, 852 HUMAN ANATOMY. 



t -ce,,s, asfar as the -a.ler bronchi. VVi.hin th^e the oHa.ec , cdls ; are replaced 



.he termination o, the latter, 



sinelT'or in Tm ted groups between the plates. Before respiration and the conse- 

 quent expansion of the ai -spaces take place, the cells lining these cavities are small 

 and probab^ of one kind. The groups of the smaller cells are larger, more numer- 

 ous, P and more uniformly distributed in young animals than m old ones, in which 

 they are often represented by single cells irregularly disposed 



X The adjacent alveoli share in common the interposed wall, which consists of the 

 two layers of delicate elastic membrane beneath the epithelium lining the alveoli and 



FIG. i575- 



.. 



Passage from 

 atrium into air-sac 



Alveolus- . . ,.__,. ^^.^ , ,,, . ^ 



-Atrium 



XfaKj WF&T' W . U^ .Alveolus 



Pulmonary arten- 

 Bronchiole 



Air-sacs 



V f 



1 



Section of lung, showing general relations of divisions of air-tubes. X 5- 



tin- intervening capillary net-work, supported by a delicate framework of elastic fibres. 

 The capillary net-work is noteworthy on account of the closeness of its meshes, which 

 are often of less width than tin- diameter of the component capillaries. The latter are 

 not confined to a single plane, but pursue a sinuous course, projecting first into one 

 alveolus and tlu-n into tin- om- on the opposite side of the interalveolar septum. The 

 capillarirs arc, thru-fore, exclmK-d from the interior of the air-cells by practically 

 only th<- atiriuiatt-d respiratory epithelium, the large plate-like cells lying over the 

 blood \e--e|> while the small cells cover the intercapillary areas. Distinct intercellu- 

 l.ir api-min--, oi Moinata. formerly described as affording direct entrance from the 

 alveoli into definite lymphatic^, probably do not exist. That, however, inspired 

 Ionian particles may pass between the epithelial cells into lymph-spaces within the 

 lar \\all and thence into lymphatics, to be transported to more or less dis- 

 tant points, is shown by the gradual accumulation of carbonaceous and other parti- 

 cles within the interlobular tissue and the lymph-nodules along the course of the 

 lymphatic vends. Such accumulations may acquire conspicuous proportions, the 

 entire interlobular septum appearing almost black. In view of the very frequent 

 presence of pigment-loaded leucocytes within the alveoli, as well as outside the alve- 



