. The elasmobranch fishes . A B Fig. 210. Brain of Reterodonius francisci. (Mildred Bennett, del.) A. Dorsal view. B. Ventral view. ch., cerebellum; , restiform body; dL, diencephalon or thalameneephalon; id., inferior lobe; , median olfactory nucleus; mcd., medulla; ms., mesencephalon; old)., olfactory bulb; oJd., olfactory lobe; old., olfactory tract; opd., optic lobe; , pallial eminence; tl., telencephalon; , vascular sac; 7, olfactory nerve; //, optic nerve; IV, trochlearis or fourth nerve; VI, al»ducens or sixth cranial nerve; IX. glossopharyngeal nerve; X, vagus nerve. th
. The elasmobranch fishes . A B Fig. 210. Brain of Reterodonius francisci. (Mildred Bennett, del.) A. Dorsal view. B. Ventral view. ch., cerebellum; , restiform body; dL, diencephalon or thalameneephalon; id., inferior lobe; , median olfactory nucleus; mcd., medulla; ms., mesencephalon; old)., olfactory bulb; oJd., olfactory lobe; old., olfactory tract; opd., optic lobe; , pallial eminence; tl., telencephalon; , vascular sac; 7, olfactory nerve; //, optic nerve; IV, trochlearis or fourth nerve; VI, al»ducens or sixth cranial nerve; IX. glossopharyngeal nerve; X, vagus nerve. the compactness is the resnlt of the enlargement of the cerebellnm {ch., fig. 212) wherel)y it overlies the mesencephalon or, occasionally, even a part of the diencephalon {Scoliodon, or Myliohatis). We may now consider in more detail the different segments of the brain. In form the telencej^halon (fl.) depends largely npon the proximity of the right and left halves. In Heterodontus francisci (fig. 210) these are clearly sepa- rated by a median anterior sulcus so that the lobes have undergone but slight fusion in the middle line. In Heptanchus we have noted a similar condition
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