SENSE ORGANS OF AMIURUS 361 



geniculated root is represented in Fig. 6. In Figs, i 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 

 tuber cula 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 (1) by Mayser, is repre- 

 sented in Fig. 6. It appears to be much more developed than the 

 similar system in Gyprinus. The latter is the commissura ansulata ; 

 its posterior bundles are those which stretch towards the ganglion 

 inteipedunculare, (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 



