Early bronze forms of 牙 are usually read as a sideways picture of upper and lower teeth meshing together. Ancient writing could be rotated or reversed, so the old graph is easier to recognise when imagined on its side. This sketch separates the two uneven rows and lets their central teeth interlock; it is an ancient-form reconstruction, not a dental diagram.
A piece of culture
A daily phrase at the sink 刷牙 means “to brush your teeth”: shuā yá in Mandarin and caat3 ngaa4 in Cantonese. The same 牙 appears in 牙刷, yáshuā / ngaa4 caat3, “toothbrush”. Notice how the two characters simply swap places: the action is brush + teeth, while the object is teeth + brush.