Tunnel through which the fetus must pass during birth. The birth canal comprises the pelvic bone and the soft tissues surrounding it. The osseous section of the canal forms an irregular cylinder lying between the superior and inferior straits, also called pelvic cavity. The diameters of the bone components of the canal are of crucial importance: they determine the mechanism of birth and its very feasibility. The soft parts of the canal are the lower segment of the uterus, the cervix, the vagina, and the vulva. The changes in the birth canal—such as the dilation of the cervix and the distension of the lower segment—are the dynamic phenomena involved in birth.