128 RESPIRATION. 



face. If examined under a powerful microscope, we may 

 see, even for a considerable time after their removal from 

 the body, that these cells have minute hair-like processes 

 in motion, which wave like a field of grain under the 

 influence of a breeze (Fig. 39). This is a truly beauti- 

 ful sight; and since it is found that these little cilia, as 

 they are called, always produce currents in one direction, 

 from within outward, it is probable that they serve a 



useful purpose in catching and 

 carrying away from the lungs 

 dust and other small particles 

 drawn in with the breath (Fig. 

 39). The three diseases which 

 more commonly affect the lungs, 

 as the result of exposure, are 

 FIG. 39.-ciLi ATE i> CELLS. pneumonia, or inflammation of 

 the lungs, implicating principally 



the air-cells; bronchitis, an inflammation of the large bron- 

 chial tubes; and pleurisy, an inflammation of the investing 

 membrane of the lungs, or pleura. Among the young, an 

 affection of the trachea takes place, known as croup. 



7. The Movements of Respiration. The act of 

 breathing has two parts (1), inspiration, or drawing 

 air into the lungs, and (2), expiration, or expelling it 

 from the lungs again. In inspiration, the chest extends 

 in its length, breadth, and height, or width. We can 

 prove that this is the case as regards the two latter, by 

 observing the effect of a deep breath. The ribs are ele- 

 vated by means of numerous muscles, some of which oc- 

 cupy the entire spaces between those bones. But the 

 increase in length, or vertically, is not so apparent, as it is 

 caused by a muscle within the body called the diaphragm, 

 it being the thin partition which separates the chest from 

 the abdomen, rising like a dome within the chest. (Fig. 16). 



7. The act of breathing ? Extension of the chest by breathing ? 



