HUMAN LUNG 325 
JUSTESEN’S RULE 
In the embryonic and adult ox, Justesen found the following 
rule concerning the number of bronchial tubes: the number of 
side branches in each case corresponds to those of the continu- 
ation of the trunk and its branches. For instance, take a bronchi- 
olus whose first branch bears eight air-sacs. Then the total 
number of air-sacs attached to the continuation of the bronchiolus 
are also eight. He gives a skillful explanation of this fact. 
Now I have applied Justesen’s rule to my model and find that 
it does not hold for the alveolar ducts of the human lung. 
THE AIR-SAC 
Since (according to Oppel) Rossignol made use of the term 
infundibulum for the last divisions of the respiratory air tubes, 
the shape of this part has attracted the attention of many 
authors. 
According to Justesen, the air-sac generally arises from the 
atrium with a relatively narrow opening and then it expands 
remarkably. Oppel agrees that the air-sac takes an infundibular 
form. On the contrary, Henle, Stieda, and Ebner do not acknowl- 
edge this. Laguesse and d’Hardiviller point out that the region 
of the air-sac is often narrowed. I notice at times a certain 
degree of extension in the air-sacs, but I do not think this exten- 
sion is enough to warrant the name infundibulum. 
Miller, Justesen, and Schulze have shown that the alveolar 
septa are elongated to a certain extent in the region of the air- 
sac and divide it into two or three parts. In case the region of 
the air-sacs is separated in these parts, the elongated septa stand 
parallel or perpendicular to each other. On the other hand, 
when there is a partition into four, they cross each other. The 
alveoli may or may not be included in these parts. In the latter 
case the parts represent nothing more than deep alveoli. 
Now I will show the relation of the air-sacs to the alveolar 
ducts. If in my model I follow the alveolar ducts forward, they 
usually seem to divide themselves into two air-sacs, and some- 
times into three. But when the air-sacs are sent off as side 
