318 



THE ORGANS 



The alveolus (Figs. 215, (/, 216). The epitheUum of the alveolus 

 consists of two kinds of cells, respiratory cells and so-called "festal" 

 cells (see Development, page 321). 



The respiratory cells (Fig. 216) are some of them large, flat, non- 

 nucleated plates, while others are much smaller, non-nucleated ele- 

 ments. The absence of nuclei and the extremely small amount of 

 intercellular substance render these cells quite invisible in sections 

 stained by the more common methods. The cell boundaries are best 

 demonstrated by means of silver nitrate (technic i, p. 82). 



Fig. 216. — From Section of Cat's Lung Stained with Silver Nitrate. (Klein.) 

 (Technic i, p. 82.) Small bronchus surrounded by alv^eoli, in which are seen both flat 

 cells (respiratory epithehum) and cuboidal cells (foetal cells). 



The "fcetal" cells are granular, nucleated cells which are scattered 

 among the respiratory cells (Fig. 216). Their position appears to be 

 less superficial than that of the respiratory cells, the foetal cells lying 

 in the meshes of the capillary network, the respiratory cells covering 

 the capillaries. In the embryonic lung and in the lungs of a still-born 

 child the air passages and alveoli contain only this type of cells, the 

 small flat plates apparently resulting from a flattening out of the 

 cuboidal cells due to pressure from inspiration, and the large flat 

 plates to union of a number of small plates. Delicate elastic fibrils 

 support the respiratory and foetal cells. Around the opening of the 

 alveolus the elastic fibres are more numerous, forming a more or 

 less definite ring. The disposition of elastic tissue in the wall of the 



