The vagina is a sex organ that is part of the female genital tract. It is a muscular and tubular in structure. In humans, the vagina extends from the vulva to the uterus. The outer vaginal opening may be partly covered by a membrane called the hymen. At the deep end, the cervix (neck of the uterus) bulges into the vagina. The vagina allows for sexual intercourse and childbirth. It channels menstrual flow, which occurs periodically as part of the menstrual cycle.The location and structure of the vagina varies among species, and may vary in size within the same species. Unlike mammalian males, who usually have the urethral opening as the only opening to the urinary tract, females usually have two external openings: the urethral opening for the urological tract and the vaginal opening for the genital tract. The vaginal opening is much larger than the nearby urethral opening, and both are protected by the labia in humans. In amphibians, birds, reptiles and monotremes, the cloaca is the single external opening for the gastrointestinal tract and the urinary and reproductive tracts.While the vagina plays a significant role in human and non-human sexuality, its role with regard to human female sexuality (including heterosexual and lesbian sexual activity) is especially documented. Less is known about the vagina of other animals. In human females and a number of other female mammals, vaginal moisture is increased during sexual arousal by vaginal lubrication, which reduces friction and allows for smoother penetration of the vagina during sexual activity. The texture of the vaginal walls create friction for the penis during sexual intercourse and stimulate it toward ejaculation, enabling fertilization. In addition, a variety of sexually transmitted infections (STIs) and other disorders can affect the vagina. Because of the risk of STIs, safe sex practices are recommended.Cultural perceptions of the vagina have persisted throughout history, ranging from viewing the vagina as the focus of sexual desire, a metaphor for life via birth, inferior to the penis, or as visually unappealing or otherwise vulgar. In common speech, the word vagina is often incorrectly used to refer to the vulva.