76 The Development of the Lungs 
Huntington, 98, says: “If we seek for an explanation of the cause 
which leads to the migratory changes of the cephalic bronchus (Lateral 
1), I admit that we enter the realm of pure hypothesis. At the same 
time, the very general development throughout the mammalia of this 
type, with the resulting greater respiratory area of the right lung, may, I 
think, not improperly be referred to the development of the mammalian 
form of the systemic and pulmonary arteries. On the left side, the 
greater quantity of blood thrown from the right ventricle into the left 
pulmonary artery passes through the Botallian duct directly into the 
aorta, only a small portion traversing the left pulmonary circulation. 
On the right side, however, with the early obliteration of the dorsal seg- 
ment of the fifth arch, all the blood entering the right pulmonary artery 
is forced to traverse the entire pulmonary circulation returning to the 
left auricle by the pulmonary veins.” This explanation, according to 
Bremer’s description of the development of the pulmonary arteries, could 
not account for the increased size of the right lung, especially in the pig 
where all of the blood to the lungs is forced to pass through the left 
pulmonary artery after the establishment of the transverse anastomoses 
and the subsequent degeneration of the proximal portion of the right 
pulmonary artery. 
We may say then in conclusion, that there is one simple possible expla- 
nation for the general asymmetry of the mammahan lung which lies 
in the asymmetry of the anlage. Owing to the fact, however, that the 
pulmonary anlage in lower animals is frequently symmetrical, it seems 
more probable to look upon this characteristic as an adaptation on the 
part of the pulmonary apparatus to its environment which may reach 
such extremes as we find in the lung of the snake. It is more probable 
then, that, with the necessity of an increased respiratory surface as we 
ascend the animal scale, the asymmetrical heart and the development 
of its adult form gives us adequate ground for a normal asymmetry of 
the respiratory apparatus, especially as the heart and liver, forming the 
principal environment of the lungs, have phylogenetic precedence and 
are of more physiological importance during intrauterine life. In its 
final form, this asymmetry consists, in the vast majority of lungs, in a 
suppression of left Lateral 1 to leave space for the descent of the aorta 
and pulmonary arch with the heart and a suppression of left Ventral 2 
to provide room for the pulmonary veins from the lower lobes. In ani- 
mals, however, where these branches are formed they are so placed that 
they do not interfere with either of these features of the development of 
the vascular system. 
