The cervix or neck is the lowest part of the uterus and the vaginal portion projects into the vault of the vagina. The cervical canal is 2.5cm. long, and the constricted area where the cervix and isthmus meet forms what is known as the internal os (mouth); the external os is a small round opening at the lowest point of the uterus, but after childbirth it is a transverse slit. In structure the cervix differs from the body of the uterus, containing less muscular and more elastic tissue. Lining the canal is mucous membrane, with glands which secrete alkaline mucous.