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Trachea
The trachea, colloquially called the windpipe, is a tube that connects the pharynx and larynx to the lungs, allowing the passage of air, and so is present in almost all air-breathing animals with lungs. Only in the lungfish, where the lung is connected to the pharynx and the larynx, is it absent. The trachea extends from the larynx and branches into the two primary bronchi. At the top of the trachea the cricoid cartilage attaches it to the larynx. This is the only complete ring, the others being incomplete rings of reinforcing cartilage. The trachealis muscle joins the ends of the rings and these are joined vertically by bands of fibrous connective tissue, the ''annular ligaments of trachea''. The epiglottis closes the opening to the larynx during swallowing.
The trachea develops in the second month of development. It is lined with an epithelium that has goblet cells which produce protective mucins. An inflammatory condition, also involving the larynx and bronchi, called croup can result in a barking cough. A tracheotomy is often performed for ventilation in surgical operations where needed. Intubation is also carried out for the same reason by the inserting of a tube into the trachea. From 2008, successful operations have transplanted a windpipe grown by stem cells, and synthetic windpipes.
==Structure==
The human trachea has an inner diameter of about and a length of about . It commences at the lower border of the larynx, level with the sixth cervical vertebra. Inside the trachea at the level of the fifth thoracic vertebra (T5) there is a cartilaginous ridge known as the carina of trachea which runs across from the front to the back of the trachea and marks the point of bifurcation into the right and left primary bronchi. The carina is opposite the sternal angle and can be positioned up to two vertebrae lower or higher, depending on breathing.
A ring of hyaline cartilage called the cricoid cartilage forms the inferior wall of the larynx and is attached to the top of the trachea. The cricoid cartilage is the only complete ring of cartilage in the trachea. Below this 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 at the back, 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 rate of air flow. The esophagus lies posteriorly to the trachea. Circular horizontal bands of fibrous tissue called the annular ligaments of trachea join the tracheal rings together. 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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