The yolk sac vestige (Meckel’s diverticulum) is noticeable and is located about midway on the small intestine. The mucosa of the small intestine becomes progressively thinner from the duodenum to the ileum as villi become shorter and crypt depth diminishes. Villi have an ellipsoid shape and are covered by enterocytes with microvilli and goblet cells whose orifice appear as pits on the surface. There are no intestinal submucosal glands in chickens, although in some species there are tubular glands that are homologous to these glands in mammals. At the posterior end of the ileum is a circular ring of muscular tissue projecting into the colon lumen as the ileal papilla that may serve as a valve at the ileo-cecal-colic junction. Entrances to the ceca are located immediately posterior to this ring.
The ceca are usually paired in birds and their size is influenced by diet (i.e., larger with higher-fibre diets). In most birds, a right and left ceca are present at the junction of the small and large intestines. Not all the food eaten by chickens and turkeys enters the ceca, and the ceca seem to have lesser importance in domestic fowls as compared with wild fowls. The most noticeable function of the ceca is related to the microbial digestion of cellulose. This is of greater importance for the energy needs of some wild species. Urine that has entered the colon from the cloaca may pass into the ceca via antiperistalsis. This kind of contraction of the intestine is the most striking feature of colonic motility and is believed to occur almost continuously. Because of antiperistalsis, the ceca are filled. The muscular ring of the ileum effectively prevents reflux of colonic material into the ileum. In the ceca, the uric acid present in the urine becomes a nitrogen source for the microorganisms associated with cellulose digestion. Also, water reabsorption from the reflux urine is another important function of ceca.
The digestive tract ends with the cloaca, the site that is common to digestive, reproductive, and urinary systems. The caudal opening to the exterior is known as the vent. The bursa of Fabricius is a dorsal diverticulum of the cloaca and is associated with the development of humoral immunity in birds. It is an important site for the preprocessing of B lymphocytes.
Another organ concern with digestion is the liver, which is bi-lobed. The left hepatic duct communicates directly with the duodenum, whereas the right duct sends a branch to gallbladder, or it may be enlarged locally as a gallbladder. Gallbladder is present in chickens, turkeys, ducks, and gees, but not in some other species, including the pigeon. The pancreas lies within the duodenal loop. It consists of at least three lobes, and its secretions reach the duodenum via three ducts, one from each lobe.