THE RESPIRATION 



lining of the bronchial tubes. It is so thin as to offer almost 

 no obstruction to the passage of oxygen out of the cell and 

 the entrance of the carbon dioxid from 

 the blood vessel (Fig. 133). 



252. Ciliated Cells. It must be re- 

 membered that mucous membrane lines 

 all cavities in the body accessible to the 

 air. Almost all of the nose and pharynx, 

 and all of the air passages as far as the 

 lobules, are lined with a mucous mem- 

 brane the cells of which are furnished 

 with cilia (Fig. 134). These are minute 

 FIG. 131. TWO Lobules at hair like filaments which are in constant 

 the End of a Bronchial mo tion. When a few of the cells are 



Tube. . 



examined under a microscope, we may 

 see the cilia in motion, even for a time after the removal of 

 the cells from the body. They make a quick stroke up- 

 ward, and move back more slowly, and this is found to 

 give them the power of moving particles of dust which 

 enter the lungs upward toward 

 the larynx. Upon reaching the 

 larynx, the dust brings about irri- 

 tation which causes it to be coughed 

 up. In the nasal passages, they 

 serve a similar purpose. At the 

 opening of the nostrils are also 

 placed ordinary hairs (hundreds of 

 times larger than cilia), which aid 

 in cleaning the air of dust as it 

 enters the nose. 



253. The Blood Tubes in the Lungs. Near where the 

 trachea divides into the two bronchi, the pulmonary artery, 

 bringing the dark blood to the lungs, divides into two 

 branches, and the subdivision continues, until, finally, a net- 

 work of capillaries is formed around each lobule, or cluster 



FIG. 132. The Wall of an Air 

 Cell. 



*, the epithelium; t, partition be- 



tween two air cells, in which the 

 capillaries lie; c, fibers of elastic 



tissue> 



