VOCAL AND RESPIRATORY APPARATUS. 251 



TRACHEA AND BRONCHI. 



The trachea, or windpipe, is a membrane-cartilaginous tube,, 

 about four and a half inches in length, three-quarters to one inch 

 in width, extending from the fifth cervical to the upper border 

 of the fifth dorsal vertebra. It divides into two bronchi, the 

 right, about an inch long and nearly at right angles, passing 

 behind the right pulmonary artery on a level with the fifth dorsal 

 vertebra ; the left, narrower, about twice as long, passes beneath 

 the arch of the aorta on a level with the fifth or sixth dorsal 

 vertebra, behind the left pulmonary artery. The trachea and 

 bronchi are formed of a series of cartilaginous rings, incomplete 

 at their posterior third, connected by fibro-elastic membrane, 

 and lined by columnar ciliated epithelium. The last ring is 

 triangular, so as to fit the rings of the two bronchi. The mucous 

 membrane contains numerous racemose glands, the largest on the 

 posterior surface, called the tracheal glands. The muscular fibres 

 consist of two layers, longitudinal, the most external and trans- 

 verse internal, both unstriated. 



Relations. The trachea has the following relations in the 

 neck : 



Laterally. 



Lobes of the thyroid body; 

 Carotid arteries; 

 Inferior thyroid arteries. 



Anteriorly. 



Skin, superficial and deep fascia; 



Anterior jugular veins; 



Left innominate vein; 



Inferior thyroid plexus of veins; 



Isthmus of thyroid body; 



Arteria thyroidea ima (sometimes). 



Posteriorly. 



(Esophagus ; 



Vertebral column; 



Recurrent and inferior laryngeal nerves. 



The following in the thorax: 



Laterally. 



Pleura; 

 Pneuniogastric nerves. 



