Line the interior walls of the organs and tubes opening to the outside of the body, such as those of the digestive, respiratory, urinary, and reproductive systems. These membranes are lined with epithelium, which are involved in absorption and secretion. The mucous membrane is composed of epithelium orverlaying a layer of connective tissue caleld lamina propria. In some instances, the lamina propria rests on a third layer of smooth muscle cells. Not every mucous membrane secretes mucous.