126 The Development of the Lungs 
dorsal bronchus. The second lateral bronchi develop in the region be- 
tween the chest wall, heart, and liver. The area in which the remainder 
of stem has to grow has in cross-section practically the shape of an 
isosceles triangle. The stem, occupying a point about the middle of the 
base, sends three sets of branches, namely, dorsal, lateral, and ventral, 
directed into the angles of the triangle where they would have the most 
freedom to develop. Between the roots of the two stem bronchi runs 
the cesophagus, leaving no place for the development of median branches 
in this region. At the level of Lateral 4, however, below the cesophagus 
more room occurs and, consequently, we observe in this region the forma- 
tion of medial bronchi. Undoubtedly the difference in the branching of 
the stem in the Lobus inferior of the human lung when compared with 
the pig may be sought in its altered topography owing to the erect posture 
which changes principally the position of the liver. 
This adaptation on the part of the lungs to their environment is to be 
expected for they are relatively late accessions to the animal economy and 
are of no known use to the organism during the period of gestation. 
Accordingly as the heart and liver are both phylogenetically older than 
the lungs and also are of known functional value during feetal life, it is 
natural that the latter should adapt themselves to the early needs of 
older organs. 
10. The growth of the main series of bronchi is monopodial in char- 
acter, that is to say, they are produced without a definite division of the 
end bud. New elements are not always produced from the end bud, but 
may be formed from the stem some distance from its terminus. 
The process is successive, that is to say, the elements are produced one 
after another from above downwards, recapitulating the method of growth 
shown in simpler animals like the reptiles, for example. When a new 
element is about to be produced, on2 notes an increase in the number of 
karyokinetic figures in the epithelium in the region of the new branch. 
The basement membrane becomes less distinct and the connective-tissue 
nuclei in the surrounding mesoderm are more closely packed together. 
In this region a slight bulging of the epithelium is then noted, which 
increases until a small elevation is raised upon the surface of the stem. 
This increases in size, yielding a rounded projection, which gradually 
emancipates itself and gives rise to a new bronchus. The process is 
essentially the same whether it occurs in the neighborhood of the termi- 
nal bud or higher up on the stem. In general, we may say, the lateral 
and medial elements are produced nearer the terminal end of the main 
