GG 



CHAPTER IV. 



THE NEEVOUS SYSTEM 



The Brain. — The brain of tlie sole has no very special features which distinguish it 

 among those of other bony fishes. The position of the brain is almost entirely 

 unaffected by the change which has taken place in the normal position of the fish ; the 

 posterior part of the skull, as was previously pointed out, has neither twisted nor 

 become asymmetrical, and the corresponding part of the brain also retains its original 

 position and symmetry. But it seems still more remarkable that the anterior part of the 

 brain has not to any great degree followed the anterior part of the skull in the rotation 

 which the latter has performed. When the brain is exjjosed from the upper (right) 

 side of the fish, the right side of the organ is seen, the extreme anterior end being 

 alone very slightly turned round towards this side. This absence of change in the 

 position of the brain is not so paradoxical when we compare carefully the relation of 

 the organ to the skull. The anterior end of the brain lies beneath the posterior 

 portions of the frontal bones, and these retain their original position. 



It might have been expected that the change in the position of the anterior cranial 

 nerves would have caused a greater twisting of the brain ; but we find that the trunks 

 of the nerves have moved while their roots have been very slightly affected. 



Viewed from the dorsal side the brain exhibits the same series of ganglionic masses 

 which ai'e seen in other bony fishes. At the posterior end is the single median globular 

 cerebellum which projects backwards above the anterior thickened continuation of the 

 spinal cord, called the medulla oblongata. In front of the cerebellum is the pair of 

 masses whence the optic nerves arise, the optic lobes, each of which is almost as large 

 as the cerebellum ; in front of these again is the pair of cerebral hemispheres which 

 are slightly smaller than the optic lobes, and immediately in front of the cerebral 

 hemispheres are the olfactory lobes. In some bony fishes the olfactory lobes are 

 removed to some distance from the brain and placed in proximity to the olfactory 

 capsules, being connected with the brain by long nervous peduncles or crura ; e.g., in 

 the cod and carp. But in the sole as in other flat-fishes, and in the perch, mackerel, 

 pike, gurnard, and others, the olfactory lobes form part of the brain and are at a 

 distance from the olfactory capsules ; with which they are connected by the olfactory 

 nerves. 



