THE ELASMOBRANCII FISHES 



231 



may take a coiii'sc more or less nearly ])arallel as in (Uirvharias and i\I i/liobatis 

 (fig. 212). 



In the greater nnmber of Elasniobranchs the brain is elongated so that in a 

 dorsal view at least parts of all the five segments are visible. In a few, how- 

 ever, the brain may appear as a more or less compact mass. As a normal thing 



A B 



Fig. 210. Brain of Reterodonius francisci. (Mildred Bennett, del.) A. Dorsal view. B. 

 Ventral view. 



ch., cerebellum; c.r., restiform body; dL, diencephalon or thalameneephalon ; id., inferior 

 lobe; im.n., median olfactory nucleus; mcd., medulla; ms., mesencephalon; old)., olfactory 

 bulb; oJd., olfactory lobe; old., olfactory tract; opd., optic lobe; p.c, pallial eminence; tl., 

 telencephalon; v.s., 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 Myliohat is). 



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 



