aie h HERBERT G. WILLSON 
marked 3a, and is so designated because in tracing it back through 
the serial sections it was found to represent the third dichotomy 
from a bronchus which contained cartilage in its wall. The 
3a dichotomizes into branchings marked 4a and 4b, of which 
4b has not been followed any further, but 4a again divides dichot- 
omously into Ja and 5b, whose walls show alveolar outbranch- 
ings, so that they are to be regarded as respiratory bronchioles. 
The 56 is followed only a short distance, but 5a again divides into 
two stems, one of which was followed for some distance, but its 
reconstruction is omitted in the photograph for the sake of sim- 
plicity. The other stem, which may be designated 6a, is com- 
pletely reconstructed, and gives rise to all that portion of the 
model which is colored. It can be followed into a further dichot- 
omy, one branch of which gives rise to the portions colored 
orange and green, while from the other all the remaining por- 
tions originate. The orange and green portions have been 
separated from the rest of the model in order that its parts might 
be more completely shown. 
A photograph of the model on this scale, though useful for a 
general orientation of its parts, does not sufficiently reveal the 
details. ‘These are more clearly shown in figure 2, which repre- 
sents a part of the portion colored orange in figure 1 at a greater 
magnification. It shows a number of infundibula or air-saes 
with their alveolar outbranchings, and it shows also how difficult 
it is to determine exactly what shall be termed an air-sac and 
what an air-cell. Thus the lower of the two portions colored 
yellow might equally well be regarded as a single air-sac with a 
number of complicated air-cells, or as at least two air-sacs with a 
common basal portion. Similarly, the upper yellow portion 
might be regarded as a single large air-sac or as three, according 
to the point of view of the observer. The terms infundibulum 
or air-sac (ductulus alveolaris B. N. A.) and alveolus or air-cell 
are all useful in conveying an idea as to the arrangement of the 
terminal air-spaces of the lung, but it must be remembered that 
in the human lung, at least, transitions exist between them; 
particular cases may be found where it is difficult to say whether 
one is dealing with an air-sac or an air-cell. 
