Histology of alimentary canal
Mucosa: surface epithelium (simple columnar), a lamina propria (aerolar connective on which the epithelial layer rests), and a muscularis mucosae (a scant layer of smooth muscle fibers that enable local movements of the mucosa). The function is to secrete (enzymes, mucus, hormones, etc), absorb digested foodstuffs, and protection (against bacterial invasion).
Submucosa: dense connective tissue containing blood and lymphatic vessels, scattered lymphoid follicles, and nerve fibers. Its intrinsic nerve supply is called the submucosal plexus. Its major functions are nutrition and protection.
Muscular externa: a bilayer of smooth muscle with the inner layer running circularly and the outer layer running longitudinally. Another important intrinsic nerve plexus, the myenteric plexus. By controlling the smooth muscle of the muscularis, this plexus is the major regulator of GI motility.
Serosa: aka visceral peritoneum. It consists of a thin layer of areolar connective tissue. In areas outside the abdominoplevic cavity, the serosa is replaced by an adventitia, a layer of coarse fibrous connective tissue that binds the organ to surrounding tissues. The serosa reduces friction as te mobile digestive system organs work and slide across one another and the cavity walls. The adventitia anchors and protectors the surrounded organ.