496 THE ORGANS 



fibres), and higher up, of the VII. A small mass of gray of a gelatinous appear- 

 ance near it is its terminal nucleus. The course of the secondary tract cannot be 

 made out and is not accurately known. The other bundle is the descending 

 vestibular root. It lies lateral to the fasciculus solitarius. Accompanying it are 

 cells which constitute its terminal nucleus. Occupying the floor of the ventricle 

 lateral to the dorsal nucleus of the X is another terminal nucleus of the vestibular 

 nerve, the nucleus medialis (triangular or chief nucleus.) Internal arcuate 

 fibres emerging from these regions ma}?- represent secondary tracts (probably 

 reflex) from these nuclei, (p. 499; Fig. 339.) 



The nucleus gracilis has disappeared. The nucleus cuneatus may be pre- 

 sent, much diminished, and give rise to some internal arcuate fibres to the 

 medial lemniscus. The lemniscus is now a tract which has become built up 

 on each side of the median line. This latter is known as the raphe {i.e., 

 "seam," .stitched by the decussating fibres). 



The ventral spino-cerebellar tract and spino-thalamic tract are in about the 

 same lateral position, but the dorsal spino-cerebellar tract has moved dorsally 

 and together with olivo-cerebellar fibres (see below) begins to form the restiform 

 body (see below). 



In the lateral part of the reticular formation, between spinal V and olive are 

 seen the nuclei laterales. In these nuclei some of the spino-cerebellar fibres end. 

 The axones of these nuclei partly enter the restiform body on the same side and 

 partly cross to the opposite restiform body (p. 513; Fig. 345). They form some 

 of the ventral external arcuate fibres seen in the section. The lateral nuclei are 

 thus partial interruptions in the spino-cerebellar path. The nuclei arcuati are 

 well marked. 



Other Afferent Cerebellar Neurones. — A new and important convoluted mass 

 of gray is the inferior olivary nucleus, forming the bulge of the lateral surface of 

 the medulla known as the olive. Near it are the dorsal and medial accessory 

 olives. The axones of the olivary cells are the olivo-cerebellar fibres. They cross 

 through the fillets, pass through or around the opposite olivary nucleus, thence 

 proceed dorso-laterally, being gathered into more compact bundles, traverse or 

 surround the spinal V and dorsal to it bend longitudinally, forming a great part 

 of the restiform body. The latter produces an eminence on the dorso-lateral 

 surface of the medulla. The restiform body, thus formed by these spino-cere- 

 bellar and olivo-cerebellar fibres, together with certain others, passes into the ' 

 cerebellum higher up, forming the major part of the inferior cerebellar arm or 

 peduncle. According to many authorities fibres from the columns and nuclei 

 of Goll and Burdach of the same side (dorsal external arcuate fibres) and opposite 

 side (ventral external arcuate fibres) may join the restiform body. (Comp, 



P- 513-) , . 



Fibres appearing on the external surface of the olivary nucleus are the term- 

 ination of a large tract descending to the olivary nucleus, the central tegmental 

 tract. Its origin in higher levels is not accurately known. 



Intersegmental Neurones. — The reticular formation is now still more 

 extensive. 



The original U-shaped mass of intersegmental tracts (and the coUiculo-spinal 

 tract) has now become widely separated into two parts. The lateral part, con- 



