﻿Note on Sioim-hlaclder and Lnngs. 



171 



, would at once explain why it is that the lung apparatus 

 of the Dipnoan invariably communicates with a ventrally 

 placed glottis round the riglit side of the cesophagus. It 

 also explains the at first purely ventral position of the lung 

 rudiment in the embryo, and the undivided condition of the 

 lung in the adult of Ceratodus, on the whole the most 

 primitive existing Dipnoan. A serious obstacle, however, 

 in the way of the acceptance of the hypothesis is found in 

 connection with the nerve supply of the lung apparatus of 

 the Dipnoi. (1) If the Sagemehl hypothesis were correct, 

 we should expect the lung apparatus to be innervated 

 entirely by the right vagus — as a matter of fact, it is 

 innervated by both vagi ; further (2), on this hypothesis we 

 should expect that if a branch of the left vagus did pass 

 to the pulmonary apparatus, it would pass to it round the 

 ventral side of the cesophagus, whereas, as a matter of fact, 

 the pulmonary branch of the left vagus crosses over to the 

 right side of the lung dorscd, not only to the cesophagus, 

 but also to the right vagus. 



The object of this note is to show that these difficulties, 

 insuperable as they appear at first sight, may, as a matter 

 of fact, be justifiably disregarded. That this is so will, I 

 think, be gathered from a study of Fig. 1, A and B showing 

 the conditions of nerve supply to the lungs in Polypterus, 

 for comparison with the lung-fishes. 



I 



Taking first objection (1), we see that in Polypterus the 

 right lung, in correlation with its increase of size, invades 

 the area of supply of the left vagus. This enlarged right 

 lung shows a large nerve trunk running down each side, 

 that on the right being a branch of the right vagus, that of 

 the left a branch of the left vagus. If, therefore, the left 

 lung were to disappear, the surviving lung, though originally 

 a purely right lung, would show an innervation from both 

 right and left vagi. 



(2) In investigating any supposed migration of the lung 

 apparatus, we should naturally look for a " trail " or track 

 marked out by the course of the arteries and nerves. Examin- 

 ing the relation of these in the Dipnoi, we find the right 

 pulmonary artery, the left pulmonary artery, and the 



