Anatomical Types of Glands
Several types of glands provide the different types of alimentary tract secretions. First, on the surface of the epithelium in most parts of the gastrointestinal tract are billions of single-cell mucous glands called simply mucous cells or sometimes gobletcells because they look like goblets. They function mainly in response to local irri-tation of the epithelium: they extrude mucus directly onto the epithelial surface to act as a lubricant that also protects the surfaces from excoriation and digestion.
Second, many surface areas of the gastrointestinal tract are lined by pits that rep-resent invaginations of the epithelium into the submucosa. In the small intestine, these pits, called crypts of Lieberkühn, are deep and contain specialized secretory cells. One of these cells is shown in Figure 64–1.
Third, in the stomach and upper duodenum are large numbers of deep tubularglands. A typical tubular gland can be seen in Figure 64–4, which shows an acid- andpepsinogen-secreting gland of the stomach (oxyntic gland).
Fourth, also associated with the alimentary tract are several complex glands—the salivary glands, pancreas, and liver—that provide secretions for digestion or emul-sification of food. The liver has a highly specialized structure. The salivary glands and the pancreas are compound acinous glands of the type shown in Figure 64–2. These glands lie outside the walls of the alimentary tract and, in this, differ from all other alimentary glands. They contain millions of acini lined with secreting glandular cells; these acini feed into a system of ducts thatfinally empty into the alimentary tract itself.