710 MB. GEBABD W. BUTLER ON THE [NoV. 19, 



usual, should be found in (some, not all of) the Amphisbaenldae, 

 which are also unique in having the right lung partially or com- 

 pletely suppressed \ 



AVith regard to the lungs of mammals — it has been suggested by 

 some^ that this inequality is due to the unsymmetrical position of thJe 

 heart. There are, however, certain considerations which induce me 

 to incline to another view ^. Tirstlj^, the lungs may, as we have 

 seen, differ markedly in size in reptiles in which the heart is 

 symmetricall}^ situated. Secondly, in the few mammals which I 

 have examined the smaller size of the left pleural space seems 

 to depend not so much on the position of the heart as on the want 

 of symmetry in the mediastinal membranes, whose line of attach- 

 ment to the diaphragm is a curve sweeping round the left border 

 of the central tendon. Thus perhaps the first cause of the inequality 

 of the lungs here, as in Snakes, may have been the leftward 

 displacement of the stomach, — which cause, however, may have only 

 come into action when, with the development of the diaphragm, the 

 mediastinum came to be Jixed in its oblique left-sided position. 

 According to this view the unsymmetrical position of the heart 

 would be due to the same cause as the inequality of the lungs, and 

 not be itself the cause of this. 



VIII. Conclusions. 



1. In all the Amphisbaenidse examined the right lung is either 

 absent or smaller than the left. 



2. In all the other vertebrates examined the right lung is fully 

 developed, and if one lung is rudimentary or absent, it is the left. 

 Thus 



3. The left lung is the smaller in many mammals, and more 

 markedly so in the Gymnophiona and many snake-like Lizards [not 

 Amphisbaenidso] and Snakes, in which last the left is usually 

 reduced to a mere rudiment or absent altogether. 



4. In the more theoretical section VII. I incline to the view 

 that in their first beginnings the lungs were in the ancestors of all 

 air-breathing vertebrates potentially paired, having their origin in 

 paired branchial pouches, and show reason to believe that they 

 were actually paired in the ancestors of at least some forms which 

 show no trace of a second. 



5. It would seem that the primary cause of the inequality of 

 the lungs, where it occurs, is that one-sided displacement of the 

 stomach and adjoining portion of the oesophagus which is seen in 



'^ To avoid needless repetition, other remarks which naturally might follow 

 here are placed only in the next section (Conclusions 5 and 6) 



^ G. L. Duvernoy, ' Le90ns d'Anat. comp. de G. Cuvier,' 2nd ed. torn. vii. 

 pp. 20, 24, 25 (1840). 



E.. Owen, ' Anatomy of Vertebrates,' vol. iii. p. 577 (speaking of Marsupials) 

 (1868). 



^ I refer only to the leftward displacement of the ventricle. I do not dispute the 

 fact that in most mammalian lungs we note that the left bronchus appears the 

 longer, owing apparently to the fact that the one-sided development of the aortic 

 root has entailed the suppression of part of the left lung in that region. 



