HEALTH AND DISEASE 



work effectually prevents the trachea from being closed by any moderate 

 pressure that may be exerted upon it, and air is consequently always 

 capable of reaching the lungs without interference. The entrance of food 

 or of any foreign body into the trachea is jealously guarded against, and 

 it is only when these parts are, so to speak, taken by surprise that such 

 an accident can happen. In the first place the mucous membrane of the 

 whole of this region is rendered extremely sensitive to touch by the distri- 

 bution of the superior 

 laryngeal nerve, which is 

 a branch of the vagus. 

 Then in the act of swal- 

 lowing, the whole larynx 

 and trachea are drawn 

 close up behind the 

 tongue by muscular ac- 

 tion, whilst at the same 

 time the epiglottis is 

 drawn downward to meet 

 them, and in this way the 

 opening into the larynx 

 is closed against the en- 

 trance of food. At the 

 same moment the vocal 

 cords are brought to- 

 gether and the space 

 between them reduced 

 to a mere chink; whilst 

 lastly, if, in spite of these 

 precautionary arrange- 

 ments, a fi'agment of food 

 should by chance enter 

 the larynx or trachea, it is immediately expelled by a violent expiratory 

 effort or cough. 



The Bronchi. — The trachea on reaching the chest divides into the 

 two bronchi, one going to each lung. As seen in fig. 200, they then 

 divide and subdivide again and again till the tubes are scarcely larger 

 than a hair, when they are termed bronchioles. These, after a short 

 course, suddenly change their character, becoming dilated into sack-like 

 ends, and presenting depressions, cups, or pits on their walls as shown 

 in fig. 201. 



The Air-cells. — These cups constitute the air-cells, alveoli, or vesicles 



Fig. 200.— The Lungs and Bioncai 



