X TELEOSTEAN FISHES 361 



As regards the nervous system, the brain (Fig. 151) shows two striking 

 differences from that of the elasmobranch. There is a large well-developed 

 cerebellum (c) which projects back as a somewhat tongue-shaped organ 

 over the roof of the fourth ventricle. If, however, the brain is cut through 

 with a sharp knife in the sagittal plane it is seen that a forward con- 

 tinuation of the cerebellum dips inwards underneath the optic lobes, 

 encroaching upon the cavity of the mid-brain. This is known as the 

 valvula cerebelli. Whereas in the elasmo- 

 branch the cerebellum as it grows in length ol. . 



bulges outwards, in the teleost on the ^ ^ , „ a_ o /? 



other hand only its posterior or tailward „^_. 



portion bulges outwards, its anterior por- 

 tion bulging inwards into the ventricular 

 cavity so as to form the valvula cerebelli. if " ™ 



There is reason to suspect that a some- "''■'■ Mf 

 what similar though more extensive modi- ^.^ -^ ™ 



fication has taken place in the region of y 



the hemispheres. The portion of brain-wall w vi|.-,j^ 0;^, 



which in the typical vertebrate bulges out ^... ,..-^^ ,vi^ 



to form the hemisphere appears to be re- |v..!!l"^li *^,r 



presented in the teleost by a solid thicken- x. 



ing which projects into the cavity of the ViM 



anterior portion of the brain from its floor 

 (Fig. 151, c.s) and is covered over by 

 a thin membranous roof {pr.). 



The last feature which need be men- '°' '''' 



J . , , ■ 1 i_ r 1 '^^^ brain of a teleost (Salmon), 



tioned is that the optic lobes are of reia- (From Wiedersheim, The Cambridge 



tively enormous size in most teleosts (Fig. ■'V''"'™' History, vol. vii.) c, Cere- 



. , . \ tj bellum ; c.s, solid mass projecting 



151, Op.l.) — in correlation with the very into cavity of fore-brain ; oi, olfactory 



high development of the eyes. '°'''; "*■'■ "p"'^ i°be; /«;°. pi^ieai 



o L J organ ; pr, thin roof of fore-brain, 



The Teleost possesses the same outfit partially removed ; sp.c, spinal cord. 



of sense organs as the Elasmobranch. R°-an figures indicate cranial nerves. 



The eyes, apart from their large , size, show an interesting pecul- 

 iarity in regard to their focusing arrangements. A projection from 

 the choroid known as the falciform process penetrates the vitreous body 

 in the neighbourhood 'of the optic nerve and passes in a meridional direc- 

 tion towards the lens, to the equator of which it is attached by a curious 

 bulb (the " campanula Halleri "). The falciform process is provided 

 with longitudinal muscle fibres which by their contraction pull the lens 

 nearer the retina so as to focus more distant objects — the eye when in 

 a state of rest being focused for objects close at hand. Here we have 



