26 
surface of the stem. They like the other series, are independent pro- 
ductions of the main bronchus. They are not originally formed on the 
lateral bronchi and subsequently transferred to the stem bronchus. 
Consequently, they are chief bronchi and not accessory in the sense of 
Aeby. In the pig and in the great majority of mammals, left Ventral 2 
is suppressed. With the absence of left Lateral 1, it destroys the 
absolute symmetry of the mammalian lung. The cause for the re- 
markable hyperdevelopment of the Ventral 2 on the right side in most 
mammals is undoubtedly due tv the effort to increase the respiratory 
area by filling the space that intervenes between the heart and dia- 
phragm with the Lobus infra cardiacus. The remainder of the ventral 
series are usually paired in the pig and like the dorsal series ordi- 
narily alternate with the larger lateral bronchi. As a rule their roots 
are placed on the ventral surface of the stem midway between the ad- 
jacent 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 principal bronchus. ‘They rarely 
occur higher than the level of Lateral 4 and are extremely irregular 
in their arrangement. 
7. Noteworthy are the great variations found in the production of 
the various bronchi. The Lateral series are by far the most constant 
elements 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 un- 
common even in the pig where these elements are unusually well de- 
veloped, to find a series complete, of course, with the exception of 
the 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 
