The digestive (gastrointestinal) tract is a long hollow tube that extends from the esophagus to the rectum. It includes the esophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (colon), and rectum. The wall of the digestive tube exhibits four layers that show a basic histologic organization. The layers are the mucosa, submucosa, muscularis externa, and serosa or adventitia. Because of the different functions of the digestive organs in the digestive process, the
morphology of these layers exhibits variations.

The mucosa is the innermost layer of the digestive tube. It consists of a covering epithelium and glands that extend into the underlying layer of loose connective tissue called the lamina propria. An inner circular and outer longitudinal layer of smooth muscle, called the muscularis mucosae, forms the outer boundary of the mucosa.

The submucosa is located below the mucosa. It consists of dense irregular connective tissue with numerous blood and lymph vessels and a submucosal (Meissner’s) nerve plexus. This nerve plexus contains postganglionic parasympathetic neurons. The neurons and axons of the submucosal nerve plexus control the motility of the mucosa and secretory activities of associated mucosal glands. In the initial portion of the small intestine, the duodenum, the submucosa contains numerous branched mucous glands.