RESPIRATION. 4t)9 



to encircle one segment of the tubes, and their extremity, 

 especially in the larger tubes, overlap each other to a con- 

 siderable extent. On the small tubes they are found scat- 

 tered irregularly over the walls, leaving considerable intervals. 

 The cartilaginous plates on the primary bronchi are nearly 

 as thick as the rings of the trachea, but on the smaller tubes 

 become gradually attenuated, and cannot be demonstrated 

 on tubes less than half a line in diameter. The cartilages 

 are enveloped in a layer of white and elastic fibres which 

 completely encircles the tube, and, with the cartilages, forms 

 its external coat. Within this is a coat of circular involun- 

 tary muscular fibres, which become gradually thinner on the 

 smaller tubes, but can be demonstrated on those of -^th to 

 -j^th of a line in diameter. The internal or mucous coat 

 has an outer layer of elastic longitudinal fibres on which lies 

 a thin homogenous layer (basement membrane) supporting 

 the ciliated epithelium. Like the coats already mentioned, 

 this becomes much attenuated on the smaller tubes. Eace- 

 mose mucous glands are numerous in the larger bronchia, 

 but disappear on those of one or one-and-a-half lines in 

 diameter. The mucous membrane of the bronchia is much 

 more irritable than that of the trachea. 



The terminal bronchia, each enter a pear-shaped mass of 

 parenchymatous tissue, constituting a lobulette or ultimate 

 pulmonary lobule. The bronchus here ends in a dilatation 

 communicating with several elongated cavities (terminal 

 cavities, Mandl; air sacs, Waters), separated from one 

 another by thin and delicate walls, and having each upon its 

 walls from ten to twenty minute depressions, likewise sepa- 

 rated from each other by thin membranous parietes. The 

 latter are the alveoli or air cells. Some air cells may like- 

 wise be noticed on the commencement of the bronchial tube 

 to which they give a moniliform appearance. Two or more 



