Genital canal in females.
Together with the cavity of the uterus , it forms the birth canal. In most virgins, its external opening is partially closed by a thin fold of tissue (hymen), which has various forms, not all of which significantly obstruct the opening. The vagina's lining thickens and thins during the menstrual cycle (see menstruation ) in response to estrogen from the sexually transmitted diseases , {{link=candida">candida ), vaginitis , sores (see ulcer ), and prolapse .