What is the volcano's mouth?

The volcano is the parlour in Montag's home.

He was eating a light supper at nine in the evening when the front door cried out in the hall and Mildred ran from the parlour like a native fleeing an eruption of Vesuvius.Mrs. Phelps and Mrs. Bowles came through the front door and vanished into the volcano's mouth with martinis in their hands: Montag stopped eating.