The invention relates to a pouch that includes a flexible casing (12) having an opening (12A), the edge of which is attached to the two opposite walls of a reinforcing sleeve (18) projecting outwardly from the casing. The pouch can assume a flattened configuration in which the walls of the sleeve are arranged against one another, and an operating configuration in which said walls form a channel that keeps the opening open. The first wall (20A) of the sleeve (18) has a first longitudinal end band (30A), whereas the second wall has a sliding edge (32B) capable of sliding against the inner surface of the first longitudinal band when the pouch shifts from the flattened configuration to the operating position thereof.
