280 HERBERT G. WILLSON 
counting the number of millimeters in the perimeter of each air- 
space in that square, and totalling the amount. The grand total 
for the fifty squares amounted to 69346 mm. Since the magnifi- 
cation was 100 diameters, the corresponding perimeter in the actual 
=a mm. and if this be multiplied by the thick- 
lung would be 
the result will be nearly 14 sq. mm. 
ness of the sections zt 
(1000) 
of respiratory surface in 1 cu. mm. of lung tissue. 
The lung tissue used in this estimation was obtained after the 
lungs had collapsed—the pleura having been opened. Vier- 
ordt estimates the volume of the lungs in this condition to be 
from 3005 to 3975 ecm. Taking the volume as the average of 
these, 3400 ccm. or 3400000 cmm., the area of the walls of the 
air-passages (respiratory and non-respiratory) is approximately 
son x aa < 3400000 sq. mm., or about 47 square meters. 
Now, the volume varies as the cube of like dimensions, while 
the area varies as the square of like dimensions, so that the area 
would not be doubled if the volume of the lung were doubled by 
expansion of air-passages. According to Arnold, the volume 
of the lung when fully inflated is 6805 cem., and Vierordt states 
that the volume is 9521 ccm. ‘bei stirkster Fillung.’ For the 
areas corresponding to these estimates the extreme limits might 
fairly be placed at 70 and 90 square meters, respectively. Vier- 
ordt’s estimate possibly refers to artificial inflation of the lungs 
after death. For the volume of the lungs on deep inspiration, 
Arnold’s estimate seems a. reasonable one, since 5500 cem., in 
the case of the adult lung, is the approximate total volume of 
complemental, tidal, supplemental, and residual air. We are 
thus led to the conclusion that on ordinary deep inspiration the 
total area of the respiratory and non-respiratory epithelium is 
approximately 70 square meters, and the respiratory area alone 
must be considerably less than this. In order to estimate how 
much less, one would have to know the proportionate amount of 
respiratory to non-respiratory epithelium in the air-passages, 
and this is not known. | 
