The mouth-piece (3) and shield (1) have a flexible duct (6) through them which is resistant to chewing pressure and which joins the outer part in front of the mouth-piece with the outer part behind the mouth-shield. The duct contains several separate, parallel single ducts. The duct is formed by a separate, flexible pipe (5) going through the mouth-piece and shield and possessing greater flexural strength than the mouth-piece through which it passes.
