According to About.com, mucous membranes line parts of the body that open to the outside and are exposed to air. They include the lining of the digestive tract, such as the mouth; the lining of the respiratory tract, such as the nose; and the lining of the urogenital tract. Mucous membranes are also found in the eyes; these membranes are called conjunctival membranes.
About.com notes that mucous membranes consist of epithelial tissue that covers and protects underlying connective tissue. They have plenty of mucous glands to keep them moist. Dry mucous membranes cause numerous health problems, such as dehydration. For instance, dry mucous membranes in the nose lining can cause frequent nose bleeding.
According to the Encyclopaedia Brittanica, mucous membranes line bodily cavities and canals, mainly the digestive, respiratory and urogenital tracts. They line various tracts and structures of the body, including the eyelids, nose, mouth, windpipe, lungs, stomach, intestines, ureters, urethra and urinary bladder. They have different structures, but they all have a surface layer that is composed of epithelial cells on top of a deeper layer of connective tissue. These membranes are called mucous, as they have cells that secrete mucin, which is a mucopolysaccharide primarily found in mucus.