362 



division of the large root of the fifth descends towards the medulla, 

 not, as has been believed, in the restiform body, but, as I formerly 

 showed, in the very substance of the caput cornu posterioris, of 

 which it constitutes many of the longitudinal bundles. 



The small, or motor root of the fifth arises from the same emi- 

 nence by three or four parallel bundles, a little anterior to the larger 

 root, being separated from it only by a small process of the middle 

 peduncle coming from the upper portion of the pons Varolii. Its 

 bundles pass obliquely to the inner side of the caput cornu, and 

 spread out their fibres through the oval group of large multipolar 

 cells already mentioned. Some of the fibres of the fifth nerve de- 

 cussate with those of the opposite side through the raphe. 



Some of the fibres which form the point of the fillet, as well as 

 others coming from the anterior border of the pons Varolii, are con- 

 nected with the grey eminence of the fifth nerve. 



In tracing the caput cornu towards the brain, it is found to ex- 

 tend obliquely outwards from the angle in the floor of the fourth 

 ventricle, beneath the superior peduncle ; the deep portion of the 

 superior peduncle, before it reaches the corpora quadrigemina, passes 

 from the side of the fillet transversely inwards in front of the caput 

 cornu to the middle line, where it decussates with the corresponding 

 part of the opposite side. Part of the grey substance of the caput 

 cornu becomes imbedded in the fibres of the fillet, and ascends with 

 them ; while the rest extends more and more backward along the under 

 part of the superior peduncle, until it arrives at its inner border at 

 the side of the valve of Vieussens. Across the thin posterior edge 

 of the superior peduncle it sends numerous grey processes outwards, 

 to meet similar processes from the grey substance in the fillet ; so 

 that the longitudinal fibres of this part of the peduncle are formed 

 into small bundles by a network of grey substance ; the testis, or 

 posterior portion of the corpora quadrigemina, on each side, rests 

 upon this structure : it is continuous with, and apparently developed 

 from it. 



While these changes are in progress, the nucleus of the motor 

 root of the fifth nerve gradually disappears, and is replaced by a 

 group of cells deeply coloured with pigment, and constituting what 

 is known as the substantia ferruyinea j at the same time the grey 

 substance which forms the floor of the fourth ventricle on each side 



