60 BRAIN AND BODY OF FISH 



lobe wliile rather more anteriorly the large lateral-line nerve enters 

 the acoustic tubercle. It is thus apparent that this lobe is part of the 

 acoustico-lateralis system. The small facial lobes (Fig. 17) can be 

 recognised by tracing the facial nerves which pass on either side into 

 a small area, triangular in section, which abuts the ventricle. From 

 these areas descending fibres pass outwards into the great longi- 

 tudinal gustatory tracts ; the course of these fibres and that of the 

 facial nerves prove that these triangular areas are the facial lobes ; 

 these lobes do not appear on the surface. This detailed description 

 is given, as the central lobe has been mistaken for a facial lobe. 



Certain interesting facts concerning the lateral-line organs must 

 now be mentioned. The herring has no lateral-line. There are 

 lateral -line organs on the head, and, as we have seen, one of these 

 makes a deep bay in the temporal region and appears to form an 

 accessory part of the auditory organ. This being the case it is clear 

 that the large central lobe cannot be explained by an increased 

 functional importance of the lateral-line system. But as the herring 

 is a surface feeding animal and has an elaborate auditory mechanism 

 the most reasonable conclusion to be drawn is that this central 

 lobe has an acoustic function and should be termed the central 

 acoustic lobe. 



The conclusion to be drawn from the above facts is that the 

 brain of the herring is characterised by large optic lobes, a large 

 central acoustic lobe, and a small facial lobe, not apparent by naked 

 eye observation. This type is also found with slight modifications 

 in the sprat and pilchard and as the latter has a central acoustic 

 lobe very similar to that of the bleak it may be assumed that it is 

 characteristic of plankton feeders, as far as our present knowledge 

 allows this generalisation. 



If this pattern is compared with a typical ground feeder, such 

 as the loach, a near relative to the Cyprinoids, it will be at once 

 evident how marked is the difference. The facial lobe is very large, 

 with a lobulated surface, and overlaps the vagal lobes. The facial 

 nerves, also very large, divide at the inferior surface of the lobe 

 into an anterior branch which sphts into two bundles, one passing 

 centrally and the other laterally to the dorsal area of the lobe. The 

 posterior branch on either side passes backwards close to the ventricle 

 and enters a large posterior lobe. Large descending fibres from 

 the lobe pass into the middle division of the great longitudinal 

 secondary gustatory tracts. The central group of cells described 

 in the bleak and herring at the base of the cerebellum does not 

 appear, but a few cells forming a narrow band across from one side 

 to the other and become slightly more prominent at the outer margin. 



