Could you unpack this a little bit? It doesn't seem to refer to "heads" in the physical sense; that would make it a neurocognitive research question, not a linguistics question. It obviously doesn't refer to the social structures that must exist for any language to be used and passed from generation to generation; although those structures are fundamental to the actual existence of language, they exist "outside" our heads.
So--what is "in our heads" that enables us to speak?
Miles