Respiration 



165 



Delicate filaments, called cilia, not unlike the threads 

 which make up the pile on velvet, spring from the epithelial 

 lining of the air tubes. Their constant wavy movement 

 drives excessive secretion upwards towards the larynx, 

 where it is finally expelled by coughing. In this way the 

 lungs are also kept quite free from particles of foreign 

 matter derived from, the air. Thus 

 these tiny cilia act as dusters which 

 Nature uses to keep the air tubes 

 free and clean (sec. 23). 

 - 250. General Structure of the 

 Lungs. The lungs, the chief organs 

 of respiration, are two pinkish-gray 

 structures of a light spongy appear- 

 ance that fill the chest cavity, except 

 the space taken up by the heart and 

 large vessels. Between the lungs 

 are situated the large bronchi, the 

 oesophagus, the heart in its pericar- 

 dium, and the great blood vessels. 



The base of the lungs rests on the FIG. 86. Bronchial Tube, 

 dome-like diaphragm, which separates with its Divisions and 

 the chest from the abdomen. This Subdivisions - 

 partly muscular and partly tendinous (Show ^ rou P s of ai j sacs 



. . . a * the termination of mm- 



partition is a most important factor ute bronchial tubes.) 

 in breathing. 



-251*. The Pleura. Each lung is covered by an elastic 

 serous membrane in a double layer, called the pleura. One 

 layer closely envelops the lung, at the apex of which it 

 is reflected to the wall of the chest cavity of its own 

 side, which it lines. The two layers thus form between 

 them a closed sac, a serous cavity (see Fig. 70, also 

 sec. 2 1 3") . ' 



