The digestive tube consists of four major layers, or tunics: an internal mucosa and an external serosa with a submucosa and muscularis, or muscular layer, in between. These four tunics are present in all areas of the digestive tract from the esophagus to the anus. Three major types of glands are associated with the intestinal tract: (1) unicellular mucous glands in the mucosa, (2) multicellular glands in the mucosa and submucosa, and (3) multicellular glands (accessory glands) outside the digestive tract.
