A STUDY OF RESPIRATION 



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to which attention has already been called. They are shaped 

 something like a letter C, the open ends toward the dorsal 

 surface being joined by the con- 

 nective tissue that surrounds the 

 windpipe and joins the rings to 

 one another. When food is not 

 being swallowed, the windpipe 

 presses dorsally and closes the 

 esophagus. As food passes down- 

 ward, however, one can feel the 

 esophagus push the air tube ven- 

 trally. 



The mucous lining of the wind- 

 pipe and its branches is especially 

 interesting. The cells are more 

 or less cylindrical or club-shaped, 

 and their inner ends, which line 

 the air passages, are covered by 

 minute cilia much like those that 

 cover a paramecium (Figs. 96 and 

 102). The cilia alternately wave 

 upward toward the throat with a 

 quick movement, and then slowly 

 recover their former position. In 

 this way any dust particles that 

 have passed the Carrier of hairs at 

 the nostril openings, and the mucus 

 secreted by the membrane, are 

 moved steadily upward until they reach a point where they 

 can be coughed out into the mouth cavity. 



The Lungs. One can get a good idea of the structure of 

 the human air-passages and lungs by securing from the 

 butcher the "haslet" of a sheep or a calf. This consists 

 of the larynx, the windpipe, the bronchi, and the two lungs, 

 between which lies the heart inclosed in the pericardium. 

 A piece of the diaphragm is often attached to these organs. 



FIG. 100. Dorsal View of 

 Larynx and Windpipe cut 

 longitudinally. 



a, c, t = cartilages of larynx. 

 6, 6' = bronchi. 

 e = epiglottis. 

 h = hyoid bone. 

 tr = windpipe. 



