172 NORMAL HISTOLOGY. 



finer ramifications, we find that the outer connective- 

 tissue layer becomes thinner, the cartilaginous plates 

 become smaller and more infrequent, and finally 

 altogether disappear ; the mucous glands, too, after 

 becoming smaller and simpler in structure, disap- 

 pear with the cartilages. The muscular rings, as- 

 sume, gradually, the form of an uniform thin layer 

 of transversely arranged muscle-cells, intermingled 

 with elastic fibres, and are finally represented only 

 by a few scattered transverse cells. The mucosa in 

 the smaller tubes becomes gradually thinner, and 

 finally merges into the fibrous layer, with the inter- 

 vention only of a few scattered muscle-cells ; the 

 lower layers of epithelium gradually disappear, leav- 

 ing a single row of ciliated cells upon the basal 

 membrane. Finally, we have, in the smallest tubes, 

 very thin connective-tissue walls, containing a few 

 muscle-cells and elastic fibres, and lined with cuboi- 

 dal, ciliated, and last with respiratory epithelium. 



We are thus led to the respiratory cavities of the 

 lungs the air-chambers. If we look at the surface 

 of a lung, we see that it is more or less distinctly 

 divided, by narrow branching lines, into irregular 

 polygonal spaces, each one of which corresponds to 

 a pulmonary lobule. These lobules have, on the sur- 

 face of the lung, where they are more uniform in 

 shape than within, a pyramidal form, and are sepa- 

 rated by narrow connective-tissue septa, and each 

 lobule is, in fact, a group of air-vesicles and air- 



