﻿Note on Swim-hladder and Lungs. 



173 



right vagus all agree in testifying to a movement of the 

 lung having taken place, such as would have happened on 

 the Sagemehl hypothesis. This being so, we should perhaps 

 be justified in disregarding the discordant evidence afforded 

 by the left vagus, and in assuming that it is due to some 

 secondary moditication in the course of this nerve, especially 

 as by no conceivable process of shifting of the lung rudi- 

 ment can the contradiction between the evidence of the left 

 vagus and that of the other vagus and the pulmonary 

 arteries be removed. Whether or not it be admitted that 

 such a course of reasoning would be admissible in the 

 absence of other evidence, all doubt about its validity is 

 removed by a consideration of the condition in Polypterus. 

 Here it will be seen from Fig. 1 A that the nerve running along 

 the left side of the right lung is continuous with the left 

 vagus directly, i.e., not ventral to the oesophagus, but dorsal 

 to it. Now I think it is clear that this condition mtcst be 

 secondary. The whole lung rudiment was originally ventral 

 in position, and it is clear that any part of the primitive 

 rudiment coming within the area of supply of the left vagus 

 would be connected up with the left vagus round the 

 ventral and left sides of the pharynx. The precise manner 

 in which the secondary connection has come about is of no 

 importance to the general argument : it may well have been 

 through short circuiting of nerve impulses through the 

 nerve plexus in the mesodermal sheath of the developing 

 pharyngeal region, the short circuit becoming eventually 

 marked off as a thick nerve trunk. 



(Such secondary anastomoses between nerve trunks are, 

 of course, well known to occur frequently, e.g., between 

 terminal branches of paired cranial nerves in vertebrates, 

 or in the condition of dialyneury in Gastropod Molluscs.) 

 The point which I desire to bring out is, that in the very 

 primitive Polypterus a condition holds which entirely does 

 away with the difficulty in the way of Sagemehl's view, 

 involved in the purely dorsal position of the pulmonary 

 branch of the left vagus in Dipnoans. Once this difficulty 

 has been got over, the X-like crossing of the pulmonary 

 branches of the vagi in Dipnoans is clearly explicable by 



