THE TEACHEA. 1081 



Structure of the Wall Of the Trachea. The walls of the trachea and bronchi 

 are composed of (1) a fibro-elastic membrane in which the cartilaginous bars are 

 embedded ; (2) within this, and on the dorsal aspect of the tube, a layer of muscular 

 tissue, termed the musculus trachealis ; and (3) the lining mucous membrane. 



The fibre-elastic membrane is strong and dense. It passes round the whole circum- 

 ference of the tube, and becomes continuous, above, with the perichondrium which invests 

 the cricoid cartilage. Embedded in its substance are the series of cartilaginous bars. 

 These vary in number from 15 to 20, and are composed of hyaline cartilage. They 

 are horseshoe -shaped, the dorsal fourth of the circumference being deficient, so that 

 dorsally each bar ends in two rounded extremities. The outer surface of each tracheal bar 

 is flat and even, and /Joes not project much beyond the level of the membrane in which 



Spinal medulla 



Trachea 



^ - (Esophagus 



_- 4th thoracic vertebra 

 ^-Innominate artery 

 Left common carotid 

 innominate vein 



Manubrium sterni 



___Synchondrosis 

 sternalis 



Right pulmonary 

 """ artery 



Pericardial cavity 



... Left atrium 

 - Aortic valve 



- Body of sternum 



Right atrio- 

 ventricular valve 



| Wall of right 

 ventricle 



(Esophagus 

 Diaphragm 



! _J Descending aorta 

 I Ji 



Xiphoid process 



r io. 860. SAGITTAL SECTION THKOTJGH THE THORAX OF AN OLD MAN. The superior border of the 

 manubrium sterni and the bifurcation of the trachea are lower than in the average adult. 



i is embedded ; the inner surface, however, is convex in the vertical direction, and 

 consequently it bulges slightly into the lumen of the tube. The intervals between the 

 bars are somewhat narrower than the bars themselves, and neighbouring bars frequently 

 show a more or less complete fusion, whilst others present other irregularities, such as a 

 tendency to bifurcate. The lowermost bar is specially adapted to the tracheal bifurca- 

 tion. In the median plane, ventrally, it inclines downwards, and from this median peak a 

 cartilaginous strip is carried backwards in the fork between the two bronchi. 



The m. trachealis is a continuous layer of involuntary muscular tissue, placed in 

 the dorsal part of the wall of the trachea internal to the fibro-elastic membrane. The 

 muscular bundles are arranged transversely, and are attached to the extremities of the 

 bars, and also to the inner surfaces of the bars for a short distance beyond their 



ixtremities. In the intervals between the bars the transverse muscular bundles are 

 attached to the fibro-elastic membrane. It is evident that, by its contraction, this 

 muscle will reduce, in a marked degree, the lumen of the tube. 



