84 The Development of the Lungs 
about the stem bronchi form the anlage of the Lobus inferior on each 
side. With the appearance of V. 2, the Bronchus infracardiacus, on the 
right, a swelling forms over it yielding the anlage of the Lobus infra- 
cardiacus. These swellings are at first surrounded by shallow grooves, 
which, with the rapid growth of the bronchi beneath, develop into the 
fissures separating the various lobes. With the further growth of these 
chief bronchi and the appearance of the series of bronchi on the stem, 
a series of swellings and fissures are formed over and between them. 
These are equivalent, in all senses except in age and size, to the earlier 
fissures and swellings, but, under ordinary circumstances, never deepen 
into distinct lobes. ‘This is partly due to the more rapid growth of the 
first bronchi, to the gradual increasing density of the mesoderm, and, 
lastly, to the environment of the several lobes of the lung. That is to 
say, the Lobus superior with L. 1 has the territory between the chest 
wall and the upper part of the heart on the right side. The right Lobus 
medius and the left Lobus superior, with L. 2, have the large space be- 
tween the chest wall and the angle formed between the heart and liver 
on each side. It is important, however, to note on the left side, owing 
to the absence of L. 1, the Lobus superior sends up the apical segment 
of the lung containing the left Bronchus ascendens. The Lobus infra- 
cardiacus, with V. 2, grows out into the space left between the heart 
and liver and the two lower lobes, while the Lobus inferior on each side 
lying in the more or less triangular space between the chest wall and 
liver and diaphragm becomes prismatic in cross-section and grows caudal- 
wards and lateralwards to fill up the rest of the pleural cavity. 
In the pig, then, we have a series of primary projections limited by a 
series of fissures some of which give rise to the permanent pulmonary 
lobes. Those projections and fissures which take part in the lobe forma- 
tion in the pig, it is well to observe, are the first to form, but in other 
animals these same conditions do not appear to obtain. In Hystrix 
cristata, for example, not only the primary fissures between practically 
all of the principal bronchi may give rise to a series of lobes, but these 
may even be subdivided by the secondary fissures formed by the secondary 
branches of these elements, while in other animals, as for example man, 
the deepening of the fissures about V. 2 usually do not produce a separate 
lobe, leaving this region of the lung included in the right Lobus inferior. 
Between these forms we have extensive individual and general variation.: 
The drawings in Fig. 20 may be used conveniently to explain the 
lobe production in all mammals; A represents the conditions in animals 
where L. 1 is present on one side or both; B, the conditions where L. 1 
