66 The Development of the Lungs 
forming the right and left stem veins, run on the ventromedial aspect of 
the stem originating from the plexus about the main bronchi. In this 
way, we have established the regular alternation of artery, bronchus, and 
vein which persists throughout the life of the tree, although it will be 
remembered that this relationship is due primarily to the position of the 
vein with reference to the anlage. 
At 12 mm. (PI. I, Figs. 13, 14) the vessels have followed the natural 
growth of the bronchi. From the capillary plexus on the dorsal surface 
of Lateral 2 on each side, the artery to that branch is formed. The 
vein (Pl. I, Fig. 13) by the rapid development of Ventral 2 is pushed 
somewhat medialwards at this point. With the marked development of 
Lateral 1, the tracheal bronchus, in a pig 13.5 mm. long, a branch (PI. 
II, Fig. 15) is given off from the right pulmonary artery, which runs up 
along the ventral surface of the bronchus to end in the plexus about that 
branch. Continuing downwards, the arteries (Pl. II, Fig. 16) on both 
sides run on the dorsolateral aspect of the stem. The branches to Lateral 
2 have increased somewhat in length, and from the right pulmonary 
artery a new branch is formed, which, passing under the root of right 
Lateral 2, ends on the lateral and under aspect of Ventral 2, the Bron- 
chus infracardiacus. The artery still maintains its position with refer- 
ence to the stem, which causes it to lie in the angle between the lateral 
and dorsal bronchi. Thus, the artery itself, however, is not responsible 
for the division of these two groups from the stem as Aeby implies when 
he says in speaking of Lateral 1, “In ihm hat offenbar die Scheidung 
des hyparteriellen Gebietes in zwei streng geschiedene Bezirke noch nicht 
stattgefunden, ein Thatbestand, der wohl damit in Verbindung gebracht 
werden darf, dass die Lungenarterie nicht sondernd einzugreifen ver- 
mocht hat.” Should we still suspect a causal relationship here, it is only 
necessary to glance at the ventral bronchi, particularly Ventral 2, to see an 
element not only originating from the stem away from the influence of the 
artery but also with its growth, developing from its capillary plexus an ar- 
tery which passes around the stem and rests on its lateral side. Interesting 
changes, at the same time, are occurring in the veins (PI. II, Fig. 15). 
From the tracheal bronchus, a branch may be observed passing down to 
the common pulmonary vein running still more ventral than the artery 
to Lateral 1, another one of the final adult relationships in the pig’s lung. 
Here, however, we have an exception to the general relationships of the 
vessels to the bronchi due to the more ventral position of the veins and 
the failure of right pulmonary artery to form behind Lateral 1, which, 
in this particular instance, gives us a Vena comes to the artery to the 
tracheal bronchus instead of the usual alternation found in other portions 
