THE RESPIRATORY SYSTEM. 



245 



are carried in with the inspired air ; the cilia, by their movement, 

 carry the fluid up the trachea into the pharynx. 



The main bronchi are similar in structure to the trachea. 



In the lungs the bronchi branch in a tree-like manner, the final 

 ramifications opening into the pulmonary air cells. The larger intra- 

 pulmonary bronchi are lined by columnar ciliated epithelium resting 

 on a basement membrane. Lying under this basement membrane are 

 longitudinally disposed elastic fibres with loose connective tissue. More 

 externally is a layer of smooth muscle fibres arranged circularly, the 



Infu.ndibu.li 





Cartilage f>Late& 



FIG. 94. Section of lung of cat, showing termination of bronchus. 

 (From Gray's Anatomy.} 



bronchial muscle. External to the bronchial muscle is a fibrous coat 

 containing scattered, irregular plates of hyaline cartilage. 



The smaller bronchi (bronchioles) have no cartilaginous plates, but 

 their muscular coat is well marked. 



Each bronchiole leads into a small number (three or four) of wider 

 thin-walled spaces, lined by flattened epithelium, and called atria. Out 

 of each atrium open two or three blind diverticula, each of which is 

 called an infundibulum. The walls of the infundibula are studded with 

 hemispherical sacs known as alveoli, which are lined by flattened, non- 

 nucleated, epithelial cells. Between adjacent alveoli there is a dense 

 network of capillaries, supported by a small amount of fine connective 



