890 



ORGANS OF RESPIRATION. 



In form the bronchi exactly resemble the trachea on a smaller scale ; 

 they are rounded and firm in front and at the sides, where they are pro- 

 vided with imperfect cartilaginous rings, and they are flattened and mem- 

 branous behind. 



Structure of the Trachea. 



The trachea presents for consideration the elastic framework of incomplete 

 cartilaginous rings, layers of fibrous, muscular, and elastic substance, and the 

 lining mucous membrane, with glands. 



The cartilages &nd fibrous membrane. The cartilages are from sixteen to 

 twenty in number. Each presents a curve of rather more than two-thirds 

 of a circle, resembling the letter C. The depth from above downwards is 

 from one line and a half to two lines, and the thickness half a line. The 

 outer surface of each is flat, but the inner surface is convex from above 

 downwards, so as to give greater thickness in the middle than at the upper 

 and lower edge. The cartilages are held together by a strong fibrous mem- 



Fig. 624. 



Fig. 624. OUTLINE SHOWING THE 

 GENERAL FORM OP THE LARYNX, 

 TRACHEA, AND BKONCHI AS SEEN 



PROM BEHIND. 



h, great cornu of the hyoid bone ; t, 

 superior, and t\ the inferior cornu of 

 the thyroid cartilage ; e, the epiglottis ; 

 a, points to the back of both the ary- 

 tenoid cartilages, which are surmounted 

 by the cornicula ; c, the middle ridge 

 on the back of the cricoid cartilage ; 

 tr, the posterior membranous part of 

 the trachea; 6, 6', right and left 

 bronchi. 



brane. This membrane is elastic 

 and extensible in a certain de- 

 gree, and not only occupies the 

 intervals between the cartilages, 

 but is prolonged over their outer 

 and inner surfaces, so that 

 they are, as it were, imbedded 

 in ifc. 



The layer covering the outer 

 side of the rings is stronger than 

 that within them ; and from this 

 circumstance, together with the 

 roundness of their inner surfaces, 

 they may be felt more pro- 

 minently on the interior than on 

 the exterior of the trachea. 



The cartilages terminate ab- 

 ruptly behind. At the back of 

 the trachea, where they are 

 altogether wanting, the fibrous 

 membrane is continued across 

 between their ends, but it is here 

 looser in its texture. 



