XX.] THE STRUCTURE OF THE LUNG. 213 



in 30 p.c. alcohol for an hour or more, then in 

 strong spirit for about an hour ; leave the other 

 (ft) in 30 p.c. alcohol for two to three days. 



Cut open (a) and observe 



a. The large central space. 



b. The somewhat short primary septa running 

 inwards from the wall of the lung, and form- 

 ing a number of polygonal chambers open 

 towards the central space. 



c. Short secondary septa, projecting into the 

 chambers from the primary septa. A lens 

 may be required to see these distinctly. 



3. Scrape the inner surface of (/3) and mount in a 

 drop of iodine. Observe with a high power 



The isolated epithelium cells, consisting of a 

 thin ground plate having near one border a 

 nucleus with a little surrounding protoplasm 

 stained with iodine. 



4. Make transverse sections from the posterior part 

 of a small mammalian trachea, which has been 

 hardened in chromic acid *5 p.c. The section 

 should be so taken as to cut through the ends of 

 a tracheal ring and include the membranous 

 part between them. Observe 



a. Externally, the fibrous coat of loose connec- 

 tive tissue, becoming denser towards the 

 tube of the trachea ; in this are embedded 

 the two ends of the ring of cartilage. 



