168 



RESPIRATION 



the lungs. All of these which have a diameter of more 

 than one-fortieth of an inch are equipped with cartilagi- 

 nous rings. They have in addition a certain number of 

 smooth muscle fibres in their walls. Their lining, like that 

 of the trachea, consists of a mucous membrane made up 



Epiglottis, 

 arynx. 



-Thyroid cartilage. 

 ficoid cartilage. 



Ring of cartilage. 

 /Left bronchus, 

 hest wall. 



FIG. 98. Front view of the trachea and its branches. 



of elongated cells, the exposed tips of which carry many 

 cilia for the purpose of whipping dust and mucus out 

 of the lungs. They are also furnished with cells which 

 secrete mucus and thereby keep their surface moist. 



When the bronchial tubes have become as small as 

 one-fortieth of an inch, their walls lose their carti- 



