Why are the cartilage rings of the trachea incomplete at the back?

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Illustration showing different animals and their respiratory organs: a spider with book lungs, a chicken with lungs, a sponge with labeled inflow and outflow pores, a fish with gills, and a caterpillar with spiracles. — Respiratory system Different methods of respiration in animals.

The trachea is supported by 16 to 20 horseshoe-shaped cartilage rings that encircle the front of the airway. These rings are incomplete at the back, where the trachea lies directly against the esophagus.

This anatomical arrangement is functional: the open ends of the cartilage are connected by muscle bands, allowing the esophagus to expand into the space behind the trachea during the swallowing of food. Furthermore, the muscle bands connecting these free ends can contract during a cough, narrowing the tracheal lumen to increase the velocity of exhaled air, which helps dislodge foreign particles from the respiratory tract.1