RESPIRATORY SPACES OF THE LUNGS 25) 
mammals consists of flat, non-nucleated cells and small nucleated 
cells. As the respiratory epithelium is essentially the same in 
the mammals which have been studied, the following description 
will apply to them all (fig. 12). 
The small, nucleated cells present in surface view a round or 
elongated form, from 2 » to 15 u» in breadth and averaging 5 yu in 
height. It has been described sometimes as flat, sometimes as 
cuboidal. As a matter of fact, it is intermediate between the two 
types and might properly be called either flat or cuboidal, but 
in order to avoid confusing it with the non-nucleated plate, it 
will be convenient to designate the nucleated type as cuboidal. 
These cells are distributed not always uniformly and the size 
of the alveoli is also variable, so that the number of nucleated 
cells in different alveoli differs very much, amounting from one 
to ten or more. It is obvious that in the guinea-pig, rat, and 
others, which have small alveoli, the nucleated cells in an alveolus 
are fewer. Though these cells usually appear scattered, they 
sometimes form groups of two or five cells. Although they border 
the non-nucleated cells, sometimes each cell appears singly in 
the midst of the non-nucleated cells. 
The large, flat, non-nucleated cells are approximately penta- 
gonal or hexagonal in shape, though their outlines are not straight. 
Their diameters are usually at least ten times as long as those 
of the small cells. Cells of intermediate size between the two 
are almost never seen. The non-nucleated cells in one alveolus 
number from three to ten or more. As there is no considerable 
difference in the size of the epithelial cells of different animals, 
they are fewer in the animals with smaller alveoli. 
According to Kolliker, the edge of the alveolus is almost en- 
tirely covered with the non-nucleated and only seldom with the 
nucleated cells. Iagree with this statement. The non-nucleated 
cells are bent over the edges of the alveolar septa, so that they 
cover two sides of the wall at the same time. 
For studying the relation between the capillaries of the lung 
and the respiratory epithelium, it is best to select either the 
preparations in which the capillaries as well as the epithelium 
are impregnated or the ones which are mounted in glycerin. 
THE AMERICAN JOURNAL OF ANATOMY, VOL. 27, NO. 3 
