128 The Development of the Lungs 
the ventral series, arteries pass around the lateral aspect of the stem 
bronchus beneath the root of the corresponding lateral bronchus to gain 
the outer aspect of the ventral bronchus along which they run. The 
medial bronchi receive their supply from branches that originate from 
the main artery and pass around the dorsal aspect of the stem to run on 
the dorsal surface of the medial bronchi. As the right pulmonary artery 
runs ventralwards to Lateral 1 the artery to that bronchus develops on 
its ventral surface. 
In the younger stages, both the aortic arch and the Ductus arteriosus 
he well above the level of Lateral 1. As the embryo increases in age, 
there is a gradual descent of the heart and with it, the great vessels. 
At 15 cm. one observes the Ductus arteriosus at the level of Lateral 1; 
at 22 cm. the aortic arch reaches this point, while at birth both vessels 
le below the bronchus. 
12. The pulmonary vein develops in pigs about 5 mm. long as an in- 
growth from the undivided portion of the Sinus venosus at the level of 
the pulmonary anlage. As the stem bronchi increase in size, right and 
left pulmonary veins develop from the capillary plexus which surround 
them. ‘These, naturally, form on the ventral surface, with the bronchi 
between them and the arteries. Similarly, as the various principal 
bronchi are produced from the stem bronchus, veins are formed from 
the capillary plexus. The veins from the lateral bronchi lie below and 
ventralwards to the bronchi, those from the dorsal elements run along 
the medial aspect of the air passages to empty into pulmonary veins 
lying ventralwards to the stems. The veins from the ventral bronchi 
extend along the medial aspect of the bronchus and terminate directly 
into the pulmonary veins; those from the medial bronchi extend along 
their ventral surface to empty in the larger veins accompanying the 
stems. ‘The vein from Lateral 1 runs along the ventral aspect of the 
bronchus somewhat ventralwards to the corresponding artery. This 
forms the single exception to the general alternation of artery, bronchus, 
and vein. As the embryo increases in age, the Vena pulmonalis, which 
originates near the midline, is gradually pushed to the left by the 
increasing asymmetry of the heart, until it finally comes to he over the 
area of the stem bronchus where a left Ventral 2 would have developed 
if such a bronchus were present. ‘The hyperdevelopment of the Bronchus 
infracardiacus associated with the development of the Vena cava in- 
ferior to the right of that bronchus aids in pushing the Vena pulmonalis 
to the left. 
13. The asymmetry of the mammalian lung is associated with the 
