BREATHING 165 



225. How Nature protects the Air Passages. The inside 

 walls of the windpipe and bronchial tubes are lined with 

 a mucous membrane, which secretes a thick, sticky fluid 

 called " mucus," to keep the passages moist. This mem- 

 brane is covered with microscopic, threadlike processes 

 called cilia, so close together that they resemble somewhat 

 the pile on velvet. They seem to wave to and fro, like a 

 field of grain under a gust of wind (Fig. 4). 



The cilia always bend upwards and outwards towards 

 the mouth with considerable force and then resume their 

 former position with a very gentle movement. They sweep 

 up bits of dust and mucus which are expelled by a sudden 

 blast of air which we call coughing. These tiny cilia are 

 simply the dusters which nature uses to keep the air pas- 

 sages neat and clean. 



Experiment 48. The respiratory sounds may be heard fairly well 

 by applying the ear flat to the chest, with only one garment inter- 

 posed. Borrow a stethoscope from a physician and listen to the 

 respiration over the chest. Note the difference of the sounds in 

 inspiration and in expiration. Do not confuse the heart sounds with 

 those of respiration. 



Experiment 49. Place a large sponge, which will represent the 

 lungs, in a paper bag which justs fits it; this will represent the 

 pulmonary layer of the pleura. Place the sponge and paper bag 

 inside a second paper bag, which will represent the parietal layer 

 of the pleura. Join the mouths of the two bags. The two surfaces of 

 the bags which are now in contact will represent the two surfaces 

 of the pleura. 



Experiment 50. Get a sheep's lungs, with the windpipe attached. 

 Ask for the heart and lungs all in one mass. Examine the windpipe. 

 Note the horseshoe-shaped rings of cartilage in front, which serve to 

 keep it open. Put pieces of the lung tissue in a basin of water and 

 note that they float. 



