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 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.