TERMINALS OF HUMAN BRONCHIOLE 281 
It might be pointed out that the method of multiplying the 
perimeter by the thickness of the section is exact only in the case 
of a tube of uniform diameter. ‘To illustrate, it is plain that tke 
cylinder formed by a pile of coppers has an area on its curved 
surface equal to the sum of the areas of the edges of the coppers, 
while the area of the curved surface of a cone is really greater 
than the total area of the edges of a number of dises of gradually 
diminishing diameter piled up to represent a cone. Here, then, 
is a source of error which tends towards making the result too 
low, while errors of omission in the counting or tracing would 
tend in the same direction. On the other hand, the cubic milli- 
meter of lung tissue on which our calculation is based contained 
only the finer branchings of air-passages, so that the result would 
be accurate only if the whole lung were made up of such fine 
branchings. This source of error, tending to make a too high 
result, can hardly be canceled by the factors referred to above. 
The figures given probably represent the extreme upper. limits 
of area corresponding to the respective degrees of expansion. 
In Hermann’s Handbuch der Physiologie there is given a cal- 
culation by Zuntz of the area of the respiratory surface. Zuntz 
assumes the average diameter of an alveolus to be 0.2 mm. when 
the lung is moderately inflated, and the total air-space of the 
lung to be 3400 to 3700 cem. He considers that at least 3000 
cem. of this space is occupied by alveoli and infundibula. He 
calculates the volume of an alveolus as though it were a sphere, 
and arrives at the following result for volume and area of a 
single alveolus: 
Volume, 0.00414 ccm. 
Area, 0.125+ sq.mm. 
Reducing the 3000 eem. to cubic millimeters, he divides this 
volume by the volume of a single alveolus, and reaches the con- 
clusion that the number of alveoli in the lung is 725 million. On 
the basis of this result and the estimated area of a single alveolus, 
he concludes that the area of the respiratory surface is $0 square 
meters, when the lung is inflated to a moderate extent. 
Aeby used the same figures as Zuntz for volume and area of a 
single alveolus, assuming the average alveolus to be of spherical 
THE AMERICAN JOURNAL OF ANATOMY, VOL. 30, NO. 3 
