The birth canal otherwise know as the vagina. Its function in childbirth is to open fully.
The birth canal is made up of ridges created by folds in the skin.  The ridges create pleasure when having sexual intercourse but during childbirth flatten out and help the canal to open up.

There are lubricating glands on either side which keep the area moist and slippery during child birth and when having sexual intercourse and lots of tiny blood vessels in this area.  The vagina is approximately 7.5 cm long at the front and approximately 10cm long at the back.