CHAPTER XIII. 



THE RESPIRATORY APPARATUS. 



THE respiratory apparatus consists of a multi- 

 tude of small cavities or chambers, on whose walls 

 the blood is brought into close contact with the air, 

 and of a system of branching tubes through which 

 the air is conducted to and from them. The con- 

 ducting tubes are the larynx, the trachea, and the 

 bronchi. We shall limit our study of the tubes to 

 the two last. 



The walls of the trachea consist of several layers : 

 commencing at the outside, we have first, a layer 

 of firm connective tissue the fibrous layer in which 

 lie imbedded, incomplete cartilaginous rings, the 

 space between whose free ends, at the posterior por- 

 tion of the tube, is bridged over by transverse bands 

 of smooth muscular tissue, which binds the ends to- 

 gether. The cartilaginous rings are of the hyaline 

 variety, and their perichondrium is continuous with 

 the looser connective tissue of the layer in which 

 they lie. This layer merges into the next, the sub- 

 mucosa, which consists of loose, fibrillar connective 

 tissue with elastic fibres, the latter running chiefly 

 in a longitudinal direction. 



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