550 THE XEKVOUS SYSTEM 



turn in the mid-region of the pons Varolii. The motor root occu- 

 pies a slightly cephalad position as compared with the sensory. 

 Though the path of the central neurones which supply this root is 

 not definitely known, from analogy it is reasonable to suppose that 

 from the motor cortex their fibres start down the pyramidal tracts, 

 as in the case of the central motor neurones of the other cranial 

 nerves, but unlike these, they leave the pyramidal tracts at a level 

 considerably cephalad from the nerve trunk for which they are 

 destined. They probably decussate through the median raphe, and 

 many of them then end about the nerve cells of the substantia fer- 

 ruginea or locus ceruleus (nuclei minores nervi trigemini), a scat- 

 tered group of large cells lying near the median line and ventral to 

 the grey matter surrounding the aqueduct of Sylvius. 



Some of the central motor neurones, however, are continued past 

 this nucleus without interruption ( Vs, Fig. 387, and Vc, Fig. 414), 

 and these fibres, together with the neu raxes from the cells of the 

 substantia ferruginea, pass caudal ward as the descending or mesen- 

 cephalic root of the trigeminus. This root above the trochlear 

 decussation is lateral to the descending root of the fourth or troch- 

 lear nerve. Caudad to the trochlear decussation it is .continued 

 downward in the same plane and is thus found just dorso-lateral 

 to the substantia ferruginea and resting upon the ventral surface 

 of the grey matter which surrounds the aqueduct and forms the 

 floor of the fourth ventricle. 



At the level of the fifth nerve its mesencephalic root turns 

 lateralward, many of its neuraxes (probably those coming from the 

 cells of the substantia ferruginea) entering the motor root, and 

 passing between the bundles of transverse pons fibres to their exit 

 in the portio minor of the trigeminus. Other fibres of the descend- 

 ing root of the fifth (probably those central neurones which come 

 from above the locus ceruleus) terminate in the motor nucleus of the 

 trigeminus (chief motor nucleus, nucleus princeps nervi trigemini), 

 a group of large nerve cells in the dorso-lateral part of the pontal 

 tegmentum. This motor nucleus is ventro-mesial from the chief 

 sensory nucleus and the incoming centripetal root (sensory root, 

 portio major) of the trigeminus. 



The nerve cells of the chief motor trigeminal nucleus are en- 

 veloped by an intricate network of collaterals derived from the 

 neurones of the pyramidal tracts and of the minor trigeminal 

 nuclei. The neuraxes of the cells of the chief nucleus, together 

 with those from the cells of the locus ceruleus, form the motor root 



