22 BRAIN AND BODY OF FISH 



sensation. There is a similar division, though more complex, in 

 the brain, and so it is found that the lobes of sensation are found 

 on the dorsal or posterior aspect. It will be understood that the 

 term posterior in man becomes superior in fish owing to the upright 

 posture of man, contrasted with the horizontal poise of fish. 



The reader must forgive this rather elementary description, 

 but it is necessary if he is to follow the more technical matter. 



It may well be asked what means are to be adopted to understand 

 the use or function of the lobes which have been enumerated ? 

 Are you justified in comparing the brain of a fish with what is 

 known about the brain of man ? As to the lobes which receive 

 impressions from peripheral sense organs that have an exact counter- 

 part in man there is no difficulty. 



For example, the eye of a fish has much the same structure 

 as in man, and is connected directly to the two optic lobes by large 

 nerve trunks which cross each other as they enter the brain. There 

 can be no doubt that this system is rightly assumed to be a visual 

 apparatus. The truth of this assumption is further established, 

 when the brains of two familiar flat-fish are compared. The plaice, 

 which feeds by day and largely by sight, has very large optic lobes 

 which tend to be convoluted, whereas the sole, which is nocturnal in 

 its habits and feeds largely by smell and touch, has very small 

 optic lobes. 



It may be stated as a general rule that fish with nocturnal habits 

 have small optic lobes ; in the cod family this generalisation is 

 well shown. The , cod, w hiting, and pollack have large optic lobes 

 and are hunters by day ; on the other hand the ling, burbot and 

 rockling have small optic lobes. 



According to Cimjiingham the ling is somewhat nocturnal and 

 the rockling entirely nocturnal in their habits, and according to 

 Tate Regan the burbot goes in active pm-suit of prey at night. 



The above examples of the variations in the size of the lobes are 

 isimple and straightforward, and show well the relation of function 

 to the central nervous system ; they further establish a i^rinciple 

 which enables us to unravel the more complex areas of the brain 

 such as those concerned in the appreciation of taste which are 

 known as the gustatory centres. These centres are very important 

 in fish as the substances that give rise to the sensations of taste 

 are always in solution. Therefore it is not surprising to find that 

 fish, living entirely in a fluid medium, are furnished with an extensive 

 gustatory system. Taste is a chemical sense, and the organs of 

 taste enable the animal to recognise sweet, sour, salty and bitter 

 substances. 



