160 PROFESSOR MARSHALL, 



not develop from the neural crest. The roots of origin do not shift 

 downwards, but are from their first appearance in the adult position. 

 The course of the nerve is nearly parallel to, and certainly not perpen- 

 dicular to the axis of the head. It has not the definite relations to 

 the visceral clefts and arches, and to the head cavities, characteristic 

 of a segmental nerve. And it has no ganglion cells at any point in 

 its length. 



As the nerve is not an independent segmental nerve, it must either 

 belong to one of the segmental nerves or else be a nerve of altogether 

 exceptional nature. The latter supposition should, I think, only be 

 adopted as a last resource if all the other attempts at explanation fail, 

 and I therefore propose now to consider the relations of the sixth to 

 the segmental nerves, or rather to the fifth and seventh nerves, which 

 are clearly the only ones which could claim it. 



By the majority of writers who have discussed this point, the sixth 

 is referred to the fifth. Thus, Gegenbaur considers the sixth to be an 

 independently arising motor root of the fifth, a view which Schwalbe 1 

 also adopts. Wiedersheim speaks of the fifth and sixth nerves as to- 

 gether making up a segmental nerve ; while Huxley 3 is disposed to 

 view the sixth as primarily part of the fifth. 



Notwithstanding the weight of authority against me, I think that 

 the sixth nerve should be grouped with the seventh, and not with the 

 fifth, for the following reasons : — 



1. In the early stages of both chick and dog-fish the roots of the 

 sixth are completely behind those of the fifth nerve. Indeed, the 

 majority of the roots are even behind the roots of the seventh ; and 

 although a transverse section may, as in fig. 7, pass through the roots 

 of both sixth and seventh nerves, yet the root of the sixth in such a 

 section is the most anterior of the series, the other roots being further 

 back, and completely behind the seventh root. 



2. In adult vertebrates, also the sixth nerve usually arises beneath 

 or slightly behind the seventh, very rarely in front of it. 



3. Though the sixth nerve may, beyond its root, be closely con- 

 nected with the fifth, yet it is important to notice that all the cases — 

 Petromyzon, Lepidosiren, Pipa, Sana, Anura — in which it is described 

 as fusing with the fifth, are also cases in which the seventh and fifth 

 nerves are very closely connected together, so that the connection between 



1 Schwalbe, Das Ganglion Oculomotoni, p. 74. 

 s Wiedersheim, Morphologische Studien, p. 23. 

 3 Huxley, Anatomy of Vertebrated Animals, p. 73, note, 



