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 



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