A. Cervix: The narrow, tubular, inferior portion (“neck”) of the uterus that protrudes into the vagina; it contains the external os. B. Perimetrium: The outer serosal covering of the uterus (visceral peritoneum); not a tube and doesn’t project into the vagina. C. Vestibule: The external genital space between the labia minora with the urethral and vaginal openings; not part of the uterus. D. External os: Merely the opening of the cervix into the vagina—not the entire projecting tubular structure.