RESPIRATORY SPACES OF THE LUNGS 347 
4. Respiratory epithelium in Aves 
In the respiratory apparatus of birds, the bronchi, which are 
always located near the surface of the lung, give rise to the 
parabronchi which branch sideways into the interior of the lung. 
The parabronchi divide into the lung-flutes, whose walls are 
filled with small cavities. These cavities have hitherto been called 
bronchioli, but it seems to me more fitting to call them flute- 
holes. From the bottom of each flute-hole extend several small 
canals which are true respiratory spaces and were erroneously 
called alveoli by some authorities. I will designate the canals 
as respiratory canaliculi. 
While the flute-holes are lined by continuous pavement epi- 
thelium, the wall of the respiratory canaliculi consists of close 
blood-capillaries, whose spaces have no coating and admit free 
passage of inspired air, resulting in the communication of all 
the respiratory canaliculi with each other. Between two neigh- 
boring respiratory canaliculi there are usually capillary networks 
of several layers, so that the spaces are highly complicated and 
difficult to study microscopically. The intercapillary spaces have 
about the same diameter as the blood-capillaries. 
The respiratory epithelium of birds has been studied by various 
investigators. Baer studied the lung of the pigeon and came 
to the conclusion that the intercapillary spaces are coated by 
delicate, flat ‘Belag,’ while the capillaries themselves are naked 
in every case. According to Eberth, the blood-capillaries of 
the finest air-passages (respiratory canaliculi) are especially 
uncovered, with flat cells attached here and there. I think that 
these conclusions are doubtful, for the methods used were in- 
efficient. Most authors, as Elenz, Oppel, Schulze, are of the 
opinion that the respiratory epithelium of birds, though not 
actually demonstrated, must be present. 
If the respiratory spaces of birds were bare, it would be the 
only case in higher animals in which either an external or an 
internal surface of the body has no coating of epithelium. 
In studying this question several methods were used.  In- 
jection of the lungs of the duck, pigeon, and sparrow with 0.25 
