Complex sentence: There are from fifteen to twenty incomplete C-shaped tracheal rings of cartilage that reinforce the anterior and lateral sides of the trachea to protect and maintain the airway, leaving a membranous wall ("pars membranacea") dorsally without cartilage.
Split into simpler sentences: There are from fifteen to twenty incomplete C-shaped tracheal rings of cartilage that reinforce the front and sides of the trachea to protect and maintain the airway. This leaves a membranous wall ("pars membranacea") at the back, without cartilage.