TERMINALS OF HUMAN BRONCHIOLE 279 
alveoli represented in the model are elongated to a greater ex- 
tent than is usually described, though, as has been mentioned, 
Rossignol noted that certain alveoli had unusual depth. The 
models confirm the observations of Rossignol regarding alveoli. 
In the construction of the models and in the examination of 
the sections, careful search was made for evidence of interalveo- 
lar communications, but no evidence of their existence was found. 
The air-sacs interlock with wonderful closeness, so that there is 
absolutely no waste of space, and because of this close interlock- 
ing it sometimes requires great care to satisfy oneself of the ab- 
sence of interalveolar communications, but in no case could such 
communications be demonstrated. 
THE AREA OF PULMONARY AIR-SPACES 
During the course of this work the interesting question of the 
total area of the respiratory air-spaces naturally suggested it- 
self, and an attempt was made to answer it by estimating the 
total area of the respiratory epithelium in a cubic millimeter of 
lung tissue and then multiplying this by the total volume of the 
lung, expressed in cubic millimeters. It is evident that such a 
method can give only approximately the actual respiratory sur- 
face in the lung, since it takes no account of the variations that 
may occur in the size and number of the air-spaces in various 
cubic millimeters of the lung, and it fails to make allowance for 
the larger non-respiratory bronchioles and bronchi. Yet the 
calculation seemed worth carrying out, as it promised, at least, 
a maximum figure beyond which the total respiratory surface 
could not possibly extend. 
In order to estimate the area represented in one cubic milli- 
meter of lung tissue, a square of 100 mm. side was marked out on 
each of fifty successive drawings in the series of adult lung, the ~ 
squares being oriented so that the series of squares represented 
successive sections of tissue. Since the sections were 20 yu thick, 
the fifty squares together represented sections totaling 1 cu. 
mm.in volume. The total perimeter of the various air-passages 
in each of these squares was measured. This was done by trans- 
ferring the drawings within each square to millimeter paper, and 
