Walk down the thyroid cartilage with your fingers until you come to the horizontal groove which separates it from the cricoid cartilage (the first tracheal ring). You should be able to feel a small indentation (it barely accepts the tip of your finger) between these 2 structures, directly in the mid-line. This is the crico-thyroid membrane, the site for emergent tracheal access in the event of upper airway obstruction.