1852 



HUMAN ANATOMY. 



goblet-cells, as far as the smaller bronchi. Within these the ciliated cells are replaced 

 by simple columnar elements which, in turn, give place to low cuboidal cells within 

 the proximal part of the terminal bronchi. Towards the termination of the latter, 

 transition into a simple squamous epithelium takes place. 



The walls of the air-spaces — the atria, the air-sacs, and the alveoli — have es- 

 sentially the same structure, consisting of a delicate fibro-elastic framework which 

 supports the blood-vessels and the epithelium. Within the adult lung the latter is 

 simple and is represented by two varieties of cells, the large, fiat, plate-like elements 

 (.020-.045 mm. ) and the small nucleated polygonal cells (.007-.015 mm.) occurring 

 singly or in limited groups between the plates. Before respiration and the conse- 

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

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

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

 they are often represented by single cells irregularly disposed. 



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. 1575. 



Air-sac 



Passage from 

 atrium into air-sac 



Alveolus 



Terminal bronchus 



Pulmonary artery 

 Bronchiole 



Air-sacs 



Atrium 



Alveolus 



Air-sacs 



Section of lung, showing general relations of divisions of air-tubes. X 50. 



the 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 the 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 then into the one on the opposite side of the interalveolar septum. The 

 capillaries are, therefore, excluded from the interior of the air-cells by practically 

 only the attenuated respiratory epithelium, the large plate-like cells lying over the 

 blood-vessels while the small cells cover the intercapillary areas. Distinct intercellu- 

 lar apertures or stomata, formerly described as affording direct entrance from the 

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

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

 alveolar wall 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 vessels. 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- 



