In addition, the glycocalyx consists of glycoproteins that project from the apical plasma membrane of epithelial absorptive cells. It provides additional surface for adsorption and includes enzymes secreted by the absorptive cells that are essential for the final steps of digestion of proteins and sugars. The epithelium selectively absorbs the products of digestion both for its own cells and for transport into the vascular system for distribution to other tissues.
The secretory function of the mucosa provides lubrication and delivers digestive enzymes, hormones, and antibodies into the lumen of the alimentary tube.
Secretion is carried out largely by glands distributed throughout the length of the digestive tube. The various  secretory products provide mucus for protective lubrication, as well as buffering of the tract lining and substances that assist in digestion, including enzymes, hydrochloric acid, peptide hormones, and water (see Fig. 17.1). The mucosal epithelium also secretes antibodies that it receives from the underlying connective tissue.
The glands of the alimentary tract (see Fig. 17.1) develop from invaginations of the luminal epithelium and include  mucosal glands that extend into the lamina propria.  submucosal glands that either deliver their secretions directly to the lumen of mucosal glands or via ducts that pass through the mucosa to the luminal surface.  extramural glands that lie outside the digestive tract and deliver their secretions via ducts that pass through the wall of the intestine to enter the lumen. The liver and the pan creas are extramural digestive glands (see Chapter 18) that greatly increase the secretory capacity of the digestive sys tem. They deliver their secretions into the duodenum, the first part of the small intestine.
The lamina propria contains glands, vessels that transport absorbed substances, and components of the immune system.
As noted, the mucosal glands extend into the lamina propria throughout the length of the alimentary canal. In addition, in several parts of the alimentary canal (e.g., the esophagus and anal canal), the lamina propria contains aggregations of mucus-secreting glands. In general, they lubricate the epithelial surface to protect the mucosa from mechanical and chemical injury. These glands are described below in relation to specific regions of the digestive tube.
In segments of the digestive tract in which absorption  occurs, principally the small and large intestines, the absorbed products of digestion diffuse into the blood and lymphatic vessels of the lamina propria for distribution. Typically, the blood capillaries are of the fenestrated type and collect most of the  absorbed metabolites. In the small intestine, lymphatic capillaries are numerous and receive some absorbed lipids and proteins.
The lymphatic tissues in the lamina propria function as an integrated immunologic barrier that protects against pathogens and other antigenic substances that could potentially enter through the mucosa from the lumen of the alimentary canal. The lymphatic tissue is represented by  diffuse lymphatic tissue consisting of numerous lym phocytes and plasma cells, located in the lamina propria, and lymphocytes transiently residing in the intercellular spaces of the epithelium. lymphatic nodules with well-developed germinal centers.  eosinophils, macrophages, and sometimes neutrophils.
The diffuse lymphatic tissue and the lymphatic nodules are referred to as gut-associated lymphatic tissue (GALT). In the distal small intestine, the ileum, extensive aggregates of nodules, called Peyer’s patches, occupy much of the lamina propria and submucosa. They tend to be located on the side of the tube opposite the attachment of the mesentery. Aggregated lymphatic nodules are also present in the appendix.
The muscularis mucosae forms the boundary between mucosa and submucosa.