A soft, pink blanket that lines all of the natural openings/holes in the body. The mouth, eyes, nose, throat, vagina, anus and the hole in the penis (the urethra) have these linings. MEMBRANES have small holes in them. If a STD gets on it then it can go inside someone's body. You can look inside your mouth and see MUCOUS MEMBRANES. They are on the inside of your cheeks.