162 Scientific Proceedings, Royal Dublin Society, 



descending aorta — the abnormal subclavian — came to look like a small 

 branch from the left descending aorta. In the normal human subject the 

 remains of the right descending aorta exist as the arteria aberrans. 1 



Invariably the recurrent branch of the vagus nerve supplying the larynx 

 isentrapped in the arms of the arches because these pass dorsalwards from a 

 ventral position, and the nerves are pulled back in loop fashion by them. On 

 the left the nerve evidently loops round arch v, because it there persists round 

 the ductus arteriosus, which is the persistent fifth left arch. On the right it 

 presumably loops round the fifth also, but that vessel afterwards disappears. 

 If from the right side all the arches were to disappear and the recurrent 

 still assumed the same position, the fact would suggest that the nervous 

 system was well developed previous to the disappearance of the arches. 

 It is evident that all the five arches were severed on the right side of this 

 particular rabbit in such a way that the recurrent, if it had not continued 

 to grow in length as in a normal specimen, might have receded to its original 

 direct position. It lies, however, in the normal situation, and its position 

 there proves that at one time this recurrent nerve must have embraced a 

 blood-vessel which branched from the aortic bulb through one of the visceral 

 arches, probably the fifth, but at least as far back as the fourth. Then as the 

 aortic bulb with the heart moved caudalwards the recurrent was brought to- 

 its present position, where it remained after the vessel which distorted its 

 course had been absorbed. Correlated with a similar abnormal development 

 of the right subclavian in man there is generally a direct, not recurrent, 

 course of the right inferior laryngeal nerve. 2 



1 Macalister's " Human Anatomy," 1889, p. 555. 2 Ibid., p. 556. 



