Cervical canal
The cervical canal is a tube that connects with the uterine cavity through the internal os with the vagina through the external os. The internal os is smaller than the external os. The external os is a small, circular opening on the rounded end of the cervix. The external os opens into the vagina. The cervical cavity is located at the top of the vagina. The external os is surrounded by two lips, an anterior lip and a posterior lip. The anterior lip is shorter and thicker. Both lips are touching the vaginal wall. Before pregnancy, the external os has a rounded shape.  After pregnancy the os changes shape.