plexus, the myenteric plexus (Auerbach’s plexus), is present between the outer and inner muscle layers. As in the submucosal plexus (Meissner’s plexus), nerves and ganglion cells are present here. This plexus innervates the muscularis externa and produces peristaltic activity.
As noted, the esophagus is fixed to adjoining structures throughout most of its length; thus, its outer layer is composed of adventitia. After entering the abdominal cavity, the short remainder of the tube is covered by serosa, the visceral peritoneum.
Mucosal and submucosal glands of the esophagus secrete mucus to lubricate and protect the luminal wall.
Glands are present in the wall of the esophagus and are of two types. Both secrete mucus, but their locations differ.
FIGURE 17.4 • Photomicrograph of an esophageal submucosal gland. This photomicrograph shows a mucicarmine-stained section of the esophagus. An esophageal gland, deeply stained red by the carmine, and an adjacent excretory duct are seen in the submucosa. These small, compound, tubuloalveolar glands produce mucus that lubricates the epithelial surface of the esophagus. Note the stained mucus within the excretory duct. The remaining submucosa consists of irregular dense connective tissue. The inner layer of the muscularis externa (bottom) is composed of circularly arranged smooth muscle. 110.
 Esophageal glands proper lie in the submucosa. These glands are scattered along the length of the esophagus but are somewhat more concentrated in the upper half. They are small, compound, tubuloalveolar glands (Fig. 17.4). The excretory duct is composed of stratified squamous epithelium and is usually conspicuous when present in a section, because of its dilated appearance.
 Esophageal cardiac glands are named for their similarity to the cardiac glands of the stomach and are found in the lamina propria of the mucosa. They are present in the terminal part of the esophagus and frequently, though not consistently, in the beginning portion of the esophagus.
The mucus produced by the esophageal glands proper is slightly acidic and serves to lubricate the luminal wall. Because the secretion is relatively viscous, transient cysts often occur in the ducts. The esophageal cardiac glands produce neutral mucus. Those glands near the stomach tend to protect the esophagus from regurgitated gastric contents. Under certain conditions, however, they are not fully effective, and excessive reﬂux results in pyrosis, a condition more commonly known as heartburn. This condition may progress to fully developed gastroesophageal refux disease (GERD).
The muscle of the esophageal wall is innervated by both autonomic and somatic nervous systems.
The striated musculature in the upper part of the esophagus is innervated by somatic motor neurons of the vagus nerve, cranial nerve X (from the nucleus ambiguus). The smooth muscle of the lower part of the esophagus is innervated by visceral motor neurons of the vagus (from the dorsal motor nucleus). These motor neurons synapse with postsynaptic neurons whose cell bodies are located in the wall of the esophagus.
The stomach is an expanded part of the digestive tube that lies beneath the diaphragm. It receives the bolus of macerated food from the esophagus. Mixing and partial digestion of the food in the stomach by its gastric secretions produce a pulpy fluid mix called chyme. The chyme then passes into the small intestine for further digestion and absorption.
The stomach is divided histologically into three regions based on the type of gland that each contains.