The digestive system consists of two parts: the alimentary canal and the digestive glands. The digestive duct is a long muscular duct that starts from the mouth and continues to the pharynx, esophagus, stomach, small intestine, large intestine and finally the anus, including the mouth, pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum) and large intestine (cecum, colon, rectum). There are two types of digestive glands: small digestive glands and large digestive glands. The small digestive glands are scattered in the walls of each part of the digestive duct, while the large digestive glands include three pairs of salivary glands (parotid, submandibular, and sublingual), liver, and pancreas, all of which discharge secretions into the digestive duct through ducts. The basic function of the digestive system is to digest and absorb food to provide the body with the materials and energy it needs. The nutrients in food, except for vitamins, water and inorganic salts, can be absorbed and used directly. The process of breaking down food into small, simple, absorbable molecules in the digestive tract is called digestion. The process by which these small molecules pass through the mucosal epithelium of the digestive tube into the blood and lymphatic fluid is called absorption. For the unabsorbed residue, the digestive tract passes through the large intestine and is excreted in the form of feces.