24 BRAIN AND BODY OF FISH 



It may well be asked what is this new nerve, the glossopharyngeal, 

 which has not been mentioned hitherto. This nerve and the small 

 lobe into which it runs, which may be regarded as a forward extension 

 of the vagal lobe, supplies an anterior gill-arch so that for om: 

 present piu-pose it may be looked upon as functionally part of the 

 vagal lobe, supplying a small anterior part of the gill -arches and 

 neighbouring structures. But it is important to remember its 

 existence, as in man as we shall mention later, it supplies a particular 

 part of the pharynx and has a specialised function in the more 

 complex structure of the higher vertebrates. 



We are now in a position to apply the same process of reasoning 

 to the vagal and facial lobes as was used in the interpretation of the 

 function of the optic lobes. For this purpose the examples will be 

 taken from three species of the large family of carps or Cyprinoids, 

 of which there are many members in British freshwaters. Let us 

 see what can be learnt from the comparison of the hind-brains of the 

 bream, the roach and the gudgeon, to be more concise their facial 

 and vagal lobes. 



The habits of these fish are familiar to any coarse fisherman. 

 The bream is found on the bottom in muddy waters, where reeds 

 grow, and he sucks at the bait rather than bites, and lifting the bait 

 up, tilts the float and drags it slowly along the surfa'ce before finally 

 taking it into its mouth. It is able to extract food-stuffs from the 

 mud. The roach is a more lively fish and often bites freely, but 

 usually investigates the bait before actually closing its lij)s ; as a 

 rule it feeds near the bottom, but it also takes a fly and feeds at times 

 near the surface ; the gudgeon frequents gravelly bottoms and 

 searches for its food among the stones with its sensitive barbels. 



When the brains of these fish are examined the difference in 

 size of their lobes is very evident. The bream has a very large vagal 

 lobe, the roach a small one, and the gudgeon one of moderate size ; 

 on the other hand the gudgeon has a very large facial lobe, the roach 

 a very small one and the bream a small one. It has been observed 

 that the vagal lobe is for mouth-tasting, so that the question arises 

 what is the reason for this great enlargement of the vagal lobe in the 

 bream, which is stiU more marked in the carp. The answer to this 

 question is that these fish have a speciaUsed organ on the palate, 

 that consists of ridges lined with taste-buds which enables the animal 

 to sift and sort, retain, or reject nutrient material from the mud and 

 decaying vegetable matter, on which it feeds : this organ with its 

 taste-buds is represented in the vagal lobe and is the cause of its 

 great development. The size of the facial lobe in the gudgeon is 

 stiU more easy to explain ; this fish has a pair of barbels riclily 



