The gastrointestinal tract (GIT) is divided into distinct subunits, with morphological and functional differences: mouth including teeth, esophagus, stomach, pancreas and biliary tract, liver, duodenum, jejunum, ileum, colon, and anus. The different functions are reflected in macroscopic and microscopic anatomy. Macroscopically, the stomach, for instance, has longitudinal, oblique, and circular muscle layers to mix and churn food and so aid digestion. Microscopically, the cells lining the stomach include specialized H cells that produce hydrochloric acid.