γελανόω
From LSJ
γεγόναμεν γὰρ πρὸς συνεργίαν ὡς πόδες, ὡς χεῖρες, ὡς βλέφαρα, ὡς οἱ στοῖχοι τῶν ἄνω καὶ κάτω ὀδόντων. τὸ οὖν ἀντιπράσσειν ἀλλήλοις παρὰ φύσιν → we are all made for mutual assistance, as the feet, the hands, and the eyelids, as the rows of the upper and under teeth, from whence it follows that clashing and opposition is perfectly unnatural
