THE TRACHEA. 361 



the superior laryngeal, a branch of the vagus, which supplies the 

 mucous membrane, the cricothyroid and arytenoid muscles; and 

 the inferior laryngeal, or recurrent laryngeal, also a branch of the 

 vagus, which supplies all the other muscles, and also the arytenoid 

 muscle. 



THE TRACHEA. 



The trachea or windpipe (Fig. 202) is about 11 cm. in length, 

 and 2.5 cm. in diameter, and extends from the cricoid cartilage to 

 its division into the bronchi, which corresponds to the fourth or 

 fifth dorsal vertebra. 



Structure. It is composed of rings of cartilage, from 16 to 

 20 in number, which are incomplete behind, where the trachea is 

 in contact with the esophagus. These cartilaginous rings are situ- 



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FIG. 202. From longitudinal section of human trachea, stained in orcein (Huber). 



ated within the two layers of an elastic fibrous membrane. In 

 the spaces between the rings these layers unite, thus forming 

 a single membrane. The membrane behind is also single. Be- 

 tween the ends of the cartilaginous rings is also a transverse layer 

 of unstriped muscular tissue, trachealis muscle, and posterior to 

 this are some longitudinal fibers of the same kind. The trachea 

 is lined with mucous membrane covered with columnar ciliated 

 epithelium, and in it are mucous glands, tracheal glands, whose 

 secretion lubricates the membrane, and there are elastic fibers 

 arranged longitudinally. 



Blood-vessels. The artery supplying the trachea is the 

 inferior thyroid, and its veins terminate in the thyroid venous plexus. 



Nerves. The nerves are branches of the vagus and sympa- 

 thetic. % 



