The trachea is a tube extending from the larynx to the bronchi in mammals, and from the pharynx to the syrinx in birds. The trachea has an inner diameter of about 25.4 millimeters (1.00 in) and a length of about 10 to 16 centimeters (3.9 to 6.3 in). It commences at the lower border of the larynx, level with the sixth cervical vertebra, and bifurcates into the primary bronchi at the vertebral level of thoracic vertebra T5, or up to two vertebrae lower or higher, depending on breathing. There are about fifteen to twenty incomplete C-shaped cartilaginous rings 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. The trachealis muscle connects the ends of the incomplete rings and contracts during coughing, reducing the size of the lumen of the trachea to increase the air flow rate. The esophagus lies posteriorly to the trachea. The cartilaginous rings are incomplete to allow the trachea to collapse slightly so that food can pass down the esophagus. A flap-like epiglottis closes the opening to the larynx during swallowing to prevent swallowed matter from entering the trachea. The natural airway formed by the trachea may be damaged or closed off because of illness or injury. Intubation is the medical procedure of inserting an artificial tube into the trachea to permit breathing. Diseases of the trachea are Tracheobronchitis, Tracheomalacia, Tracheal fracture, Airway obstruction, Malignancy.