* Curling of the upper [[Lips_-_Anatomy_%26_Physiology|lip]] closes the '''nostrils''' and allows negative pressure to form in the '''nasopalatine duct'''. | * Curling of the upper [[Lips_-_Anatomy_%26_Physiology|lip]] closes the '''nostrils''' and allows negative pressure to form in the '''nasopalatine duct'''. |