(ESOPHAGUS AND STOMACH OF BIRDS.
The cesophagus is of immense size in many carnivorous birds ; considerably larger than the intestinal The capaciousness of this tube enables it to hold for a time the entire fish and large bones which these birds swallow, and which cannot be contained in the sto mach ; and to facilitate the discharge, by vomiting the indigestible remains of the food, which form balls of hair, feathers, and bony matter.
The cesophagus expands just before the sternum into the crop, (ingInvies, prolobus, Is jabot,) which is furnished with numerous mucous or salivary glands, disposed in many cases in regular rows. In such birds as nourish their young from the crop the glands swell remarkably at that time, and secrete a greater quantity of fluid. This part is found in land birds only, but not in all of these ; it ex ists in all the galline, and in some birds of prey.
There is another glandular and secre tory organ, much more common than the crop, belonging, indeed, most probabl3 to the whole class. This is the bulbus glandulosus, (echinus, infundibulum, pro. ventriculus, corpus tubulosum,) which is situated before the entrance of the ceso phagus into the proper stomach, and whose form and structure vary consider ably in the different genera and species. In the ostrich, for example, its magni tude and form give it the appearance a second stomach. In some other birds, as the psittaci, ardee, (crane, stork, &c.) its appearance is different from that of the proper stomach, but its size is larger ; while, on the contrary, in gallinaceous fowls it is much smaller.
This bulbus glandulosus consists of a vast congeries of glands. The cesopha gus, of which it is a dilatation, has a vast number of glandular bodies inter posed between its tunics, and entirely surrounding the tube, so as to constitute the " Zone of gastric glands" of Mr. Ma cartney. These bodies have a hollow internally, and they open into the ca vity of the bulbus by numerous very plain apertures. The fluid secreted by them passes into the gizzard, and mixes witb the food.
A deviation from the natural struc ture, which is completely unparalleled, occurs:in the stomach of the cuckoo. The gizzard of the bird is covered, internally, with an abundance of short, bristle, and spiral hairs, lying close toge ther.
The structure of the stomach differs most widely in the different orders and genera of this class. It appears merely as a thin membranous bag, in several of those which feed on flesh and insects, when compared with the thick mus cular globes of the granivorous genera. But there are in both many interme diate links between these extremes, and at the same time considerable analogies in the structures, which are apparent ly the mos'. opposite. This is particular ly observeable in the course of the muscu lar fibres, arid in the callous structure and appearance of the internal coat ; in which points, many of what are called membra nous stomachs have a great resemblance to those of the galling.
Both parts, but particularly the mus cular, are very strong in the gizzard (ventriculus billbosus) of granivorous birds. We find here, instead of a mus cular coat, four immensely thick and powerful muscles, viz. a large hemi spherical pair at the sides (laterales,) and two smaller ones (intermedii) at the two ends of the cavity. All the four are dis tinguished, by the unparalleled firm ness of their texture, and by the pecu liar colour, from all the other muscles of the body.
The internal callous coat must be con sidered as a true epidermis ; since, like that part, it becomes gradually thicker from pressure and rubbing. It forms folds and depressions towards the cavity of the stomach ; and these irregularities are adapted to each other on the oppos ed surface. The cavity of this curious stomach is comparatively small ; its lower orifice is placed very near the upper. Every part of the organ is, indeed, calculated for producing very powerful trituration ; and this is still further promoted by the well known in stinctive practice of granivorous birds, of swallowing small hard stories with their food.