THE LUNGS. 



579 



Fig. 238. 



In the air-cells, I cannot admit the existence of more than two layers 

 a fibrous membrane and an epithelium. The former is manifestly the 

 much attenuated mucous membrane and fibrous tunic of the bronchia, 

 entirely deprived of the smooth muscles, and consisting of a homoge- 

 neous matrix of connective tissue, together with elastic fibres and 

 numerous vessels. The elastic fibres, 0*0005 0'002 of a line in size, 

 present the form chiefly of separate trdbeculse and filaments, running 

 especially at the angles of the air-cells, which have been flattened in 

 the distended condition, as well as around their openings ; they anasto- 

 mose with each other on every side and thus constitute a firm frame, on 

 which the softer, vascular parts of the air-alveolce, composed of con- 

 nective tissue, are stretched. The structure of these elastic trabeculce, 

 which, at the points where the air-cells abut upon each other, mutually 

 coalesce, so that the boundaries of the separate cells cannot for the 

 most part be recognized, is al- 

 most everywhere one of the 

 most close elastic networks 

 possible, the interstices of which 

 appear only as extremely nar- 

 row fissures, although occasion- 

 ally the fibres are more loosely 

 united, so that they are plainly 

 recognizable as elastic elements 

 of the usual kind. From the 

 trabeculce also, but everywhere 

 sparingly, elastic fibres, in part 

 very fine, proceed into the re- 

 mainder of the walls of the 

 air-cells, in which, by their 

 union, they constitute a wide 

 network. The connective tissue 

 of the air-cells, which appears 

 to be altogether homogeneous, 

 is quite a subordinate element 



in their composition, compared with the elastic elements and vessels, 

 presenting itself, as it may be said, only in the walls of the alveolce, 

 between the elastic trdbeculce, as a connective medium between the 

 numerous capillaries. 



The epithelium of the air-cells is of the common tessellated kind, 

 without cilia, and composed of polygonal, pale, granular cells, in mor- 



Fio. 238. Human air-cell, with the surrounding tissues, magnified 350 diameters: a, 

 epithelium ; 6, elastic trabecula ; c, more delicate wall between the latter, with finer elastic 

 fibres. 



