GENERAL STRUCTURE OF THE RESPIRATORY ORGANS. 365 





twenty times greater than that of the capillaries which arc distributed upon 

 their parietes; varying (according to the measurement of Weber) from the 



The inner surface as well as the whole of the 



to the ? ' th of an inch. 1 



FK;. 158. 



FIG. 159. 



System of alveolar passages and in- 

 fundihula from the margin of the 

 Lung of a Monkey (Cereopethecus) in- 

 jected with mercury, magnified 10 di- 

 ameters, o, Terminal bronchial twig; 

 6, b, infundibula; c, c, alveolar pas- 

 sages. 



Air-cells of Human Lung, with intervening tissues : a, epithelium ; b, elastic trabecuke ; 

 c, membranous wall, with fine elastic fibres. 



infuudibula and alveolar passages are invested by a continuous and in the 

 foetus homogeneous epithelium. In adult mammals, however, it is not homo- 

 geneous. In the mature foetus the cells are flat, 4-6 angled, with granular 

 contents and a bright spherical nucleus. In the adult such cells are 

 arranged in groups of two or four separated by large transparent thin struc- 

 tureless cells, which are either irregularly angular or sinuous in outline. 

 These cover the capillaries and all the folds and prominent edges. The 

 granular spheroidal cells cover the 

 alveolar walls between the meshes of 

 the capillaries, and in adults constant- 

 ly contain spheroidal black pigment- 

 granules. The capillary plexus ( Fig. 

 160) is so disposed between the two 

 layers which form the walls of two 

 adjacent air-cells, as to expose one of 

 its surfaces to each ; by which provi- 

 sion the full influence of the air upon 

 it is secured. The network of ves- 

 sels is so close, that the diameter of 

 the meshes is scarcely so great as that 

 of the capillaries which inclose them, 

 and they often project into the cavity 

 of the alveoli covered only by the 



FIG. 160. 



Arrangement of the Capillaries of the Air-cells of 

 the Human Lung. 



1 The dimensions given by Moleschott (De Vesiculis Pulmonum Malpighianis) are 

 very much less than these; the range of diameter being stated by him at between 



T i- -th and 



^oth of an inch. The Author's own observations, however, lead him to 

 regard Weber's and Kolliker's statements as very near the truth. 



