Joseph Marshall Flint 47 
dorsoinferior branch, which is the equivalent of the apical branch on L. 2 
of the left side. In this specimen, however, the ventral fork becomes the 
ventroinferior branch and the dorsal fork continues as the main bronchus, 
giving rise to the dorsoinferior branch only after undergoing another 
subdivision. In a much smaller percentage of lungs, the same thing 
happens on the left side, the ventral fork giving rise to a ventroinferior 
branch, while the dorsal grows on as the stem, producing the apical or 
stem only after passing through another division at the end. When this 
state of affairs occurs, we have the so-called “ cardiac bronchus of Hasse,” 
which d’Hardiviller believes is formed on the stem bronchus in the space 
for left V.2, and then wanders up to Lateral 2. Of course in some 
animals Ventral 2 is formed regularly on the left side, and in others as a 
variation which establishes the symmetry of this segment of the tree. 
In the pig, however, owing to the relations of the pulmonary vein to 
this part of the stem (see chapter on pulmonary vessels), I have never 
seen a left Ventral 2. This power of the bronchi gives us a suggestive in- 
sight into the adaptations of the growing branches. The selection of the 
division to continue as the stem is probably governed largely by the physi- 
cal environment in which the branches find themselves. As the conditions 
are usually the same, the same branches ordinarily become the stem, but 
if these are changed, what generally forms the stem is shunted off to 
become a side branch of relatively small size, while the division which 
usually constitutes the side branch, grows out as the stem and produces a 
numerous progeny of lateral divisions. In other words, the extent of the 
growth of a branch depends to some degree upon the nature of its physi- 
cal environment. As I have stated above, owing to the generally fixed 
conditions, the major branches, especially such important ones at Lateral 
2, have ordinarily a fixed type of division, but further out on the laterals 
or in the lower divisions, like Lateral 4 or 5 for example, this interchange 
of forks frequently takes place, as almost every specimen shows variations 
in the order of the branching. 
The next division of the L. 2 is the ventrosuperior (Pl. II, Fig. 19; 
Pl. III, Fig. 20, L. 2, s), projecting from the main bronchus just ex- 
ternal to the dorsoinferior branch, while a short distance lateralwards and 
dorsalwards is given off a dorsosuperior branch (PI. II, Fig. 19; Pl. II, 
Fig. 20, L. 2, ds), which already shows indications of division. These 
branches represent apparently branches of the second order, but in reality, 
after a dichotomous division, each segment of the stem between the suc- 
cessive branches is equivalent in its order to that of the last lateral 
division. In the adult lung these branches are all easily recognizable. 
