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SENSE ORGANS OF AMIURUS 361 
geniculated root is represented in Fig. 6. In Figs. 4 and 5 the 
patches of ganglion-cells lateral to the ventral columns are the nuclei 
for the anterior and posterior roots of the sixth nerve. 
From the floor of the fourth ventricle vessels (v) are distributed 
up the sides of the vagus and trigeminal lobes as well as up the 
posterior face of the laminated bridge of grey matter joining the 
tubercula acustica (com. tub. ac.) 
This appears to give place gradually to the cortex of the cerebel- 
lum without again exposing the fourth ventricle, the roof of which is 
thus formed in the posterior part of this region by the cerebellar 
cortex, (Fig. 6), which is, however, gradually encroached on by the 
molecular layer until it is confined to the periphery. (Fig. 7). 
Two great transverse ventral commissural systems are readily seen 
in sagittal sections of the brain, one behind, the other in front of the 
ganglion interpedunculare ; the former of these which appears to be 
equivalent to the fibres marked pons varoli (?) by Mayser, is repre- 
sented in Fig. 6. It appears to be much more developed than the 
similar system in Cyprinus. The latter is the commissura ansulata ; 
its posterior bundles are those which stretch towards the ganglion 
interpedunculare, (Fig. 7), its anterior form the base of the brain 
immediately behind its junction with the lobus inferior (Fig. 8). 
Between the planes represented in Figs. 6 and 7, the fourth ventricle 
gradually becomes’ slit-like in section, its wall being formed of 
vertical fibres which connect the outer part of the ‘ Uebergangs- 
ganglion’ of Mayser, (‘transitionary,’ because, according to Mayser’s 
conception, it is situated partly in the hind and partly in the mid- 
brain) with the molecular layer of the cerebellum. The slit-like sec- 
tion of the ventricle is soon altered by the decussations of fibres in 
this region, by which a dorsal part is separated off belonging to the 
cerebellum (Figs. 7 and 8). Most posteriorly is the decussation of 
the secondary vago-trigeminal fasciculi, some fibres of which are re- 
presented approaching the middle line in Fig. 7. 
Fig. 7 is from a plane immediately behind the optic lobes, the tip 
of one of which is just caught in the section figured, with the 
fourth nerve emerging below and behind it. The nucleus of that 
nerve is in a more anterior plane (Fig. 8), as well as its decussation 
between the valvula cerebelli and the ventricle. From the plane re- 
presented in Fig. 8, as far as that in Fig. 11, the valvula cerebelli is to 
be met with, cortical substance at first predominating, but afterwards 
