The walls of the digestive tract have four layers of tissue: mucosa, submucosa, muscularis externa and serosa. The inner-most layer is the mucosa, a membrane that forms a continuous lining of the GI tract from the mouth to the anus. In the large bowel, this tissue contains cells that produce mucus to lubricate and protect the smooth inner surface of the bowel wall. Connective tissue and muscle separate the muscosa from the second layer, the submucosa, which contains blood vessels, lymph vessels,