from the inner to the outer side of the foot, obtains in almost all birds, and enables us readily to detect which digit is suppressed, when the normal four are not all present. Variations of different kinds exist, however, in the number and disposition of the toes. In many birds - such as the Parrots - the outermost toe is turned backwards, so that there are two toes in front and two behind, whilst in the Trogons the inner toe is turned back with the hallux, and the outermost toe is turned forwards. In others, again, the outer toe is normally directed forwards, but can be turned backwards at the will of the animal. In the Swifts, on the other hand, all four toes are present, but they are all turned forwards. In many cases - especially amongst the Natatorial birds - the hallux is wholly wanting, or is rudimentary. In the Emeu, Cassowary, Bustards, and other genera, the hallux is invariably absent, and the foot is three-toed. In the Ostrich both the hallux and the next toe (" index ") are wanting, and the foot consists simply of two toes, these being the third and fourth digits. The toes are mechanically flexed during the sleep of most birds by the action of a special muscle which runs from the pubis outside the knee to join one of the flexors of the toes (the flexor digi-torum perforatus), and which is therefore put on the stretch whenever the leg is bent upon the thigh.
The digestive system of birds comprises the beak, tongue, gullet, stomach, intestines, and cloaca. Teeth are invariably wanting in living birds, and the jaws are encased in horn, constituting the bill. Dental papillae, sometimes covered with a cap of dentine, have, however, been observed in the embryos of some Parrots. In the extinct Odontopteryx, moreover, the osseous substance of the jaws is prolonged into tooth-like processes of two sizes; and in the Odontornithes of the Cretaceous period the jaws are furnished with true teeth implanted in distinct sockets. The form of the bill varies enormously in different birds, and it is employed for holding and tearing the prey, for prehensile purposes, for climbing, and in some birds as an organ of touch. In these last-mentioned cases the bill is more or less soft, and is supplied with filaments of the fifth nerve. In many birds, too, in which the bill is not soft, the base of the upper mandible is surrounded by a circle of naked skin, constituting what is called the "cere," and this, no doubt, serves also as a tactile organ.
The tongue of birds can hardly be looked upon as an organ of taste, since it is generally cased in horn like the mandibles. It is, in fact, principally employed as an organ of prehension ; but in some cases - as in the Parrots - it is soft and fleshy, and then, doubtless, is to some extent connected with the sense of taste. It is essentially composed of a prolongation of the hyoid bone (the glosso-hyal), which is sheathed in horn, and is variously serrated or fringed.
Salivary glands are invariably present, but they are rarely of large size (they are very large in the Woodpeckers and Swifts), and they have often a very simple structure.
In accordance with the structure of the neck, the gullet in birds is usually of great length, and it is generally very dilatable. In the carnivorous, or Raptorial, and in the granivorous birds, the gullet (fig. 326, 0) is dilated into a pouch, which is situated at the lower part of the neck, just in front of the merry - thought. This is what is known as the "crop" or "ingluvies" (c), and it may be either a mere dilatation of the tube of the gullet, or it may be a single or double pouch. The food is detained in the crop for a longer or shorter time, according to its nature, before it is subjected to the action of the proper digestive organs. The oesophagus, after leaving the crop, shortly opens into a second cavity, which is known as the "proventriculus" or "ventriculus succenturiatus" (p). This is the true digestive cavity, and its mucous membrane is richly supplied with gastric follicles which secrete the gastric juice. The proventriculus, however, corresponds, not with the whole stomach of the Mammals, but only with its cardiac portion; and it opens into a second muscular cavity, which corresponds to the pyloric division of the Mammalian stomach. The gizzard (g) is situated below the liver, and forms in all birds an elongated sac, having two apertures above, of which one conducts into the duodenum, or commencement of the small intestine, whilst the other communicates with the proventriculus. The two chief forms of gizzard are exhibited respectively by the Raptorial birds, which feed on easily-digested animal food, and the Rasores and some of the Natatores, which feed on hardly-digested grains. In the birds of Rapine the gizzard scarcely deserves the name, being, as a rule, nothing more than a wide membranous cavity with thin walls. In the granivorous birds, whose hard food requires crushing, the gizzard is enormously developed; its lining coat is formed of a thick, horny epithelium, and its walls are extremely thick and muscular. This constitutes a grinding apparatus, like the stones of a mill; whilst the "crop" or oesophageal dilatation may be compared to the "hopper" of a mill, since it supplies to the gizzard "small successive quantities of food as it is wanted" (Owen). Supplementing the action of the muscular walls of the gizzard, and acting in the place of teeth, are the small stones or pebbles, which, as is so well known, so many of the granivorous birds are in the habit of swallowing with their food, or at other times. In fact, there can be no doubt but that the gravel and pebbles swallowed by these birds are absolutely essential to existence, since the gizzard, without this assistance, is unable properly to triturate the food.