THE LUNGS AND RESPIRATION. 



203 



ing downward from that is the trachea proper. This con- 

 sists essentially of a dense fibrous tissue in which are im- 

 bedded horseshoe-shaped pieces of cartilage which serve to 



Fig. 87. THK AIR PASSAGES AND LUNGS FROM BEFORE. L,UNG TISSUE REMOVES ON 



RIGHT LUNG TO SHOW RAMIFICATIONS OF BRONCHIAL TUBES. 



a, larynx; b, trachea; d, bronchial tube. 



keep the trachea open. The open ends of these horseshoes 

 are backward, that is, next to the gullet, and the absence 

 of bands of cartilage here no doubt materially facilitates 

 swallowing. 



The trachea is lined on the inside with several layers of 

 epithelial cells, of which the innermost layer is ciliated. 

 These cilia move in regular rhythm and in such a way that 

 any material resting on them is driven forward toward the 

 mouth. In this way the mucus, or phlegm, is removed. 

 The trachea at its lower end divides into two branches, 

 called the bronchial tubes, and these in turn divide and sub- 

 divide repeatedly until finally a perfect system of ramifica- 

 tions of tubes is the result. At the end of each of these 

 finer ramifications there is a sack-like dilation called the 

 alveolus. The wall of this alveolus is thrown into little 

 pouches, each pouch being called an air cell. It is neces- 

 sary to remember here that the term "cell" is not used in 



