The swelling of grain, peas, etc, in the crop may be due only to the action of moisture and warmth, and is therefore a physical effect. It is stated by Hasse that a similar milky secretion occurs in some species of Parrots.
The lower oesophagus has an epithelium similar to that of the upper find of the crop, but there are a small number of glands in the mucous membrane with an alkaline or neutral secretion. In the proventriculus the epithelium is reduced to a single layer of columnar cells. The glands of this region secrete the acid gastric juice. In the Pigeon they are small and simple: in the Fowl and Goose they have lateral loculi. But their size and character vary a good deal in different birds. They are largest in Rhea and the Ostrich.
The gizzard is well developed in the Pigeon as in the Fowl and the Lamel-lirostres s. Chenomorphae (Ducks, &c). A short tube, which is always pale as compared with the vascular proventriculus, connects that organ to the gizzard. The pylorus is placed on the right side and close to the entrance of the proventriculus. The walls of the gizzard show two tendinous spots which lie one on the right, the other on the left side in the natural position of the organ. The two tendinous spots are the centres whence radiate the musculi laterales which make up the bulk of the organ. They are composed of smooth muscle fibres which in transverse section appear to lie in columns, the fibres in each column being connected to the fibres in the adjoining columns by short tendinous fibres. Two softer muscular bundles lie one close to the entrance of the proventriculus, the other at the opposite pole. These are the musculi intermedii. The mucous membrane is glandular; and the glands secrete the horny internal lining. This lining is discoloured by the food, and it is continually worn away by the attrition of stones, etc, swallowed with the food; and it is continually formed anew by the action of the glands.
If it is stripped off by force, the attached surface appears as if covered by very fine short villi, or processes which have been pulled out of the gland-tubes. In sections of the gizzard these processes can be readily made out dipping into the gland-tubes; they are conical, more transparent, and apparently softer than the superficial layers. Vertical lines, apparently formed either by irregularities of structure or by imbedded cells, are traceable nearly through the thickness of the horny layer. In many birds, e. g. flesh-eating birds, the muscular walls of the gizzard are thin and its secreted lining soft and tenacious. The degree of development of the muscles and the lining is closely connected with the character of the food - as was shown by Hunter's experiment of feeding a Sea-gull with barley. The muscles then became at least double the thickness of those in a Gull which had lived on fish. Cf. Catalogue of Physiological Series, Royal College of Surgeons' Museum, i. p. 49, preps. 522 D, and 523.
For the bile and pancreatic ducts, see description of Plate II.
The two caeca appended to the commencement of the large intestine are very small in the Pigeon - a contrast to the long caeca of the common Fowl, Pheasant, Grouse, etc. The large intestine is short and straight, as in all birds except the Ostrich. The rectal aperture lies at the apex of a cloaca common to it and the urogenital ducts. The rectal region of the cloaca is large and is separated by an annular ridge, which in some birds is but feebly indicated, from a small middle or urogenital chamber into which open the ureters and genital ducts on the dorsal wall, the genital apertures externally to those of the ureters. An annular fold always present separates the urogenital chamber from the third, outer or posterior chamber, the external opening of which is guarded by a strong sphincter muscle. An aperture on the dorsal wall of this outer chamber leads in young Pigeons into the Bursa Fabricii - an ovoid sac with a narrow neck lying dorsally to the cloaca. In the Ratitae (? Apteryx) the urogenital chamber opens into the Bursa owing to the fact that the neck of the latter is not constricted, and its aperture is commensurate with the dorsal aspect of the outer chamber of the cloaca.
In Plotus anhinga (Darter), a Carinate, Garrod found a large aperture to the Bursa, and Forbes has confirmed the fact. The Bursa commences to atrophy in the Pigeon at the sixth, in the Fowl at the eighth, month according to Martin Saint-Ange. There seems to be much variety in this respect among birds, and it is possible that it occasionally persists. As a rule, however, its aperture closes, its cavity is obliterated and its walls atrophied - a more or less fibrous remnant persisting. It is large in the embryo. Its function and homology are unknown: its cavity contains only remnants of faeces or concretions, the origin of which is not certain. It appears early in development as a solid outgrowth of cells from the dorsal wall of the proctodaeum ( = cloaca) before the rectal aperture opens into it. The central cells atrophy and thus form the cavity of the organ. In the adult, according to Stieda, the walls consist of a fibrous outer coat with internal prolongations which form the axes of the primary and secondary longitudinal folds which project internally; and an internal epithelium which has several layers of cells the superficial columnar, the deep angular, separated from the fibrous coat by a membrana propria.
In addition there are the 'follicles' so-called, which are imbedded in the longitudinal folds. Each follicle consists of a central mass of minute rounded nucleated cells, continuous at the apex of the follicle with the deep layer of the epithelium, and probably never cut off from it; of a membrana propria continuous with that of the epithelium; and of a surrounding investment of adenoid or reticular tissue with numerous capillaries developed from the same embryonic cells as the fibrous coat.