RESPIRATORY SPACES OF THE LUNGS 349 
epithelium can be seen with those methods by which even the 
endothelium of the capillaries themselves were impregnated by 
injection through the trachea.. Therefore, from histological evi- 
dence it seems reasonable to conclude that no respiratory epithelium 
is present in the respiratory spaces of birds. 
As described later, the reticular fibers pursue their course 
along the blood-capillaries of the respiratory spaces of birds; 
therefore, there must be a thin membrane, which involves the 
fibers in the circumference of the blood-capillaries, although 
no epithelial covering is found here. I treated preparations, 
which were fixed in Flemming’s fluid, with Heidenhain’s iron- 
hematoxylin and looked for such thin membranes, but, as was 
to be expected, direct demonstration was impossible. 
5. Respiratory epithelium in Mammalia 
The animals used for this study were the mole, bat, rat, 
guinea-pig, rabbit, cat, and dog, among which the first two 
mentioned show considerable differences in the structure of 
respiratory spaces, so that I will describe them separately. 
A. Respiratory epithelium of mole and bat. The respiratory 
epithelium of these animals has not been described before. 
The branching of the air passages of these animals is almost 
the same as that of other mammals, that is, into bronchioli and 
alveolar ducts. But the finer structure of respiratory spaces 
themselves are comparable with those of the bird in several 
respects. For the intercapillary spaces have no substance at 
all, the neighboring alveoli communicate with each other freely. 
The diameter of the intercapillary spaces in the mole is in general 
two or three times as large as that of the blood-capillaries there, 
but as each alveolus has its capillary network, the latter appears 
in surface view to be the one over the other, and accordingly the 
space seems narrower. The capillary networks of adjoining 
alveoli communicate with each other. 
In the bat there is only one layer of common capillary net- 
work between two alveoli, the spaces being of the same diameter 
as the blood-capillaries there. 
