THE THORAX AND RESPIRATION. 239 



omission. Whilst all admit that a copious secre- 

 tion is poured into this passage, it is not shown 

 how the mucus is thrown off. 



" There is a fine and very regular layer of mus- 

 cular fibres on the back part of the trachea, exte- 

 rior to the mucous coat, and which runs from the 

 extremities of the cartilages of one side to those 

 of the other. This transverse muscle is beauti- 

 fully distinct in the horse. 



" When a portion of the trachea is taken out, 

 and every thing is dissected off but this muscle, 

 the cartilages are preserved in their natural state, 

 but the moment that the muscular fibres are cut 

 across the cartilages fly open. This muscle, then, 

 is opposed to the elasticity of the cartilages of the 

 trachea. By its action it diminishes the calibre 

 of the tube, and by its relaxation the canal widens 

 without the operation of an opponent muscle. 



" The whole extent of the air-passages opens or 

 expands during inspiration, and then the trachea 

 is also more free ; but in expiration, and especially 

 in forcible expectoration and coughing, the trachea 

 is diminished in width. The effect of this simple 

 expedient, is to free the passage of the accumu- 

 lated secretion, which, without this, would be 

 drawn in and gravitate towards the lungs. When 

 the air is inspired, the trachea is wide, and the 

 mucus is not urged downwards. When the air is 

 expelled, the transverse muscle is in action, the- 

 calibre of the tube is diminished, the mucus occu- 



