CHAPTER VII 

 THE NERVOUS SYSTEM 



[HE Nerves of the Fish. — The nervous system in the fish, 

 as in the higher vertebrates, consists of brain and 

 spinal cord with sensory, or afferent, and motor, or 

 efferent, nerves. As in other vertebrates, the nerve substance 

 is divided into gray matter and white matter, or nerve-cells and 

 nerve-fibres In the fish, however, the whole nervous system is 

 relatively small, and the gray matter less developed than in 

 the higher forms. According to Gunther the brain in the pike 

 (Esox) forms but y^^-^ part of the weight of the body; in the 

 burbot (Lota) about ^^tj part. 



The cranium in fishes is relatively small, but the brain does 

 not nearly fill its cavity, the space between the dura mater, 

 which lines the skull-cavity, and the arachnoid membrane, which 

 envelops the brain, being filled with a soft fluid containing 

 a quantity of fat. 



The Brain of the Fish. — It is most convenient to examine the 

 fish-brain, first in its higher stages of development, as seen in 

 the sunfish, striped bass, or perch. As seen from above the 

 brain of a typical fish seems to consist of five lobes, four of 

 them in pairs, the fifth posterior to these and placed on the 

 median line. The posterior lobe is the cerebellnm, or metenceph- 

 alon, and it rests on the medulla oblongata, the posterior portion 

 of the brain, which is directly continuous with the spinal cord. 

 In front of the cerebellum lies the largest pair of lobes, each 

 of them hollow, the optic nerves being attached to the lower 

 surface. These are known as the optic lobes, or mcseiiceplialou 

 In front of these lie the two lobes of the cerebrum, also called 

 the hemispheres, or prosencephalon. These lobes are usually 

 smaller than the optic lobes and solid. In some fishes they are 

 crossed by a furrow, but are never corrugated as in the brain 



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