228 NERVOUS SYSTEM. 



transversely laminated structure, as in higher classes, but also 

 small hemispheres extending laterally like tubercles from its 

 base. Its magnitude here corresponds, as in higher classes, 

 with that of the corpora restiformia, which are conspicuous 

 in the plagiostome fishes, and it presents internally an ar- 

 borescent appearance of white diverging fasciculi, arising 

 from its laminated structure. 



All these lobes contained within the capacious cranium of 

 fishes are covered with cineritious substance, and derive their 

 internal white fibrous parts distinctly from the great fasciculi 

 of the medulla oblongata, the corpora pyramidalia, olivaria, 

 and restiformia. From the great size of the olfactory and 

 the optic nerves in this class, as seen in the perch, (Fig. 96. 



FIG. 96. 



ff, q,) and also of the fifth pair (96. m,) and from the small- 

 ness of the three pairs of lobes from which these nerves ori- 

 ginate, the cerebral hemispheres (e,) the optic lobes (d,) and 

 the medulla oblongata (b,) appear only like small ganglia ap- 

 propriated to these nerves. The olfactory tubercles (96./,) 

 are generally contiguous to each other and to the hemi- 

 spheres, and the white fibres of the olfactory nerves (96. g,) 

 pass forwards on their lower surface from the cerebral lobes 

 (96. e.) Two commissures are generally perceptible in the 

 cerebral lobes, and beneath the small lobules in the optic 

 lobes a third ventricle is seen leading downwards to the in- 



