CHAP. XXIX.] 



ANATOMY OF THE LUNG. 



395 



Fig. 208. 







of an inch in thickness. In some of the mammalia, as the kan- 

 garoo, the rat, and mouse, the terminal air spaces are too minute to 

 contain even a single particle of epithelium, and cannot therefore be 

 lined by a pavement of such particles. The elastic tissue, described 

 as forming the cell wall in the human lung, is in such instances 

 very imperfect, being deficient in the areolse of the vascular net- 

 work, so as to render the cell walls cribriform, or rather to reduce 

 the terminal air receptacles into a congeries of inosculating and 

 most minute and irregular passages. 



In the bird's lung, the elastic tissue, according to the same anato- 

 mist, is even more scanty. The mucous membrane lining the 

 bronchia, ceases at the commencement of the lobular passages, and 

 these passages seem hollowed out in the substance of a solid capil- 

 lary plexus, or one in which the capillaries extend and anastomose 

 indifferently in all directions. This vascular plexus forms the 

 only wall of these passages, and the air has access from the pas- 

 sages into the interstices of the plexus, so as everywhere to sur- 

 round and touch the whole surface of every capillary. Instead, 

 therefore, of the terminal air-spaces being cells with a plane net- 

 work of capillaries on their walls, they are the mere interstices of a 

 solid vascular plexus. Thus 

 a more abundant intermix- 

 ture of the blood and air is 

 secured, and the ultimate 

 tissue of the lung is reduced 

 to the simple capillary wall, 

 arranged so that air has 

 access to its exterior. The 

 adhesion of the lungs to 

 the costal surface, and the 

 support afforded by the car- 

 tilaginous and other tissues 

 of the bronchial tubes, as 

 well as by the areolar tissue 

 bounding the elongated lo- 



, f\2 - Slightly oblique section through a bronchial tube. 



DllleS, are SUmCient tO main- Showing at a, the cavity of the tube. b. Its lining mem- 



., . . .. n p M brane, containing blood-vessels with large areolse. c c. 



tain the integrity OI SO trail Perforations in this membrane, where it ceases at the 



, mi i j. j. orifices of the lobular passages, d d. e e. Spaces between 



a Web. 1 he aerial interstices contiguous lobules, containing the terminal pulmonary 



-P 4--U ! !,, r, arteries and veins supplying the capillary plexus,//, to 



OI tne VaSCUlar pleXUS are the meshes of which the air gains access by the lobular 



usually even smaller in dia- p 



meter than the capillaries themselves, and, according to Mr. 

 Rainey, average Tj -^-^ of an inch. The above (fig. 208) is from 



D D 2 



