The vagina is a fibromuscular tubular sex organ that is part of the female genital tract. In humans, the vagina extends from the vulva to the uterus. At the vulva, the vaginal orifice may be partly covered by a membrane called the hymen, while, at the deep end, the cervix (neck of the uterus) bulges through the anterior wall of the vagina. The vagina facilitates sexual intercourse and childbirth. It also channels the menstrual flow, consisting of blood and pieces of mucosal tissue, that occurs periodically with the shedding of lining of the uterus in menstrual cycles. 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 orifice as the sole external urogenital orifice, mammalian females usually have two external orifices, the urethral orifice for the urological tract and the vaginal orifice for the genital tract. The vaginal orifice is much larger than the nearby urethral opening, and both openings are protected by the labia in humans. In amphibians, birds, reptiles and monotremes, an opening called the cloaca functions as a single external orifice for the gastrointestinal tract, urological tract, and reproductive tract. The vagina plays a significant role in human female sexuality and sexual pleasure. During sexual arousal for humans and other animals, vaginal moisture increases by way of vaginal lubrication, to reduce friction and allow for smoother penetration of the vagina during sexual activity. The texture of the vaginal walls can 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, health authorities and health care providers, recommend safe sex practices. 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. Colloquially, the word vagina is often used incorrectly to refer to the vulva.