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RESPIRATORY SPACES OF THE LUNGS 355 
It will perhaps not be out of place here to discuss the methods 
used by pathologists in studying respiratory epithelium, though 
they have already been considered by Miller. The pathological 
changes in respiratory epithelium have been determined by the 
study of thin sections of the lung, stained in the ordinary way, 
and this method is exceedingly inadequate because it does not 
reveal the non-nucleated cells, which make up the greater part 
of the coating of the respiratory spaces. The small nucleated 
cells may at first seem easy to recognize in these preparations, 
but in fact this is not the case, because, owing to the thinness 
of non-nucleated cells, it is necessary to make a careful examina- 
tion before deciding whether a nucleus, which appears within 
the alveolar wall, belongs to the small, non-nucleated cells or 
to the subepithelial cells. 
C. Criticism of the opinion that the respiratory epithelium is made 
up of one kind of cell. According to Oppel, the large non-nucle- 
ated cells of the respiratory epithelium are not separate cells, 
but parts of small nucleated cells, the impregnated cement lines 
being the borders between these portions and the blood-capil- 
laries. Seymonowicz and Osawa agree with this statement. 
My reasons for objecting to this opinion are as follows: 
1. If the non-nucleated cell is considered as a part of a nucle- 
ated cell, then in what way can the border lines, which appear 
with impregnation, be explained? In Amphibia the border 
between the flat portion and the nucleated portion undoubtedly 
follows the contour of blood-capillaries; but this is not the case 
in mammals, because it can be easily shown in silver preparations, 
which are mounted in something like glycerin, that the cement 
lines of the epithelial cells do not correspond to the edges of the 
blood-capillaries. Oppel believes that the presence of border 
lines between the two parts of the cells may also be explained 
in some other way, but he does not himself explain it. 
2. The large, non-nucleated cells are usually adjacent to the 
small, nucleated cells, but sometimes the former do not touch 
the latter and are enclosed entirely by non-nucleated cells. 
(For example, non-nucleated cells in the middle of figure 16.) 
' In such cases the non-nucleated cells can hardly be considered 
as parts of nucleated cells. 
