802 



THE NERVOUS SYSTEM 



wise, those passing from the restiform body to the opposite olive are deflected by the 

 olive of the same side and pursue a similar course to the raphe. While out of the hilus 

 of each olive streams a dense mass of white substance, yet many of the fibres 

 concerned with the olive pierce its walls from all sides. 



Many of the external arcuate fibres are said to be interrupted in the nucleus 

 arcuatus. This is a thin sheet of grey substance, variable in amount, which lies on 

 the ventral aspect of each pyramid, and, though it decreases inferiorly, it may be 

 evident down to the decussation of the pyramids. The nucleus receives its name 

 from the fact that its larger portion is interpolated in the course of the external 

 arcuates. 



FIGS. 597 AND 598. DIAGRAMS SHOWING THE COMPOSITION OF THE CEREBELLAH PORTIONS 

 OF THE INTERNAL AND EXTERNAL ARCUATE FIBRES. 



ynCLEUS in-- XUCLKUSOF COMMISSURAL NUCLEUS 



FUNICULUS CUNEATUS FUXIi'UI.UH OF ALA CINEUKA 



GRACILIS DOSSAL EXTERNAL ARCUATS 



FIBRES 



RESTIFORM BODY 



SPINAL TRACT OF 

 TRIGEMINUS 



Fig. 597. 



NUCLEUS OF ALA CINEREA 



\-J-:\TI:A L EXTERNAL 



ARCUATE FIBRES 



NUCLEUS OF TRACTUS KOLITARIl* 

 ! H IWIAL NUCLEUS AND J)EWK\ HI. \<; 

 ROOT OF VESTIBULAR NKi: I /: 



XUCLEUS OF FUMI'U- 

 LUS <T.\/:.t I'l x 



.NUCLEUS AM/HI; rrs 



-X" RESTIFORM Hi i II Y 



ROOT FILUM OF YAHl'S 



~ CEREBELLO-OLIVARY 

 FIBRES 



VENTRAL EXTERNAL 

 ARCUATE 



Fig. 598. 



The arcuate fibres of longer course, like the olives with which they are largely 

 concerned, have no homologues in the spinal cord. 



The central canal of the closed portion of the medulla is surrounded by a greater 

 amount of grey substance, substantia grisea centralis, than is the canal in the spinal 

 cord. This is largely gelatinous substance, the central gelatinous substance, and the 

 nerve-fibres in coursing through the grey substance are partially deflected by it, 

 leaving it as a cylindrical, more evident area of grey substance than in other regions. 

 In the open portion of the medulla the substantia grisea centralis naturally forms a 

 more transparent lamina just under the floor of the fourth ventricle. In the mesen- 

 cephalon it asrain surrounds the reformed canal or aquffiductug cerebri. 



