• Esophagus, esophageal-cardiac junction, stomach and pylorus.
• Duodenum and the rest of the small intestine, the large intestine (appendix and colon) and the anal canal.
• The two large glands, the pancreas and liver, which deliver their secretions into the duodenum.

The pancreas and liver have both exocrine and endocrine secretions. The exocrine pancreas is composed of glandular acini secreting many digestive enzymes into excretory ducts. The endocrine pancreas is composed of islets of Langerhans that secrete insulin and glucagon, which regulate the metabolism of carbohydrates. The liver cells (hepatocytes) secrete bile in an elaborate system of canaliculi and ducts that carry bile to the gallbladder and, via the common bile duct, to the duodenum. The same hepatocytes metabolize a wide variety of nutritive or other substances (hormones, drugs, etc.) and deliver metabolites directly into the blood circulation.

The histology of the stomach and intestines and of large glands mentioned above is complex because of the multiplicity of cell types, along with their close packing and variable arrangements. Gaining a clear notion of the histological characteristics of these tissues therefore requires scanning and accurately reading a great many sections and fields at low and high magnifications of the light microscope. This explains why this chapter presents a large number of microphotographs.