270 HERBERT G. WILLSON 
at the base of a four-sided pyramid, the apical angle of which 
is occupied by the atrium. In other words, the air-saes do not 
arise as accidental growths from the atria, but are formed by 
two successive dichotomies in planes at right angles. In the 
adult ox he found occasionally but three air-sacs on an atrium— 
a condition which he explains by supposing that in the case of 
the primary air-sacs the secondary dichotomy had failed owing 
to space exigency. 
Justesen holds that the bronchial branchings occur in a definite 
mathematical plan and are fundamentally dichotomies, but 
that in the majority of the branches the dichotomy becomes modi- 
fied into a sympodial arrangement, the terminal branches still 
retaining the dichotomous plan. If this be so, and the mathe- 
matical regularity of the dichotomies persist, the lateral branches 
of each sympodial stem might be expected to show a decreas- 
ing number of air-sacs as they were traced peripherally, one 
arising from an earlier dichotomy having twice as many air- 
sacs as that which arose from the succeeding dichotomy. Jus- 
tesen believed that he obtained evidence in favor of this arrange- 
ment in his observation on the pig where the eparterial 
bronchus gave rise to as many lateral branches as did the stem 
branches for the rest of the lung. 
F. E. Schulze, writing in 1906, takes the view that Miller’s 
atria are not new spaces, but only those parts of the ductuli 
alveolares into which the sacculi open. He states: 
So wenig, wie man an einem sich unregelmissig verzweigenden Baum- 
ast diejenigen Stellen, wo sich ein Ast in zwei oder auch mehrere 
Endiste teilt, als besondere typische Stellen charakterisieren und mit 
einem eigenen Namen, sondern einfach als Teilungsstellen zu bezeichnen 
pflegt, so wenig scheint mir in dem respiratorischen Gangsystem der 
Lunge die Auszeichnung dieser Stellen durch eine besondere Benennung 
(‘Atrium’) erforderlich oder auch nur zweckmiassig zu sein. 
Schulze claims that normally in man and in many mammals 
there are direct communications between alveoli—‘alveolar 
pores’—and in this view he is supported by Hansemann, Has- 
sall, Zimmerman, Nicolas, and Merkel, but is opposed by 
Piersol, W. 8. Miller, Laguesse, and Oppel. 
