The trachea is a short, flexible air tube that transports and conditions inspired air. It extends from the larynx to the middle of the thorax. At T4/T5 level, it divides into the two main primary bronchi. The lumen stays open because of the arrangement of a series of C shaped hyaline cartilaginous rings. Fibroelastic tissue and smooth muscle (trachealis muscle) bridge the gap between the free ends of the C shaped cartilages at the posterior border of the trachea. The trachea wall is developed into the following: