68 The Development of the Lungs 
just below the point of bifurcation. On the right side, the vein from 
Lateral 1 passes downwards and medianwards to empty into the Vena 
pulmonalis at a point just above the confluence of the two vessels which 
accompany the stem. 
In a pig 18.5 mm. long, the relationships of the pulmonary arteries 
to the trachea (Pl. II, Fig. 19) remain the same. Just above the point 
of bifurcation, they pass gradually behind the main bronchi to take un 
their dorsolateral position. No marked changes are observed in the 
arteries to Lateral 1, save in an increase in length. The second lateral 
branches present no changes, except on the left side where a branch runs 
up on the dorsolateral aspect of the apical division of Lateral 2 (Pl. I, 
Fig. 19). The artery to Ventral 2 arising just beneath the Lateral 2 
on the right side and passing around the stem and under the root of 
Lateral 2 to run along the outer aspect of the second ventral bronchus, 
now shows a secondary branch which follows the inferior division (PI. 
II, Fig. 19) of Ventral 2. Small arteries are given off to right and 
left Dorsal 2 which run along their lateral superior aspect. On either 
side, branches to Lateral 3 (Pl. III, Fig. 20) run from a point just 
below the origin of the arteries of Dorsal 2. Beneath the third lateral 
bronchi, arteries arise which pass around the axial bronchus, and run 
lateralwards to Ventral 3. Below this level, branches are given off on 
both sides successively to Dorsal 3, Lateral 4, Ventral 4, Dorsal 4, and 
Lateral 5 (Pl. III, Fig. 20). The pulmonary veins (Pl. II, Fig. 19) 
lie medialwards and yentral to the main bronchi. Besides the branches 
from the lateral bronchi, which have been observed in the preceding 
stages, venules, lying on the medial surface of the dorsal bronchi, pass 
around the median aspect of the main bronchus and empty into the pul- 
monary veins. Similar veins from the ventral bronchi run along their 
median aspect, and empty into the Venze pulmonales on both sides. 
Otherwise, there are no marked changes in the yenous system at this 
stage save that the veins from the Lateral 2 and Lateral 1, on the right 
side now empty into the Vena pulmonalis by a common trunk. The 
second lateral vein on the left and with it a vein from the apical branch, 
which joins it about the root of Lateral 2 empties into the main pulmon- 
ary vein at a level somewhat higher up than the one which accompanies 
the left stem bronchus. The two veins from the stems join about the 
point of origin of the main bronchi and are continuous with the Vena 
pulmonalis above. From the infracardiac bronchus, a vein empties into 
the right stem vein just above the level of L. 3. 
At this stage the main characteristics of the pulmonary vessels are 
established for life. The arterial branch to Lateral 1 runs upwards 
