9 



Physiology. 



rings, but are C-shaped. As the windpipe branches (bron- 

 chi) into the two lungs, the cartilages continue in the 

 smaller branches which extend into every part of the lungs. 



The Internal Structure of the Lungs. The lungs are 

 full of small cavities, like a loaf of light bread. The small 

 cavities are called air sacs or air vesicles, and each air sac 

 communicates with the end of one of the branches of an 

 air tube, through which air comes into and goes out of 

 every air sac. The air sacs are very thin walled, and 

 around the sacs are networks of the fine blood tubes called 

 capillaries. 



Elastic Tissue in the Lungs. The air sacs and air tubes 

 and their surrounding blood tubes are bound together by 

 elastic tissue, which fills up most of the space between them. 



The Mucous Membrane. The lining of the trachea is 

 a mucous membrane. It pours out on its surface a sub- 

 stance somewhat 

 like white of egg, 

 called mucus. This 

 keeps the air moist, 

 and catches parti- 

 cles of dust that are 

 in the inspired air. 



Fig. 54. Ciliated Cells lining the Air Tubes (x 300). -pi , 



slow current of mucus toward the throat, whence it is, from 

 time to time, hawked up. 



Ciiiums. This current of mucus is caused by the ciii- 

 ums projecting from the lining cells of the trachea. They 

 are little hairlike projections, in countless numbers, like a 

 field of grass, each cilium having the power of bending 

 back and forth, making a quick stroke toward the throat, 

 then a slower recover stroke. Thus the united wavelike 



