. The elasmobranch fishes . Fig. 213. Brain of Scymnus. (From Burckhardt.) A. Side view. B. Median sagittal view. ch., cerebellum; , restiform body; (q)., pineal stalk; ffin., median longitudinal bundles; in., infundilnilum ; , anterior lolie; Lv., lobe of the vagus; , median olfactory nucleus; , optie lobe; , pallial eminence; , posterior or inferior lobe of hypophysis; , vascular sacs; //, optic nerve. to be unusually large. In all forms the roof of the mesencephalon is composed of a right and a left optic lobe {, figs. 210 and 213) which are hollow out- pock
. The elasmobranch fishes . Fig. 213. Brain of Scymnus. (From Burckhardt.) A. Side view. B. Median sagittal view. ch., cerebellum; , restiform body; (q)., pineal stalk; ffin., median longitudinal bundles; in., infundilnilum ; , anterior lolie; Lv., lobe of the vagus; , median olfactory nucleus; , optie lobe; , pallial eminence; , posterior or inferior lobe of hypophysis; , vascular sacs; //, optic nerve. to be unusually large. In all forms the roof of the mesencephalon is composed of a right and a left optic lobe {, figs. 210 and 213) which are hollow out- pockets from the dorsal side of the mesencephalic segment. It is largely among the cells of these lobes that the fibers of the optic nerve terminate. The ventro- lateral part of this segment of the ])rain (see fig. 213b) is enlarged by longi- tudinal swellings, the lateral fiber tracts of the mesencephalon. Through the ventral walls the third cranial or oculomotor nerve ])asses, and from the roof the fourth nerve leaves the brain stem (IT, figs. 210 and 212). The metencephalon as a segment is usually well developed in the Elasmo- branchs. Dorsally it consists of the cerebellum (ch.) and ventrally it is swollen by large fiber tracts {Scy)iinus, fig. 213b). The cerebellum is usually rhomboid in shape and divided dorsally by a median longitudinal furrow into right and left halves (fig. 210a). These may be further separated into anterior and posterior parts l)y a second furrow at right angles to the first. In some forms, as has been said, the cerebellum comes to be very complex and of inniiense size.
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