BRAIN. 



97 



The Irain of fishes is relatively small; in the Burbot 

 {Lota) it has been estimated to be -q^t\\ part of the weight 

 of the entire fish, in the Pike the x 3^o5th part, and in the 

 large Sharks it is relatively still smaller. It never fills the 

 entire cavity of the cranium ; between the dura mater which 

 adheres to the inner surface of the cranial cavity, and the 

 arachnoidea which envelops the brain, a more or less con- 

 siderable space remains, which is filled with a soft gelatinous 

 mass generally containing a large quantity of fat. It has 

 been observed that this space is much less in young specimens 

 than in adult, which proves that the brain of fishes does not 

 grow in the same proportion as the rest of the body ; and, 

 indeed, its size is nearly the same in individuals of which one 

 is double the bulk of the other. 



The brain of Osseous fishes (Fig. 41) viewed from above shows 

 three protuberances, respectively termed 'prose^icephalon, mes- 

 encephalon, and metence])halon, the two anterior of which are 

 paired, the hindmost being single. The foremost pair are the 

 hemispheres, which are solid in their interior, and provided 

 W'ith tAvo swellings in front, the olfactory lohes. The second 

 pair are the optic lohes, which generally are larger than the 

 hemispheres, and succeeded by the third single portion, the 



Fig. 41. — Brain of Perch. 

 I. Upper aspect. II. Lower aspect. 



a, cerebellum ; i, optic lobes ; c, hemispheres ; e, lobi inferiores ; /, hypophysis ; 

 g, lobi posteriores ; i, Olfactory lobes ; n, N. opticus ; o, N. olfactorius ; 

 2), N. oculo-motorius ; q, N. trochlearis ; r, iV. trigeminus ; s, iV. acusticus ; 

 t, A\ vagus ; ic, N'. abducens ; v, Fourth ventricle. 



cereMlum. In the fresh state the hemispheres are of a grayish 

 colour, and often show some shallow depressions on their 



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