318 



KESPIEATION 



which are provided with cilia. (See Fig. 135.) These cilia, 

 together with the mucus secreted by the cells, serve to catch 

 any particles of dust that may have passed the nasal cavity. 

 The cilia are in constant motion, bending quicldy upward 

 and then slowly returning to place, and in this manner 

 forcing the collected particles steadily upward toward the 

 pharjmx. 



The cartilage rings do not completely inclose the tube, 

 but are shaped like the letter C, the open part being at 

 the back and leaving a portion of the tube soft and com- 

 pressible. The gullet lies against this soft groove, and 

 thus room for expansion is given as the food passes down 

 the gullet. 



Structure of the lungs. — These are merely membranous 

 sacs with an internal framework composed of the branch- 

 ing bronchial tubes and air sacs. These treelike tubes 

 are bound together with a soft, pink connective tissue, 

 rich in elastic fibers, and over the 

 whole is stretched an elastic mem- 

 brane known as the plem-a. The 

 bronchi and bronchial tubes are 

 continuations of the trachea, and 

 the larger ones resemble it in struc- 

 ture with the exception that the 

 cartilage rings are not so regularly 

 arranged. As the tubes become 

 smaller their walls become thinner, 

 and finally the cartilage rings dis- 

 appear entirely in the smaller 

 tubes, while the mucous membrane 

 is reduced to a single layer of ciliated cells. The air 

 sacs, in which the smallest tubes end, are composed of 



Fig. 148 — Two air sacs (b) 

 into wliicli a broneliial 

 tube (a) opens. 



