THE TRACHEA AND BRONCHI 



1225 



vocal folds where the mucosa is thin and tightly bound down the vessels are scarce and small 

 in size (see p. 719). 



The nerves of the larynx are the superior and inferior laryngeal branches of the vagus and 

 also certain branches of the sympathetic. Taste-buds occur and are abundant in the mucosa 

 of the posterior surface of the epiglottis. The innervation of the muscles has already been in- 

 dicated, and the description of the course and relations of these nerves will be found in the chapter 

 on the Peripheral Nervou.s System. It should be mentioned here, however, that the idea 

 of sharply hmited territories of innervation, not only for the mucosa, but for the muscles as 

 well, has been brought into question by the researches of Semon and Horsley, Exner, and others, 

 which show that the distribution and functions of the larjiigeal nerves are extremely complex. 



The development of the larynx. — The larynx is developed partly from the lower portion 

 of the embryonic pharynx and partly from the upper portion of the trachea. The cricoid carti- 

 lage represents the uppermost tracheal cartilage, while the thyreoid is formed by the fusion of 

 four cartilages representing the ventral portions of the cartilages of the fourth and fifth branchial 

 arches. The larj'ngeal muscles are derived from the musculature of these arches and conse- 

 quently their nerve-supply is from the vagus. Whether or not the arytsenoid and epiglottic 

 cartilages are also derivatives of the branchial arches is uncertain, although it seems probable 

 that they are. 



THE TRACHEA AND BRONCHI 



The tubular trachea (figs. 972, 989), or windpipe, extends from the larynx 

 downward through the neck and into the thorax to end by dividing into two 

 branches, the right and left bronchi [bronchus (dexter et sinister)], which lead to 



Fig. 989 — Trachea and Bronchi in Their Relations to the Great Vessels as seen from 



Behind. (After Gegenbaur.) 



Trachea 



Left subclavian artery 

 Superior vena cava 



Aortic arch 

 Azygos vein 



Right branch of pulmo- 

 nary artery 



Right pulmonary veins 



Inferior vena cava 



the lungs. These tubes are simple transmitters of the respirator}^ air. Their walls 

 are, for the most part, stiff and elastic, consisting in large part of cartilage. While 

 the general form of these tubes is cylindrical, a rounded contour is presented by 

 their walls only in front and at the sides, the posterior surface being flat. The 

 inner surface of the walls of the tubes presents a succession of slight annular pro- 

 jections caused by the cartilaginous rings which enter into their structure. The 

 calibre of the trachea varies at different levels, a cast of the lumen being in gen- 

 eral spindle-shaped. Its sectional area is less than the combined sectional areas 

 of the two bronchi. When the bifurcation of the trachea [bifurcatio tracheae] is 

 viewed by looking down into its cavity, a sagitally directed keel, the carina 

 tracheae (fig. 990), is seen standing between the openings which lead into the 

 bronchi. Its position is a little to the left of the mid-plane of the trachea 

 in a slight majority of cases, or in the mid-plane in a large percentage. 



