234 TEANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



No appreciation of the cranial nerves is possible, unless 

 their peripheral distribution is taken into account at the 

 same time, and we thus see that it is possible to trace the 

 phylogeny of the cranial nerves by investigating the evolu- 

 tion of the structures which they supply. We are hence 

 interested to know what has been the phylogeny of the 

 two series of structures enumerated in the preceding 

 table. It is obvious, at the very outset, that both sets of 

 nerves must have been modifications of nerves previously 

 existing, for, to put it briefly, as both the series concerned 

 are obviously modifications of old structures, it follows 

 equally that the nerves supplying these structures must 

 have become adapted to the new conditions. For example, 

 we know that the lateral sense organs have arisen by the 

 differentiation of portions of the surface epiblast, and it 

 hence results that the lateral line nerves are modified 

 somatic sensory nerves. It is, in fact, this change of 

 function that marks the distinction between the inverte- 

 brate and the vertebrate, e.g., such a change as has taken 

 place in the phylogeny of a branchial nerve, which, from 

 at one time being partly, if not largely, a somatic nerve, 

 now belongs almost entirely to the visceral system. Just 

 as the nerve " shadows " the maturation of the sense organ, 

 so the brain, which represents a ganglion on a nerve, in 

 its turn develops pari passu with the cranial nerves. To 

 say that the " lobus trigemini " of the fishes' brain is simply 

 a part of the brain that happened to become connected 

 with the lateral line nerves, is to be unphilosophical at 

 once ; but to state that the " lobus trigemini " has been pro- 

 duced to meet the growing exigencies of the lateral line 

 nerves, which themselves owe their existence to the 

 spreading of the sense organs, explains immediately all the 

 structures concerned. The archaic brain and cranial nerves 

 were but the servants of the sense organs, and it is doubt- 



