124 BRAIN AND BODY OF FISH 



joined with the cerebellum. The stratum granulosum occupies the 

 greater part of the ventral portion of the lobe extending from one 

 lateral margin to the other but divided in the middle hne by an 

 extension of the ventricle, so that the cerebellum would be divided 

 into two, it if were not for a narrow area of stratum moleculare 

 which unites it dorsally. The acoustic tubercles are still present 

 in this section. 



In the last figure (iv) the cerebellum has become considerably 

 narrower and it is no longer divided medially. The stratum 

 granulosum now occupies the lateral margins of the ventral portion 

 of the lobe, while the greater part of the lobe is formed by a central 

 area of stratum moleculare which expands dorsally into a hemis- 

 pherical prominence. This description may seem tedious but we 

 shall now suggest the possible functional significance of this unusual 

 type of cerebellum. 



We are now in a position to review the relative importance of 

 the various sense-organs represented in the brain of the eel, and 

 certain conclusions seem to be clearly indicated. The olfactory 

 system is obviously very important, as shown by the size of the 

 olfactory bulbs and the complexity of the olfactory organ. The 

 insignificant size of the optic lobes reflects the secondary importance 

 of sight in the feeding habits of a nocturnal animal. It is true that 

 the eyes of the silver eel become enlarged, previous to the trans- 

 atlantic migration, but this phenomenon is probably due to the 

 obscurity of the depths into which it will enter. The gustatory 

 system is next in importance, as shown by the large size of the facial 

 lobes, which receive all the afferent fibres from the taste-buds of 

 the lips, tongue, and the marginal barblet of the nostrils. This 

 picture is in marked contrast to the condition found in another 

 nocturnal feeder, the sole, which has poorly developed facial lobes 

 but a highly developed central acoustic lobe ; we have just seen 

 how poorly developed the central acoustic area is in the eel ; this 

 seems to be but another example of the fact that the facial lobe 

 varies inversely in size with the acoustic lobe and vice versa. 



The question which presents some difficulty is the significance 

 of the large size of the acoustic tubercles and their relations with the 

 cerebellum. Here there are three points to be considered, first, the 

 width and depth of the large-celled tissue of the acoustic tubercles, 

 secondly, the fusion of this tissue with the mesially divided stratum 

 granulosum of the cerebellum, and thirdly, the division of the cere- 

 bellum into halves, connected only by a dorsal band of the stratum 

 moleculare. The only fish that we have had an opportunity to 

 examine that has an equally large acoustic tubercle is the hake and 



