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lowest region of the uterus; it attaches the uterus to the vagina and provides a passage between the vaginal cavity and the uterine cavity. The cervix, only about 4 centimetres (1.6 inches) long, projects about 2 centimetres into the upper vaginal cavity. The cervical opening into the vagina is called the external os; the cavity running the length of the cervix is the endocervical canal; the opening of the endocervical canal into the uterine cavity, the internal os. The endocervical canal transports sperm into the uterine cavity, allows the escape of blood from the uterus during menstruation, and supplies mucus (a thick lubricating protein) to the female reproductive tract. During childbirth the canal is greatly stretched (see parturition).