The Bird Digestive System

The Bird Digestive System

Intestine and Villi

Relative Length of the Intestinal Canal

Intestinal Fold or Loop Types

Intestinal Loops and Fold Characteristics in Bird Groups

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The Digestive System consists chiefly of the Alimentary Canal and its glandular appendages, the former, beginning with the MOUTH, is successively made up of the OESOPHAGUS, the STOMACH, the small intestine or "ileum", and the large intestine or "rectum" (with the Caeca when present), which last opens into the Cloaca.  The glandular appendages are either proventricular and other mucous glands, imbedded in the walls of the Canal, or salivary glands, LIVER, and PANCREAS, communicating with it through special ducts. The function of the System is of two separate kinds: first the preparation of the food, which is effected in part mechanically and in part by chemically-acting secretions of the accessory glands; and secondly the absorption of the "chyle", or prepared nutritive fluid, by means of the LYMPHATIC SYSTEM.

The Digestive Process

The digestive process is as follows:- The food taken into the mouth is swallowed and passes through the oesophagus into the stomach, assisted in its descent by the secretions of the salivary and mucous glands. When there is a Crop, it is therein mixed with saliva and water, and assisted by the heat of the body is softened aud acted upon in a preliminary way.

It then enters the stomach, where it meets with the secretions of the proventricular or gastric glands. But beside being acted upon chemically it is crushed and triturated in the gizzard, especially in graminivorous and granivorous birds, which possess a strong muscular stomach. Thus comminuted it is known as "chyme," and passes through the pylorus into the small intestine, in the first loop of which, the "duodenum," it is mixed with the bile and pancreatic juice, these two fluids being the secretions of the liver and the pancreas.  Their principal action is to convert its soluble parts into "peptones", which are to be conveyed into the Lymphatic System, and so into the Blood.  Their absorption as chyle is effected by numerous "villi" or projections which line the walls of the whole Canal from the pylorus to the cloaca.

At the beginning of the rectum the caeca, when such are functional, receive the remaining chyme, and it is probable that in them certain hitherto undissolved matter, as cellulose and possibly chitin, is acted upon by methane, so as to extract as much nutrition as possible from the food.  After remaining a due time in the caeca, their contents return to the retum, and are finally ejected through the cloaca as faeces.

The Alimentary Canal

The walls of the Alimentary Canal are composed of five layers,of which the innermost only is of "endodermal" origin, the rest being "mesodermal" (see EMBRYOLOGY). These layers are:
  1. The tunica serosa or adventitia, which is outermost and consists of partly elastic connective tissue.
  2. A layer of smooth muscular fibres, transversely or circularly arranged.
  3. One of smooth muscular fibres, longitudinally arranged.
  4. The tunica submucosa of loose connective tissue, which contains nerves, blood, and lymphatic vessels.
  5. The tunica mucosa or innermost lining, composed of epithelial cells, which give rise to mucous and various specific digestive glands.
It is noteworthy that Birds and Reptiles differ from Mammals in the succession of the two muscular layers (2 and 3), since in the last the circular fibres are placed on the inside, next to the submucosa (4), while the longitudinal fibres together with the serosa (1) form the outer wall. These layers vary considerably in the different parts of the Alimentary Canal; thus the thickening of the walls of the gizzard is due to the excessive development of the muscular, layers, while in the oesophagus the mucosa is represented chiefly by ordinary epithelial cells, comparatively few of which form simple mucous glands, though in the region of the proventriculus its cells are transformed into large glands, often closely packed and compressed, constituting the greater part of the thickened walls. Again, in the gizzard no such specific, but only mucous glands occur, the hardened secretion of which invests its cavity with an additional cuticular lining.

Both the small and large intestines are characterized by numerous villi protruding into the canal as excrescences of the two innermost layers, and absorbing the prepared nutritive fluid. Beside the ordinary mucous glands the mucosa gives rise to two masses of specific nature which as LIVER and PANCREAS grow out of the walls of the duodenum, and thus indicate their point of origin only by their respective ducts.
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