124 THE BRAIN OF FISHES AND OF AMPHIBIA. 



The ganglia at the roots of the Olfacton^ and Optic 

 nerves are sufficiently obvious and remarkable, so that 

 no further reference need here be made to them, except 

 to point out that they, together v^^ith the ganglia at the 

 roots of the Trigeminus and Vagus, undergo a propor- 

 tionate diminution in size as the Cerebral Lobes become 

 better developed, among Eeptiles and Birds — changes 

 which seem to imply that functions previously discharged 

 by lower sensory ganglia are gradually passed on and 

 merged as products of a higher order of cerebral activity, 

 when such higher co-ordinating centres arise and come 

 into fuller action. 



The ganglia at the roots of the Auditory nerves, how- 

 ever, do not seem to attain their maximum size till we 

 come to Eeptiles, a fact which may be accounted for by 

 the probably rudimentary state of this sense endowment 

 among Fishes. 



It will be found, therefore, to be a peculiarity of all 

 Sensory Nerves in vertebrate animals that their fibres pass 

 through such Ganglia before they impinge upon the great 

 nerve centres — a fact originally noticed by Sir Charles 

 Bell. No corresponding ganglia exist in connection with 

 motor nerves, outside the anterior cornua of the spinal cord. 



they are lodged outside the cranial cavity, sometimes in the walls 

 of the cranium, and sometimes half within and halt outside this 

 cavity. Their structure is extremely simple, and in some fishes 

 they are only a very little more complex than the 'auditory 

 saccules ' met with in the Cuttle-fish. In the fact that in 

 Fishes, as in other vertebrates, the auditory organs are always 

 situated in the head, we have a departure from the rule so commonly 

 obtaining among Invertebrates. Perhaps, in its simplest forms, 

 this apparatus may have as much to do with the organism's Space 

 relations as with Hearing (see p. 218). 



