The word "瞳仁" is formed by combining the character "瞳", meaning pupil and constructed from the radical "目" (eye) indicating its category and the phonetic component "童" suggesting pronunciation, with the character "仁", which here denotes kernel or core, metaphorically describing the pupil as the essential part of the eye. This results in a compound where both characters semantically reinforce the specific meaning of the eye's pupil.