THE BRAIX 



161 



The cerebellum is always very large, overlapping the optic lobes 

 and the medulla oblongata to a greater or less extent : it is divided 

 up into several folds lying one behind the other, and usually con- 

 tains a metacoele opening into the fourth ventricle (Figs. 130 and 

 181). In Sharks the medulla oblongata is an elongated cylindrical 

 body, while in Rays it is more compressed and triangular ; at its 



melcL 



FIG. 131. BRAIX OF Cheilo- 

 scyllium. (From Parker and 

 Haswell's Zoology.) 



Viewed from the dorsal side, 

 the roof of the various ven- 

 tricles removed so as to show 

 the relations of the cavities 

 (semi-diagrammatic). 



cer, dilatation from which the 

 metaccele is given off; dia, 

 thalamoccele the reference 

 line points to the opening 

 leading into the infundibu- 

 lum ; iter, aqueduct of Sylvius 

 (mesocffile), into which the 

 optocoeles (opt) open ; meta, 

 myelocrele ; para, lateral ven- 

 tricle ; pros, median part of 

 prosocrele ; rh, rhinoccele. 



TTl.O 



FIG. 132. BRAIN OF 



Lepidosteus. (Dorsal 

 view.) (After Balfour 

 and Parker. ) 



cbl, cerebellum ; c. h, pro- 

 sencephalon ; di, thal- 

 amencephalon ; m.o, 

 medulla oblongata ; 

 off. I, olfactory lobes ; 

 opt. I, optic lobes ; prs, 

 lobes of prosencepha- 

 lon. 



anterior end are two lateral lobes, the corpora restiformw. In 

 electric Rays (p. 146) a pair of lobi electrici arise from the 

 gray matter of the floor of the fourth ventricle, and these en- 

 close a mass of giant nerve-cells. 



Ganoidei. The pallium covering the median prosocoele 

 consists mainly or entirely of epithelial and connective tissue 



M 



