NERVOUS SYSTEM. 209 



dullary band, or anterior commissure ; the ganglia themselves cor- 

 respond principally to the hemispheres of the higher Vertebrata ; 

 they are usually called the olfactory lobes, or lobes of the hemispheres. 

 These ganglia are solid, and almost always smaller than the meso- 

 cerebral ganglia ; for the most part they are composed of two kinds 

 of substances, an anterior and a posterior ; the first of these, the 

 largest, presents a delicately tuberculated surface, or, as it were, 

 traces of convolutions ; this is distinctly to be seen in the Pike, and 

 also in Gadus morrtma, the SalmonidcE, and others. From, or in 

 front of, these ganglia of the hemispheres arise the nerves of smell ; 

 not, however, directly from them, but usually from small interposed 

 round or oval ganglia, which are occasionally themselves of some 

 considerable size, and have received the names of olfactory ganglia 

 or tubercles ; they are present in most, probably all, the Osseous 

 Fishes, and are very distinct in all our indigenous Fishes, e.g., Esox, 

 Salmo, Clupea, Perca, Pleuronectes, Gobius, Gadus, Labrus, Urano- 

 scopus, &c. They are, however, frequently so small and elongated 

 as to be easily overlooked, e. g., in the Carps. A small, vascular, 

 and frequently all but membranous lobule, which is situated freely 

 exposed, and directly in the middle line between the mesocerebrum 

 and the ganglia of the hemispheres, may be regarded as the pineal 

 gland. The pituitary appendage is of considerable size, and depends 

 from its stem or the irifundibulum ; it is particularly large in Pleu- 

 ronectes and Cyclopterus ; indeed, generally speaking, Fishes have 

 the largest pituitary body of all the Vertebrata. 



The above-described is the usual arrangement of the structure of 

 the brain in the Osseous Fishes, but it is subject to many varieties in 

 the several families and genera, these consisting chiefly in the num- 

 ber of the several hemispheres or superficial ganglia being increased, 

 or in their being severally enlarged and subdivided to a striking 

 degree. 



In Fishes of the Eel kind we find two pairs of olfactory lobes or 

 ganglia introduced between the small olfactory tubercles and optic 

 lobes. The posterior pair is largest, and manifestly identical in its 

 whole structure with the olfactory lobes of the remaining Osseous 

 Fishes ; for, as in them, they exhibit two divisions of structure, their 

 anterior portion being rendered uneven by rudimentary convolutions, 

 the posterior presenting a smoother surface. This structure is par- 

 ticularly distinct in the common Eel, but occurs also with modifi- 

 cations in the Conger and Muraenophis. In the Electrical Eel 



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