26Q THE TEACHEA. 



oyer the front of the gullet and descending aorta : the arch of the aorta 

 turns backwards and to the left over it, and the left pulmonary artery 

 lies first above it and then on its anterior surface. The remaining 

 connections of each bronchus, as it lies within the root of the corre- 

 sponding lung, and the mode in which it there subdivides will be 

 afterwards described. 



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 

 provided with imperfect cartilaginous rings, and flattened and mem- 

 branous behind. 



STRUCTURE OF THE TRACHEA AND BRONCHI. 



The trachea consists of the elastic framework of incomplete carti- 

 laginous rings or hoops, layers of fibrous, muscular, and elastic tissue, 

 and the lining mucous membrane, with glands. 



The cartilages are from sixteen to twenty in number. Each pre- 

 sents a curve of rather more than two-thirds of a circle, resembling the 

 letter C. The depth from above downwards is from li to 2 lines, and 

 the t>hickness half a line. The outer surface of each is flat, but the 

 inner 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 membrane, which is elastic and 

 extensible up to a certain point, and not only occupies the intervals 

 between them, but is prolonged over their outer and inner surfaces, so 

 that they arc, as it were, imbedded in the membrane. The layer cover- 

 ing 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 readily on the interior than on the 

 exterior of the tube. 



The cartilages terminate abruptly behind by rounded ends, but the 

 fibrous membrane is continued across between them, and completes 

 the tube behind ; it is here looser in its texture. 



The first or highest cartilage, which is connected by the fibrous 

 membrane with the cricoid, is broader than the rest, and often 

 divided at one end. Sometimes it coalesces to a greater or less 

 extent with the cricoid or with the one below. The lowest carti- 

 lage, placed at the bifurcation of the trachea, is peculiar in shape ; 

 its lower border being prolonged downwards, and at the same time 

 bent backwards so as to form a curved projection between the two 

 bronchi. The cartilage next above this is slightly widened in the 

 middle line. Sometimes the extremities of two adjacent cartilages are 

 united, and not unfrequently a cartilage is divided at one end into 

 two short branches, the opposite end of that next it being likewise 

 bifurcated so as to maintain the parallelism of the entire scries. The 

 use of these cartilaginous hoops is to keep the windpipe open, a con- 

 dition essential for the free passage of air into the lungs. 



Within the fibrous membrane at the posterior flattened part of 

 the trachea, is a continuous pale reddish layer of unstriped mus- 

 cular fibres, which pass across, not only between the ends of 

 the cartilages, but also opposite the intervals ; they doulitless serve 

 to diminish the area of the tube by approximating the ends of 

 the cartilages. Those opposite the hoops are attached to the ends 



