124 The Development of the Lungs 
suppressed. With the absence of left Lateral 1, it destroys the absolute 
symmetry of the mammalian lung. The cause for the remarkable hyper- 
development of the Ventral 2 on the right side in most mammals is 
undoubtedly due to the effort to increase the respiratory area by filling 
the space that intervenes between the heart and diaphragm with the 
Lobus infracardiacus. The remainder of the ventral series are usually 
paired in the pig and like the dorsal series ordinarily alternate with the 
larger lateral bronchi. As a rule their roots are placed on the ventral 
surface of the stem midway between the adjacent lateral elements and 
opposite the corresponding dorsal bronchi. The first ventral element is 
designated Ventral 2 on account of its topographical relationship to 
Lateral 2. 
6. The medial bronchi are, like the other series, produced by medial 
outgrowths from the stem. They are not formed on the dorsal bronchi 
and then transferred to the stem. They rarely occur higher than the 
level of Lateral 4 and are extremely irregular in their arrangement. 
7. Noteworthy are the great variaticns found in the production of the 
various bronchi. ‘The lateral series are by far the most constant ele- 
ments of the tree. Still, it is not uncommon to find either an extra 
element formed or else to see one of the usual elements suppressed. As 
the common number of lateral elements is six on the right side and five 
on the left, the extremes may vary between five and seven on the right 
and four and six on the left. In the case of the dorsal series, the 
variation is even more marked than in the lateral, thus, one element may 
be suppressed, leaving the dorsal area between two adjacent lateral 
bronchi naked or, else, an extra element may be formed, giving two dorsal 
elements in a single interspace. The ventral series is still more variable 
than the dorsal, so much so, in fact, as to make it uncommon even in 
the pig where these elements are unusually well developed, to find a 
series complete, of course, with the exception of left Ventral 2, which is 
always suppressed. It is not uncommon to find several elements of 
this series absent at once. Like the dorsal bronchi, they may also be 
reduplicated in a single interspace. The medial bronchi are the most 
variable of the four types. They may not be present at all, they may be 
present only on one side, or they may be reduplicated in a single inter- 
space, but, in the pig, they never occur higher on the stem than the level 
of the fourth lateral bronchus. The reason for this fact lies in the 
presence of the cesophagus above this point, which allows no space for 
the development of medial elements from this portion of the stem 
bronchus. 
