CRANIAL NERVES AND SENSE ORGANS OF FISHES. 237 



anterior or cranial nerves have been constantly under- 

 going modifications, whilst the posterior or spinal nerves 

 have, relatively speaking, remained fairly constant. 



We have now to enquire into the essential nature of 

 the lateral and branchial nerves, and to ascertain, if 

 possible, the bearing their structure has upon the problem 

 of the phylogeny of the head. Omitting the eye-muscle 

 nerves, which are the least primitive of the cranial nerves, 

 and involve altogether special problems, and also the 

 first and second pairs, of which much has still to be 

 learned, we may consider the auditory and its associated 

 nerves first, as it is on these nerves that recent investi- 

 gation has been most prolific and most successful. And 

 here we at once see that the metameric incubus has 

 weighed heavily on the shoulders of advancing knowledge. 

 It is, in fact, only within the last few years that this 

 retarding influence has been shaken off, and the inves- 

 tigator has been free to follow where the majority of the 

 facts undoubtedly led him. The older morphologists 

 naturally regarded the auditory and lateral sense organs 

 as forming a series of metameric structures, and it was 

 hence incumbent on them to show that their innervation 

 was also metameric. The structures concerned being 

 sensory organs, a number of nerves had to be found 

 which would correspond to the dorsal sensory branches 

 of the spinal nerves. It is, perhaps, superfluous to remark 

 that these nerves were found, and we meet with descriptions 

 of two dorsal branches of the trigeminus, two of the 

 facial, one of the glossopharyngeal, and a variable 

 number for the vagus. And here it may be observed 

 that the ground was at first almost entirely worked by 

 embryologists, and great as the work of such renowned 

 morphologists as for example Dohrn and Alexander 

 Goette undoubtedly was, it is in this connection to be 



