188 PRACTICAL HISTOLOGY [XXIV 



towards the basement membrane (probably the network 

 formed by the elastic fibres will not be seen; some 

 adenoid tissue may be present amongst the inner fibres), 

 (b) the basement membrane, and (c) the epithelium, 

 made up of columnar ciliated cells, and of two or 

 three rows of small cells between the bases of the 

 ciliated cells ; some mucous cells also will probably be 

 seen. 



2. Longitudinal vertical section of trachea, taken 

 through the tracheal rings. Compare it with the 

 transverse section, noting especially the elastic fibres 

 of the mucous coat. 



3. Piece of mammalian lung taken from the more 

 central portion. (Distended with, and hardened in, 

 chromic acid '2 p.c.) Pass through alcohols down to 

 30 p.c. Place in hsematoxylin for an hour or two. 

 Imbed in paraffin (Lesson VII. 5) and cut sections. 

 Observe 



The larger bronchioles (if present in the section) 

 resemble in general features sections of the trachea 

 and bronchi except that, the cartilages are irregular 

 in form, are irregularly scattered throughout the fibrous 

 coat, the circular muscular fibres form a more or 

 less complete ring at the base of the mucous coat, the 

 epithelial layer, and all the coats are thinner. The 

 fibrous coat is continuous with that of the surrounding 

 lung tissue. 



The smaller bronchioles have no cartilage and 

 no glands, the epithelium consists of a single layer of 

 ciliated cells, longitudinal elastic and circular muscular 

 fibres are present. 



