Like people a pig has a mouth with teeth, which it uses to grind food up and mix with saliva before swallowing. The mixture of saliva and feed passes down the esophagus into the stomach. The mixture at this point has begun to breakdown or be digested by the enzymes in the saliva. Once in the stomach there is stimulation for the secretion of gastric juices from the stomach's lining. The ph in the stomach is low due to the presence of acids. Therefore although some additional breakdown of the food occurs the passing of the partially digested food into the small intestine is important for complete digestion.
The small intestine maintains a more alkaline environment, which is useful in digestion. The digestive enzymes some of which were excreted in the stomach and others in the small intestines can now work. Bile is also added to help the digest of fats by the gall bladder in this part of the digestive tract. The goal of the relatively long small intestines (compared to other parts of the digestive tract) is to break large feed particles into the tiniest components, which can then be absorbed thru the intestinal wall (villi).
After passing thru the small intestines the food reaches the large intestines where water is absorbed. The remaining products are waste and are excreted via the rectum.
Poultry have a slightly modified version of the digestive tract described above. The first important change is in the mouth. Poultry have a beak but do not use teeth. They mix the food with saliva but virtually no mastication (breaking of food particles into smaller pieces) occurs in the mouth.
Once a bird swallows its food it passes down the esophagus a short distance to a large pouch. This pouch called a crop is a kind of storage area in the bird's digestive tract. It is here that saliva begins to work at breaking down the food. The food is next moved downward to the proventiculus. It is still like the crop a portion of the esophagus, but it is also sometimes referred to as the glandular stomach. This name comes from its role in introducing hydrochloric acid and digestive enzymes to the food.
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