The intestine is short, and rather wide. It is usually divided into small and large by a circular constriction or valve.
In the Batrachian division, however, the separation into these tu o segments is some times absent. While sometimes, as in the Toad and Frog, there is a distinct large intestine, into which the smaller portion opens laterally, so as to form a cmcum. In the former of these two genera there is no valve.
In the Ophidian the two portions are generally distinct and short. But their relative extent varies considerably : the small intes tine being sometimes lengthened, and often presenting a very peculiar appearance in the shortness of its mesentery and the closeness of its folds. The indistinct ilio-cmcal valve is chiefly marked by a change in the diameter of the tube. The large intestine is often sub divided into distinct portions by one or two transverse valves.
In the C'helonia the intestine is longer and much more muscular. There is generally an ilio-ccal valve, and often a well-marked cmcum. The valve is also present in most of the Sauria. But in the Crocodile the m erlin is absent.
Birds.—In this class, the stomach is generally complex ; being separated into three distinct cavities, which differ greatly in their form, structure, and office.
The cesophagus, which leads to the first oF those cavities, has a length corresponding to that of the neck which it occupies. Its width and dilatability mainly depend on the nature and form of the food. Thus, in some of the birds of prey, or those which swallow large fish entire, it is very lax and dilatable. And in this respect, as well as in the direct con tinuity of its cavity with that of the stomach, it offers a great similarity to the gullet of the Ophidian reptiles and rnany fishes. Its mucous membrane is follicular, and folded longitudinally.
The ingluvies, or crop, is a dilatation of the cesophagus, somewhere about the middle of its length. In some of the smaller Rap tores it is but small ; in the larger and more voracious it is a considerable enlargement, that affects one side of the tube more than the other ; in the Gallinaceans it is a distinct sac, appended to the canal by a narrower neck ; and, finally, in the Pigeons, it attains its maximum size, and becomes double. Its
muscular and mucous membrane are similar to those of the cesopliagus. The food which it contains undergoes a kind of insalivation and maceration. And the highly-developed forrn of crop, which is seen in the Pigeon, pours out a milky fluid (luring that period of the year in uhich this bird feeds its young by regurgitation. At this time its mucous membrane also acquires a thicker and more glandular character.
The proper stomach, or proventriculus, com municates with the inferior part of the ceso phagus, and corresponds, both in structureitnd function, with the true stomach of the Mani nzalia. The glandular tubes which open on its free surface secrete a fluid that possesses all the properties of gastric juice. In the degree of complication these glands differ considerably ; varying from simple tubes in the carnivorous birds, to tubes that open between prorninences and prolongations, and finally form primary and secondary branches. The shape and size of this organ are subject to great variety .in different genera. In those that swallow a large prey, it is wide and straight, like the stomach of the Serpent. In others, it ap proaches the spherical form, or passes towards the right side to join the gizzard. The com parative size of these two organs also varies considerably.
The gizzard is a flattened ovoid of highly muscular texture. It is lined by a dense horny cuticle, and contains sand, grai, el, or other bard inorganized matters, which are the passive agents in the trituration of the food. Its size varies greatly. Its apertures both occupy the upper part of the organ, so that its cavity tertninates below in a blind ex tremity. Its walls contain a variable amount of muscle, the arrangement of which is usually that of the radiation of fibres from a central tendon, such as was previously noticed in speaking of the Cephalopoda. Its epithelium is hardest in the granivorous birds. And even in the same individual, it offers an increased density at the precise situations of most pressure. In like manner, Hunter noticed that a thickening, both of cuticle and muscle, was produced by- feeding a Sea-gull upon grain.