CRANIAL NERVES AND SENSE ORGANS OF FISHES. 229 



nerves of vertebrates into a number of pairs,* of which 

 ten could be distinguished in the lower vertebrates 

 and twelve in the higher. That this arrangement is 

 partly arbitrary and partly scientific is obvious enough. 

 Arbitrary, since these nerves do not arise serially as ten or 

 twelve definite pairs, but, to a considerable extent, overlap 

 and intermix, so that in many forms it is still impossible to 

 homologise several of the roots ; scientific, since, for 

 example, the nerve supplying the spiracular cleft in any 

 one form must be held to be homologous with the nerve 

 supplying the same cleft or its homologue in any other 

 form. Eecently a severe and perhaps fatal blow has been 

 struck at the serial arrangement of the cranial nerves by 

 the results obtained from an investigation of the central 

 origins of the nerve fibres in many of the lower vertebrata. 

 These researches have shown conclusively that the cranial 

 nerves are not so many independent units, but aie them- 

 selves complexes of fibres derived from various parts of 

 the brain. For example, to adhere to our former illustra- 

 tion, the so-called " seventh" nerve has, in many of the 

 lower vertebrates, not only three centres in the brain, but 

 these three centres also do duty for the "ninth" and 

 "tenth " nerves. The precise way in which the nerve fibres 

 leave the periphery of the brain is therefore a secondary 

 question, and, as far as central origin is concerned, it would 

 be more philosophical to denominate as the seventh, 

 ninth, and tenth nerves, not the "roots" of the nerves, 

 but respectively the three nuclei from which those roots 

 are compiled. And if we follow this method to its logical 



* Largely based, of course, on the assumption that the roots were final 

 quantities. It is an excellent illustration of the conservatism of science that 

 the discovery that the fibres of the roots could be traced to ultimate regions 

 in the interior of the brain has hitherto had absolutely no effect on the 

 classification of the cranial nerves. 



