236 THE HUMAN BODY. 



Structure of the Windpipe and its Branches. The 

 trachea, bronchi, and bronchial tubes are lined by mucous 

 membrane, outside of which is a supporting stratum com- 

 posed of connective and plain muscular tissues. Their walls 

 also contain cartilaginous rings or half-rings which keep 

 them open. Below the projection on the throat known as 

 Adam's apple (due to the larynx, see Chap. XXII.) there 

 may readily be felt in thin persons the stiff windpipe pass- 

 ing down to the top of the chest. 



The Cilia of the Air-Passages. The mucous membrane 

 of the trachea and its branches, down to almost the smallest, 

 has a layer of ciliated cells on its surface. Each of these 

 cells has on its end turned towards the cavity of the tube a 

 tuft of from twenty to thirty slender threads which are in 

 constant motion; they lash forcibly towards the throat, 

 move gently back again, and then once more violently to- 

 wards the outlet of the air- passage. These moving threads 

 are called cilia. Swaying in the mucus secreted by the 

 membrane which they line, they sweep it on to the throat, 

 where it is coughed or "hawked " up. 



Imagine a man rowing in a boat at anchor. The 

 sweep of the oars will drive the water back and not the 

 boat forwards. So these little oars, the cilia, being an- 

 chored on the mucous membrane drive on the secretion 

 which bathes its surface. 



With what are the windpipe and its branches lined? What lies 

 outside this lining? What do these walls also contain? What is 

 the use of the cartilages? What is " Adam's apple"? What may be 

 felt below it in front of the neck? 



What lines the mucous membrane of the windpipe and its sub, 

 divisions? What does each ciliated cell bear on its free end? How 

 do the threads move? What are they called? What is the use of 

 the cilia of the air-passages? 



.Illustrate how they push on the liquid they move in. 



