38 DR. A, MILNES MARSHALL, 



cranial and spinal nerves are in any way comparable, then we 

 must either consider — (1) the whole vagus and glosso-pharvngeal 

 as together equivalent to a single s])i(ial nerve — this is obviously 

 unsatisfactory — or {i) regard the glosso-phnryngeal and each of 

 the main branches of the vagus as equivalent to so many spinal 

 nerves. 



On the latter hypothesis, which we may consider as almost 

 demonstrated by the relations of the branches of the vagus to the 

 hinder visceral clefts, it is obvious, when we consider the very 

 short length of the neural canal from which the whole of the 

 vagus is derived, that very considerable concentration must have 

 occurred at the hinder end of ihe brain. Since the concentra- 

 tion occurs in the chick in the very ea^'liest stage of its deve- 

 lopment, it is probable that approximation and fusion of the 

 several vagus roots, with accompanying concentration of the 

 hinder end of the brain, was gradually acquired by the ancestors 

 of the chick; while in the chick this fusion and concentration 

 have been thrown back to the very earliest ontogenetic stage at 

 which they could possibly occur, i.e. at the very first appearance 

 of the nerve rudiments. At the opposite end of the brain the 

 opposite process seems to occur, judging from the distances 

 between the roots of the segmental nerves, and instead of con- 

 centration we have expansion. 



Transverse sections through the part of the hind brain from 

 which the vagus arises show, about the end of the fourth day, a 

 number of small outgrowths from the ventral surface of the brain, 

 close to the median line on either side. These are separated by 

 a considerable interval from the roots of the vagus, from which 

 they are perfectly distinct. In appearance, position, and rela- 

 tions, these small ])aired outgrowths are precisely similar to the 

 anterior roots of the spinal nerves, to which I believe them to 

 be strictly equivalent. The earliest date at which 1 have observed 

 them is the sixty-seventh hour. I have not succeeded in tracing 

 their ultimate fate. They extend forwards nearly as far as the 

 auditory capsule. Balfour suggests that some similar roots 

 noticed by himself and by others in adult or embryonic Elasmo- 

 branchs are " ventral roots of spinal nerves whose dorsal roots 

 have beenlost.^'' I cannot accept this suggestion, which has no 

 direct evidence in its favour, as far as the roots I have just 

 described are concerned. These roots belong, unquestionably, 

 to the vagus; and the description I have given above of the sixth 

 nerve shows that there is every probability that we have true 

 anterior roots considerably further forwards in the brain. 



The multiple character of the roots of the glosso-pharyngeal 



* ' Journal of Anatomy and Physiology,' loc. cit., p. 471. 



