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cae) 
The Development of the Lungs 
is placed above and ventralwards to the bronchus, receiving tributaries 
from its side branches. The main dorsoinferior branch of Lateral 2 
lies ventralwards to its bronchus, while the corresponding artery is 
placed dorsalwards and above. This vein crosses behind Lateral 2 to 
join the main venous trunk, which accompanies Lateral 2 until, in com- 
mon with the vein to the tracheal bronchus, it empties into the common 
pulmonary. 
The veins from Lateral 3, 4, and 5 have shifted now so that they lie 
a short distance ventralwards from the corresponding bronchi. They 
pass medialwards under the ventral bronchi and empty into the right 
pulmonary stem vein; those from Ventral 3, 4, and 5 lie medial to the 
respective bronchi and run dorsalwards to the stem vein. Two veins 
now accompany Ventral 2, one above running medialwards and upwards 
and emptying into the large trunk formed by the fusion of the two stem 
veins, and another lying behind the branches of Ventral 2 which passes 
upwards and joins the common vein from the lower part of the tree on 
its right side at the point of junction of the veins from the right and 
left stems. 
From Dorsal 2, 3, +, 5, and 6 the veins, lying medial to their stems, 
run ventralwards past the stem bronchus to empty into the large stem 
veins opposite their corresponding branches. The veins from the medial 
branch lie yentralwards to them and pass lateralwards to the stem vein. 
The relationships of the veins on the left side of the tree below Lateral 2 
are, with the exception of those from the Lobus infracardiacus, similarly 
arranged to those on the right. 
Throughout the whole tree to this stage, we note with the single ex- 
ception of Lateral 1 the constant relationship, which was indicated in 
the earlier embryos, of the regular alternation of artery, bronchus, and 
vein. In the earlier stages, the vessels were placed relatively close to the 
bronchi; but with the increasing age of the embryo, the position of the 
artery and vein has gradually shifted giving them a position at some dis- 
tance from it. In some cases, this wandering may be so marked, especially 
below Lateral 2, that the main veins and their chief branches may 
occupy a position approximately midway between the adjacent bronchi. 
The arteries, however, always lie closer to the air passages. 
In the first part of embryonic life, the left pulmonary arch with a por- 
tion of the right connects the right ventricle and the aorta, and the pul- 
monary arteries, after the manner described by Bremer, finally take 
origin from the left by a common stem arising from its under surface. 
The aortic arch lies above, and both arches are situated superior to the 
