The digestive system consists of a long muscular tube, extending from mouth to anus, and a large number of associated glands which secrete substances that aid food digestion. The digestive tube is generally made up of four structurally and functionally distinct layers: the mucosa, the submucosa, the muscularis, and the serosa (or the adventitia, if not covered by mesothelium). The mucosa, which is in contact with the luminal contents, is constructed to resist abrasion and to perform secretory and absorptive functions. The muscular layer which serves to propel the food through the tube is attached to the mucosa by way of a loose CT layer, the submucosa. The serosa or adventitia carries blood vessels and nerves to the wall of the digestive tube.

The large number of glands associated with the digestive canal range from unicellular components and small tubuloalveolar units residing in the mucosa to very large organs such as the pancreas and liver. Although their secretions can be quite dissimilar, they all function to promote the digestive process by imparting enzymes or mucus into the ingested food. Bile, the exocrine secretion of the liver, is stored and concentrated in another organ, the gall bladder.

Learning Objectives:

• Recognize and understand the general structural characteristics of the gastrointestinal tract: mucosa, submucosa, muscularis, and serosa.
• Understand the regions and parts of a tooth and how these structures are formed.
• Understand the functional significance of structures associated with the tongue.
• Recognize and understand the function of the different glands associated with the gastrointestinal tract.
• Recognize the various regions of the gastrointestinal tract (esophagus, stomach, small and large bowels), the major specific cell types specific to each region and how they contribute to digestive function.
• Understand the structure and exocrine function of the pancreas.
• Understand the micro architecture and functions of the liver with regard to its circulatory system and the production of bile. Recognize the structure and function of the gall bladder.

Let's start with the oral cavity.