The portion of the alimentary canal that extends from the proximal part of the esophagus to the distal part of the anal canal is a hollow tube of varying diameter. This tube has the same basic structural organization throughout its length. Its wall is formed by four distinctive layers. From the lumen outward (Fig. 17.1), they are as follows:  Mucosa, consisting of a lining epithelium, an underlying connective tissue called the lamina propria, and the muscularis mucosae, composed of smooth muscle  Submucosa, consisting of dense irregular connective tissue  Muscularis externa, consisting in most parts of two layers of smooth muscle underlying connective tissue. An adventitia consisting only of connective tissue is found where the wall of the tube is directly attached or fixed to adjoining structures (i.e., body wall and certain retroperitoneal organs).
The structure of the esophagus and gastrointestinal tract varies considerably from region to region; most of the variation occurs within the mucosa. The epithelium differs throughout the alimentary canal and is adapted to the specific function of each part of the tube. The mucosa has three principal functions: protection, absorption, and secretion.
FIGURE 17.1 • Diagram of general organization of the alimentary canal. This composite diagram shows the wall structure of the alimentary canal in four representative organs: esophagus, stomach, small intestine, and large intestine. Note that villi, a characteristic feature of the small intestine, are not present in other parts of the alimentary canal. Mucosal glands are present throughout the length of the alimentary canal but sparingly in the esophagus and oral cavity. Submucosal glands are present in the esophagus and duodenum. The extramural glands (liver and pancreas) empty into the duodenum. Diffuse lymphatic tissues and nodules are found in the lamina propria throughout the entire length of the alimentary canal (shown here only in the large intestine). Nerves, blood vessels, and lymphatic vessels reach the alimentary canal via the mesenteries or via the adjacent connective tissue (as in the retroperitoneal organs).
The histologic characteristics of these layers are described below in relation to specific regions of the digestive tube.
The epithelium of the mucosa serves as a barrier that separates the lumen of the alimentary canal from the rest of the organism.
The epithelial barrier separates the external luminal environment of the tube from the tissues and organs of the body. The barrier aids in protection of the individual from the entry of antigens, pathogens, and other noxious substances. In the esophagus, a stratified squamous epithelium provides protection from physical abrasion by ingested food. In the gastrointestinal portion of the alimentary tract, tight junctions between the simple columnar epithelial cells of the mucosa serve as a selectively permeable barrier. Most epithelial cells transport products of digestion and other essential substances such as water through the cell and into the extracellular space beneath the tight junctions.
The absorptive function of the mucosa allows the movement of digested nutrients, water, and electrolytes into the blood and lymph vessels.
The absorption of digested nutrients, water, and electrolytes is possible because of projections of the mucosa and submucosa into the lumen of the digestive tract. These surface projections greatly increase the surface area available for absorption, and vary in size and orientation. They consist of the following structural specializations (see Fig. 17.1):  Plicae circulares are circumferentially oriented submu cosal folds present along most of the length of the small intestine.  Villi are mucosal projections that cover the entire surface of the small intestine, the principal site of absorption of the products of digestion.  Microvilli are tightly packed, microscopic projections of the apical surface of intestinal absorptive cells. They fur ther increase the surface available for absorption.