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.t 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.