n.
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 ovaries , being thickest and most elastic during ovulation and pregnancy. Its normally slightly acid environment discourages disease-causing microorganisms. Thick, elastic muscle walls accommodate movement of the penis during intercourse and passage of a child during delivery. A mucuslike fluid seeps through them for lubrication during sexual arousal. Vaginal disorders include bacterial and fungal infections (e.g., sexually transmitted diseases , candida ), vaginitis , sores (see ulcer ), and prolapse .