278 HERBERT G. WILLSON 
towards the periphery. There is, indeed, sometimes an increase 
even before the air-sacs are reached. 
The air-sacs themselves show a great diversity of shape and 
size and frequently they are recurrent. Figure 2 shows plainly 
the tendency of the air-sacs to widen out to a greater diameter 
than the bronchiole from which they arise. It was, no doubt, 
this widening-out tendency which caused the early investigators 
to use the term ‘infundibulum,’ though the air-sacs are not 
funnel-shaped. Calculations of the actual size of eight air-sacs 
gave the following results, the measurements being taken in 
three dimensions: 
0.4x0.8x0.4 mm. 
0.3 x0.6x 0.3 mm. 
0.7x0.4x0.3 mm. 
0.6x0.3x0.4 mm. 
1.0x0.4x0.5 mm. 
0.3 x0.4x 0.3 mm. 
OF5eEx0ksix O22 mnie 
0.4 x 0.6 x 0.2 mm. 
The air-sacs or alveoli, as shown in the model, vary greatly 
in size and shape. The following estimates were made of the 
actual diameters in three directions of the alveoli of the lung: 
(All the measurements given above have reference to the adult 
lung.) 
0.05 x 0.06 x 0.07 mm. 
0.08 x0.08 x 0.12 mm. 
0.08 x0.10x0.13 mm. 
0.06 x0.08 x 0.10 mm. 
ONL x08 bsx7 0.20) mm: 
0.08 x0.10x0.13 mm. 
0.08 x0.05x0.15 mm. 
0.08 x0.10x0.10 mm. 
Average: 0.075 x 0.09 x 0.125 mm. 
Extremes: 0.05 and 0.20 mm. 
Ogawa in the case of a man of thirty-one years found an average 
of 0.1 mm. for depth and breadth of an alveolus, and in the case 
of a man of fifty-six years, his estimates are 0.15 mm. depth and 
0.19 mm. breadth. Ogawa’s extreme estimates, counting both 
his cases, are 0.04 and 0.21. It will be seen that many of the 
