THE LUNGS 



235 



The bronchi and larger bronchial tubes are like the trachea in structure, 

 consisting of fibrous and elastic tissue with incomplete rings of cartilage. In 

 the smaller tubes the rings become irregular plates or discs, and in the bron- 

 chioles the cartilage is absent altogether. The walls are here very thin and 

 contain circular muscle fibers (non-striated), the bronchial muscle. 



The entire tract from the trachea down to the air cells is lined 

 with mucous membrane, bearing ciliated epithelium (Fig. 156) as 

 far as the smallest tubes. 



Thyroid cartilage 



Cricoid cartilage 



Cartilage 

 rings of trachea 



Left bronchus 



FIG. 158. LARYNX, TRACHEA, AND BRONCHI. (Morris, modified from B our gery.) 



The cilia of the air passages are fine hair-like projections from the surface; 

 they have a waving motion, exerted forcibly in a downward direction. 



THE LUNGS 



The lungs are two in number, right and left, situated in the 

 right and left sides of the thorax, occupying the space enclosed 

 by the ribs (not that portion between the sternum and the spinal 

 column). They resemble a flattened cone in shape, the apex 



