THE ALIMENTARY CANAL AND ITS APPENDAGES 177 
In dealing with the lung of the Primates, considerable 
importance attaches to the growing together of the pericardium 
and the diaphragm, for this brings about a constancy, or, if I 
may be allowed the expression, a certain rigidity in the form of 
the pleural cavities. As a consequence of this, a stricter limit is 
placed upon the extension of the lobes of the lungs than in the 
lower Mammals, in which the lung is able, either constantly or 
during inspiration, to penetrate between the heart and the 
diaphragm, into the sinus subpericardiacus. This applies especially 
to the right lung, at the base of which a special lobe may be 
more or less distinctly developed. This, the lobus subperi- 
cardiacus (or azygos impar), is occasionally present in Man, 
most frequently, it appears, in the lower races and in micro- 
cephalous individuals. The probability that its presence may 
be indicative of atavism is not lessened by the fact that indica- 
tions of it often occur, in the form of a blunt process lying in 
front of the ligamentum pulmonale, which sinks into a depression 
in the mediastinum, just as in the Orang. 
Hasse has not only confirmed Aeby’s observations in all 
essential points, but, by the aid of very ample material, has 
extended and revised them. According to him, the principal 
bronchi of the human lung run downwards, backwards, and 
slightly outwards, the direct current of inspired air following the 
same course. He raises the question whether this has always 
been the disposition of these bronchi, and inquires into its cause. 
The first question he answers in the negative, and seeks to prove 
that a very gradual change took place in the position of the 
bronchi; indeed, that the position which has been acquired in 
the course of Phylogeny is exactly the reverse of the primitive one. 
The facts discovered by His in the study of the human embryo 
lend support to this view. In other words, comparison of the 
embryonic with the adult condition shows most clearly that a 
depression of the right and an elevation of the left chief bronchus 
takes place. The condition of the adult, so far as the branching 
of the bronchi is concerned, is effected as early as the end of the 
second month of intra-uterine life, the change being in the main 
due to the twisting of the heart upwards, backwards, and to 
the left. 
Hasse is, however, unable to prove any more satisfactorily 
than his predecessors why the right lung-sac is from the first 
more spacious than the left, and what caused the right eparterial 
bronchus to appear. He has, however, made an attempt at 
N 
