SEA-EISHERIES LABORATORY. 279 



large complex in the brain easily recognisable by the large 

 size of the fibres, and the density with which they stain. 

 There can hardly be said to be a single root to the 

 acusticus, its fibres becoming associated into at least two 

 rami just before or on leaving the medulla. As in 

 Menidia the very minute ganglion cells are not found on 

 the acusticus until it breaks up into its ramuli. In front, as 

 above described, the auditory nerve leaves the medulla in 

 conjunction with the motor vii., and is confused with it. 

 Its division into an anterior Ramus vestibularis and a 

 posterior R. cochlearis is not so obvious as in other fishes, 

 on account of the manner in which it emerges from the 

 medulla. Its further divisions or ramuli are therefore 

 now described. 



1. R. acusticus ampullae anterioris (r. a. a.). — Most 

 anterior branch, and courses forwards wedged in between 

 the motor vii. and the utriculus. It then passes upwards 

 and outwards to the outer wall of the ampulla, the sense 

 organ of which it enters from the front. 



2. R. acusticus recessus utriculi (r. r. u.). — Some- 

 what diagrammatic in the figure, as it really passes back- 

 wards to the floor of the utriculus to reach its sense organ. 



3. R. acusticus ampullae externae (r. a. e.). — Passes 

 almost straight outwards underneath the floor of the 

 utriculus towards the outer wall of the external ampulla 

 direct to its sense organ. 



4. R. acusticus sacculi (r. sac). — Courses at first 

 straight downwards at right-angles to the preceding 

 ramulus, and then backwards and downwards internal to 

 the sacculus to reach its sense organ. As it passes back- 

 wards it was connected with a small nerve bundle, the 

 nature of which was not determined. 



The nerve extending backwards to supply the two 

 posterior sense organs of the ear is separated off from the 



