LESSON XXIV. 



THE LUNG. 



1. Transverse section of the posterior part of a 

 mammalian trachea (chromic acid *2 p.c., hsematoxylin), 

 taken so as to include the ends of a tracheal ring, and 

 the membranous part between them. Observe, 



Externally the fibrous coat of loose connective tissue, 

 becoming denser internally, and having imbedded in 

 it the two ends of the ring of cartilage. 



Stretching between and outside the ends of the 

 cartilage, a transverse band of unstriped muscular 

 fibres ; there may also be visible outside this the cut 

 ends of muscular fibres running longitudinally. 



The submucous coat (probably much folded) 

 continuous with the fibrous coat, but of finer fibres. 

 In this coat note the small glands consisting chiefly 

 of mucous cells, though some serous cells are usually 

 present ; the ducts are not very evident, but they may 

 be seen running towards the surface, and occasionally 

 opening on it. 



The mucous coat, consisting from without inwards 

 of (a) longitudinal elastic fibres, becoming finer 



