Cricoid cartilage

The cricoid cartilage is the only complete ring of cartilage in the trachea, the rest being C-shaped and open in the back.  The cricoid is immediately inferior to the thyroid cartilage, and serves as the posterior attachment site for the vocal ligaments.  

Because the cricoid cartilage is complete, it can be pushed on (“cricoid pressure”, or the Sellick maneuver) to move the airway posteriorly, which is often done when inserting an endotracheal tube (breathing tube).  The pressure compresses the esophagus, which prevents gastric reflux and reduces the risk of of aspiration.  Currently there is mounting criticism of the standard use of cricoid pressure (see figure below).