 Inferior (lower) esophageal sphincter. As the name implies, this sphincter is located at the lower end of the esophagus; its action is reinforced by the diaphragm that surrounds this part of the esophagus as it passes into the abdominal cavity. It creates a pressure difference between the esophagus and stomach that prevents reflux of gastric contents into the esophagus. Abnormal relaxation of this sphincter allows acidic content of the stomach to return (reﬂux) into the esophagus. If not treated, this condition may progress into gastroesophageal refux disease (GERD), characterized by  inﬂammation of the esophageal mucosa (reﬂux esophagitis), strictures, and difficulty in swallowing (dysphagia) with accompanying chest pain.
 Pyloric sphincter. Located at the junction of the pylorus of the stomach and duodenum (gastroduodenal sphincter), it controls the release of chyme, the partially digested contents of the stomach, into the duodenum.
 Ileocecal valve. Located at the junction of the small and large intestines, it prevents reflux of the contents of the colon with its high bacterial count into the distal ileum, which normally has a low bacterial count.
 Internal anal sphincter. This, the most distally located sphincter, surrounds the anal canal and prevents passage of the feces into the anal canal from the undistended rectum.
Serosa or adventitia constitutes the outermost layer of the alimentary canal.
The serosa is a serous membrane consisting of a layer of simple squamous epithelium, called the mesothelium, and a small amount of underlying connective tissue. It is equivalent to the visceral peritoneum described in gross anatomy. The serosa is the most superficial layer of those parts of the digestive tract that are suspended in the peritoneal cavity. As such, the serosa is continuous with both the mesentery and the lining of the abdominal cavity.
Large blood and lymphatic vessels and nerve trunks travel through the serosa (from and to the mesentery) to reach the wall of the digestive tract. Large amounts of adipose tissue can develop in the connective tissue of the serosa (and in the mesentery).
Parts of the digestive tract do not possess a serosa. These include the thoracic part of the esophagus and portions of structures in the abdominal and pelvic cavities that are fixed to the cavity wall—the duodenum, ascending and descending colon, rectum, and anal canal. These structures are attached to the abdominal and pelvic wall by connective tissue, the adventitia, which blends with the connective tissue of the wall.
The esophagus is a fixed muscular tube that delivers food and liquid from the pharynx to the stomach.
The esophagus courses through the neck and mediastinum, where it is attached to adjacent structures by connective tissue. As it enters the abdominal cavity, it is free for a short distance, approximately 1 to 2 cm. The overall length of the esophagus is about 25 cm. On cross section (Fig. 17.2), the lumen in its normally collapsed state has a branched appearance because of longitudinal folds. When a bolus of food passes through the esophagus, the lumen expands without mucosal injury.