26 BRAIN AND BODY OF FISH 



how discriminating this fish is as regards baits. It seems, therefore, 

 that the large size of the vagal in the bream is solely due to the large 

 palatal organ and the large size of the facial lobe in the gudgeon is 

 mostly due to the possession of a pair of barbels ; and that the 

 absence of these specialised structures in the roach is the cause of 

 the smallness of both lobes. 



In the spinal cord of man there are considerable enlargements in 

 the cervical and lumbar regions and the mid-dorsal region is small 

 in comparison. This is due to the large nerve trunks that go to 

 the arms and legs respectively from these areas ; but as a rule in 

 fish the spinal cord is of uniform calibre throughout. But there is 

 one family of fish, the gurnards, that gives us an example of a new- 

 function arising in a fin, associated with the formation of globular 

 swellings of the cord. The gurnard appears to have adopted the 

 method of protection employed by the crustaceans, and to have 

 clothed its large head with a carapace like a lobster, and like a 

 lobster has found it necessary to provide itself with feelers or 

 antennae ; the three posterior rays of the pectoral fins have become 

 separate and form three slender processes on either side, which 

 possess extensive motion on a double row of joints, not connected 

 with the fins. These processes or fingers are supplied with 

 peculiar nerves, and are consequently in possession of special 

 functions. 



That they are organs of feeling cannot be doubted ; but the fish 

 has also been seen, according to Couch, " when resting on the ground, 

 to close the pectoral fins and to creep by the help of these processes 

 as if they were organs of motion that could be employed without 

 exciting alarm to the prey which the motion of the fins might 

 possibly do." What interests us particularly is the fact that the 

 additional function which the anterior spinal nerves of the gurnard 

 have to jDerform in supplying the sensitive pectoral appendages 

 and their muscles has caused the development of a paired series 

 of globular swellings of the corresponding portion of the spinal 

 cord. On opening the spinal column it is easy to expose the cord, 

 which is then seen to have six pearl-like swellings on either side 

 immediately posterior to the medulla. The three posterior of 

 these are the central connections of the nerves from the tliree 

 sensory filaments, while the tlu-ee anterior receive by a common 

 trunk the nerves from the pectoral fin. 



We see here in a simple way the principle, that is still further 

 developed in the brain, namely an increase in the size of a nerve 

 centre when it is called upon to deal with a new function. In the 

 gurnard as soon as certain rays of the pectoral fin took on the new 



