126 



HUMAN BIOLOGY 



178. The throat and larynx. Except when something 



is being swallowed, the 

 glottis is always open, 

 thus allowing a free 

 passage for the air from 

 the throat, through the 

 larynx, into the wind- 

 pipe. When food is 

 swallowed, it is of 

 course important that 

 the windpipe be closed, 

 and this is accom- 

 plished by a little trap- 

 door called the epiglottis 

 (Fig. 39). If one puts 

 the finger on the larynx 

 region and then swal- 



FIG. 40. Windpipe and lungs. 



lows, one can feel this 

 organ rising to meet the 

 epiglottis. Within the voice box are two thin membranes 

 that may be stretched "with more or less 

 tension and set in vibration by the in- 

 spired or expired air. These vocal cords 

 help to produce the various tones of the 

 voice. 



179. Lining of the air passages. The 



mucous lining of the nasal cavities and of 

 the windpipe and its branches is especially 

 interesting. The cells that cover these 

 passageways are covered by minute hair- 

 like projections called cilia, much like those on the outside 

 of a paramecium (A. B., 120), which wave upward toward 



FIG. 41. Two air sacs 

 with their branches. 



