part of head mouth BTO:0001090 BrendaTissueOBO The natural opening through which food passes into the body of an animal and which in vertebrates is typically bounded externally by the lips and internally by the pharynx and encloses the tongue, gums, and teeth. buccal cavity cavitas oris cavum oris mouth cavity oral cavity oral fissure oral vestibule rima oris vestibule of mouth vestibulum oris