150 ELEMENTARY PHYSIOLOGY. [XV. 



a group of cells more or less surrounded 

 by connective tissue, continuous with that of 

 the septa. 



c. At intervals oblique and transverse sections 

 of the small ducts. They are usually stained 

 darker than the alveoli, are not surrounded 

 by a sharp ring of connective tissue, and 

 have a well-defined lumen. 



2. Under a high power observe that 



a. The alveoli vary considerably in size, and 

 frequently have no obvious lumen; when 

 visible the lumen is usually an irregular 

 central space between the cells. 



I). The mucous cells are comparatively large ; 

 most have a disc-shaped nucleus situated in 

 the outer part of the cell near the basement 

 membrane ; in some, the nucleus is spherical 

 and lies farther from the basement mem- 

 brane. Around the nucleus is a small amount 

 of stained cell-substance, from this a stained 

 network may more or less clearly be seen 

 stretching throughout the cell, the rest of 

 the cell-substance stains little or not at all. 



c. The demilune cells, occurring often in 

 groups, lie immediately beneath the mem- 

 brana propria, and are stained throughout: 

 they are generally half-moon shaped, and 

 often have two nuclei; they frequently send 

 processes in between the mucous cells, and 

 then appear simply to fill up the spaces 



