58 The Development of the Lungs 
an average relative value. Those which have preceded are, however, suffi- 
ciently constant to serve as a general guide to the direction taken by the 
main branches of the adult tree. 
It may be well, however, to show pictorially the subsequent evolution 
of the tree without taking up the details of the branching, as a good 
corrosion of the bronchial system holds the general form of the lung 
quite as well as a hardened specimen of the lung itself. The tree of a 
pig 7 cm. long is shown in Pl. IV, Fig. 22. Besides the increasing com- 
plexity of the branching, one notes the ventral curvature of the lateral 
bronchi parallel with the chest wall. This is most marked in Lateral 2, 
less so as we proceed to Lateral 6. There are some peculiarities on this 
tree which are of great interest, for Ventral 3 on the left side is sup- 
pressed and in its place a prominent division of the second ventral or in- 
fracardiac branch has grown medianwards to take its place. A branch 
from Lateral 3 also runs to this region, giving an appearance as though 
it might be a ventral bronchus which had not left the lateral series. It 
is, however, a simple substitutive process on the part of the lateral branch 
for an element which has not developed in the earher stages. This speci- 
men also shows an instance where the dorsal fork of the first division of 
Lateral 1 continues as the stem, leaving the ventral fork, which usually 
serves that purpose, as a ventrosuperior branch, while the large dorso- 
inferior branch which is usually comparable to the apical branch on the 
opposite side rises from the next division. A median bronchus occurs 
on the left side opposite Lateral 4. On the right side, median divisions 
are not present. 
In the corrosion of a tree from the lung of a pig 18 cm. long (PI. IV, 
Figs. 23, 24) a number of interesting features may be observed, which 
serve to illustrate some of the developmental characteristics of the grow- 
ing bronchial tubes. In the first place, we ordinarily have five paired 
lateral bronchi, while in this specimen there are but four. This indi- 
cates the suppression of the last of the lateral elements which is com- 
pensated for by an hyperdevelopment of Lateral 5 to supply the region 
usually tributary to Lateral 6. Accordingly the terminal forking of 
the stem bronchus, which usually occurs between Lateral 6 and the 
continuation of the stem, takes place in this instance between it and 
Lateral.5 (Pl. IV, Figs. 23, 24). While this tree shows the suppres- 
sion of one of the lateral branches, I also have some specimens which 
present a series of six paired lateral bronchi below L. 1, indicating a 
possible variation in these elements between these limits with 5 as the 
average. Ventral 3 is suppressed on both sides, on the right it is 
