286 HERBERT G. WILLSON 
lung tissue to an average perimeter of 8.713 mm. Using a more 
direct method of calculation than previously, this average perim- 
eter of 8.7138 mm. multiplied by 1 (millimeter) gives 8.713 
sq.mm., the approximate area of lining epithelium in each cubic 
millimeter of emphysematous lung. The readings of eight other 
sections of emphysematous lung, taken from near the pleura, 
gave a total of 4728 mm., or an average of 591-mm., corresponding 
in the actual lung to 5.91 mm., or nearly 6 mm. This average 
perimeter in 1 sq.mm. corresponds to an area of 6 sq.mm. per 
cubic millimeter of lung tissue. 
It will be seen that the average for all the readings of the em- 
physematous lung indicates that a man with emphysema might 
possibly have only half the normal amount of respiratory epithe- 
lium per unit of lung volume. 
CONCLUSIONS REGARDING THE HUMAN LUNG 
1. In the branching of the respiratory bronchioles there is 
far greater complexity, irregularity, and a greater degree of inter- 
locking than is usually described. 
2. There is no spherical space, or ‘atrium,’ such as has been 
described by Miller. 
3. The method of branching of the bronchioles is dichotomous 
until the terminals are approached, and then the branching 
becomes irregular. 
4. Counting as the first branch, a respiratory bronchiole aris- 
ing from a non-respiratory one, the air-sac is usually reached at 
the fifth to seventh branch. 
5. There are normally no direct communications between 
adjacent alveoli. 
6. The bronchioles do not decrease in diameter as the periph- 
ery is approached, but remain of fairly uniform size until the 
air-sacs are reached, and the air-sacs are, as a rule, of greater 
diameter than the tubes from which they arise. 
7. Waters’ rule, that the planes of successive dichotomies cut 
one another at right angles, is only exceptionally confirmed. 
8. The lung of the child is just as complex in structure as that 
of the adult. 
