28 
This response on the part of the growing bronchi to their space 
relationships is also shown in the course or direction of the principal 
elements as well as their secondary branches. We have, therefore, 
Lateral 1 produced and growing into the area between the upper part 
of the heart and chest wall. Owing to the larger space just beside 
the vertebral column and the antagonism between it and Lateral 2, the 
lower branches of Lateral 1 are forced dorsalwards until it resembles 
superficially a dorsal bronchus. The second lateral bronchi develop in 
the region between the chest wall, heart and liver. The area in which 
the remainder of stem has to grow has in cross section practically the 
shape of an isosceles triangle. The stem, occupying a point about the 
middle of the base, sends three sets of branches namely; dorsal, lateral, 
and ventral, directed into the angles of the triangle where they would 
have the most freedom to develop. Between the roots of the two stem 
bronchi runs the oesophagus, leaving no place for the development of 
median branches in this region. At the level of Lateral 4, however, 
below the oesophagus more room occurs and, consequently, we observe 
in this region the formation of the medial bronchi. Undoubtedly the 
difference in the branching of the stem in the Lobus inferior of the 
human lung when compared with the pig may be sought in its altered 
topography owing to the erect posture which changes principally the 
position of the liver. 
This adaptation on the part of the lungs to their environment is 
to be expected for they are relatively late accessions to the animal 
economy and are of no known use to the organism during the period 
of gestation. Accordingly as the heart and liver are both phylogene- 
tically older than the lungs and also are of.known functional value 
during foetal life, it is natural that the latter should adapt themselves 
to the early needs of older organs. 
10. The growth of the main series of bronchi is monopodial in 
character, that is to say, they are produced without a definite division 
of the end bud. New elements are not always produced from the end 
bud, but may be formed from the stem some distance from its terminus. 
The process is usually successive, that is to say, the elements are pro- 
duced one after another from above downwards, recapitulating the 
method of growth shown in simpler animals like the reptiles, for example. 
When a new element is about to be produced, one notes an increase 
in the number of karyokinetic figures in the epithelium in the region 
of the new branch. The basement membrane becomes less distinct and 
the connective tissue nuclei in the surrounding mesoderm are more 
closely packed together. In this region slight bulging of the epithelium 
