GENERAL FEATURES OF THE DIGESTIVE TRACT

-A. Components: The digestive tract is a series of organs forming a long muscular tube whose continuous lumen opens to the exterior at both ends. The organs include the oral cavity, oral pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (cecum and appendix; ascending, transverse, and descending colon), rectum, and anal canal.
-B. General Structural Features: The walls of each organ consist of 4 concentric layers: the mucosa, submucosa, muscularis externa, and serosa or adventitia
-1. Mucosa, This layer borders the lumen and has 3 parts. The epithelium (mucous membrane) derives from endoderm. It is stratified squamous in the oral cavity, oral pharynx, esophagus, and anal canal; it is simple columnar in the stomach, intestines, and rectum. The lamina propria is a layer of loose connective tissue beneath the endothelium; it contains small blood and lymphatic vessels. The muscularis mucosae is a thin layer of smooth muscle bordering the submucosa.
2. Submucosa, This dense, irregular connective tissue layer contains blood and lymphatic vessels and the submucosal (Meissner's) plexus of nerves. Some organs are characterized by glands and lymphoid nodules in this layer.
3. Muscularis externa, This consists of 2 layers of smooth muscle--an inner circular and an outer longitudinal-through most of the tract. Between them lies the myenteric (Auerbach's) plexus. The muscle around the oral cavity is skeletal; where it is absent leg, hard palate, gingiva) the submucosa binds tightly to bone. In the upper esophagus, this layer contains mainly skeletal muscle, which is replaced by smooth muscle in the lower portion. The stomach's muscularis externa has 3 layers: outer longitudinal, middle circular, and inner oblique. The colon's outer longitudinal layer is gathered into 3 bands, the taeniae coli. The smooth and skeletal muscles encircling the anal canal form involuntary and voluntary sphincters, respectively.
4. Serosa and adventitia. The tract's outer covering differs by location. The esophagus and rectum are surrounded and held in place by a connective tissue adventitia like that around blood vessels. Intraperitoneal organs (stomach, jejunum, ileum, transverse and sigmoid colon) are suspended by mesenteries and covered by a serosa composed of a thin layer of loose connective tissue covered by simple squamous epithelium (mesothelium). Retro peritoneal organs (duodenum, ascending and descending colon) are bound to the posterior abdominal wall by adventitia and covered on their free (anterior) surfaces by serosa.