230 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



extreme, and endeavour to apply it to the peripheral dis- 

 tribution of the fibres, we find that the fibres from one 

 nucleus are distributed to definite tracts, which can be 

 homologised in other forms. Such a method, although it 

 is vastly more logical than the old one, is still impossible 

 of application at the present time, principally because the 

 boundaries of the nerves would be indistinguishable to 

 the naked eye, and could only be limited by the micro- 

 scope. The seventh nerve, for example, so far from being 

 a definite unit in itself, would be composed of portions of 

 three bundles, and this, of course, introduces practical dif- 

 ficulties which it were better to avoid, even at the expense 

 of some logic. A scientific classification of the cranial 

 nerves, based on the central origin and peripheral dis- 

 tribution of the nerve fibres, is, therefore, at present out 

 of the question, first, on account of the practical difficulties 

 involved, and second, because the anterior nerves require 

 much further investigation based on modern advantages 

 of knowledge and improved methods of research. Before 

 leaving this part of the subject, however, I should mention 

 one respect in which the present system of numbering 

 the cranial nerves is most strikingly inconsistent. When 

 the vertebral theory of the skull was in vogue, it was a 

 matter of no great difficulty so to group the cranial nerves 

 as to make them fall into line with the main postulates of 

 the theory ; and, indeed, they support that theory as well 

 as they can be said to support any other. It is, however, 

 a commonplace of vertebrate morphology that Gegenbaur, 

 in his renowned Hexanchus paper, completed the over- 

 throw of the vertebral theory by showing that the vagus 

 was a compound nerve, and this, of course, induced him 

 to substitute the segmental for the vertebral theory of the 

 skull. Whether the gill clefts are segmental, and there- 

 fore the nerves supplying them of the same significance, 



