342 PHYSIOLOGY. 



muscularis mucosce, while the cartilaginous elements are scattered 

 about equally in all parts of their circumference. 



As the bronchi are traced in the lungs they divide into tubes of 

 less diameter. These again subdivide into tubes growing smaller in 

 a gradual manner. After a certain stage of division each tube is 

 reduced to about one-fiftieth of an inch, and is denominated a 

 bronchiole. These bronchioles then open into blind spaces called 

 infundibula, which are lined with air-cells. Near the ending of the 

 bronchiole with the infundibulum the former ciliated epithelium dis- 

 appears and another variety of epithelium appears. This new variety 

 of epithelium consists of small, flat, polygonal nucleated cells. This 



Fig. 114. Termination of a Bronchus in an -Alveolus. 

 a, Bronchiole, b, Cavity of the alveolus, c, Air-cells. 



flat, thin epithelium also lies over the blood-vessels and even extends 

 between the blood-vessels. 



The alveoli of any group or series always communicate with one 

 another to open by a common orifice into a terminal bronchus. In 

 size they average roughly one one-hundredth of an inch in diameter. 

 Form is given to the air-cells by the presence of a fine membrane of 

 slightly fibrillated connective tissue which contains some corpuscles. 

 This is closely surrounded by a great many fine, elastic fibers which 

 give to the pulmonary parenchyma its characteristic elasticity. 

 Some nonstriped muscular fibers are apparent in the connective 

 tissue between the cells; in certain diseases these become abnormally 

 developed. The number of alveoli has been estimated to be seven 

 hundred and twenty-five millions, whose superficial area is sixty times 

 greater than that of the body. 



