NERVOUS SYSTEM. 



51 



while in others they remain but a cell or two in thickness. In 

 the cerebral hemispheres the lower or ventral surface develops 

 a large ganglionic mass, the corpus striatum, in either hemi- 



I y 



Fig. 53. Nearly median section of brain of trout, after Rabl-Riickhard. a, 

 aqueduct; ac, anterior commissure; b, bulbus olfactorius; c, ventriculus communis 

 (composed of first three ventricles of typical brain); cb, cerebellum; cs, corpus 

 striatum; /% frontal bone ; /;, habenular ganglion ; /zr, hypophysis; z, infundibulura; 

 /, lobus inferior; vi, medulla; /, pinealis ; pa, pallium of cerebrum; fc, posterior 

 commissure ; s, saccus vasculosus ; t, torus longitudinalis ; te, tectum of optic lobes ; 

 ■0, valvula cerebelli; /, //, olfactory and optic nerves; I]', fourth ventricle. 



sphere. The rest of the cerebral wall is known as the pallium 

 or mantle, and undergoes great modifications in the different 

 groups. In some fishes (cyclostomes, ganoids, and teleosts) 

 it is epithelial in character. In other vertebrates it is largely 



Fig. 54. Brain of dog (after Wiedersheim), showing fissures and gyri of 

 cerebrum. II-XII, cranial nerves. 



nervous in nature,' its outer surface (cortical substance) being 

 composed of ganglion cells. In all the lower vertebrates the 

 surface of the cerebrum is smooth, but in the higher mammals 



1 Even in mammals a portion — the septum pellucidum — retains an epithelial character. 



