38.1a The mucosa is the innermost lining of the digestive tract (Applegate, 2000, 330). In some regions the surface is folded to increase surface area. The mucosa contains different types of cells that secrete mucus, digestive enzymes, and hormones. Ducts from other glands penetrate the mucosa to open into the lumen. In the stomach and intestines, the epithelium is simple columnar for secretion and absorption, but in the mouth and anus, where protection against abrasion is required, the epithelium is stratified squamous. The mucosa is a mucous membrane consisting of epithelium lying over a layer of loose connective tissue known as the lamina propria, and a thin layer of smooth muscle called the muscularis mucosa.
b The submucosa consists of a thick layer of loose connective tissue containing blood vessels and lymphatic vessels, nerves, and some glands. Blood vessels provide nourishment to the surrounding tissues, and the blood and lymph vessels carry away absorbed nutrients resulting from digestion. The nerves of the submucosa form a network known as the submucosal plexus, or Meissner's plexus, and provide autonomic nerve impulses to the muscle layers of the digestive tract.
c The muscularis externa layer of the digestive tract consists of two layers of muscle fibers, the inner layer of which are arranged in a circular fashion around the diameter of the digestive tract, and an outer layer of muscle fibers arranged longitudinally along the length of the tract (231). This layer also contains a network of nerves - Auerbach's plexus (the myenteric plexus) which lies between the two muscle layers. Together with Meissner's plexus, Auerbach's plexus controls movements of the digestive tract, stimulated by the parasympathetic nervous system and inhibited by the sympathetic nervous system.
d The outermost layer of the digestive system is called the adventitia above the diaphragm and the serosa below the diaphragm. The...