The lower portion of the uterus forms a narrow structure which forms the [[1]]. 

The cervix is the lower part of the uterus that connects the uterus to the vagina. 

It is cylindrical in shape and forms a narrow passageway between the uterus and the vagina. 

The opening of cervix can dilate or open during childbirth to allow the baby to pass through the birth canal. 

The cervix also plays an important role in preventing infection, as it produces mucus that blocks the entry of bacteria into the uterus.