510 THE ORGANS 



olive (secondary cochlear). The exact mode of connection with the pyramids 

 is not well known. 



Afferent Roots, Their Terminal Nuclei and Secondary Tracts. — The lateral 

 (Deiters') and medial vestibular nuclei arc usually still present, also possibly fas- 

 tigio-bulbar fibres. The ventral cochlear nucleus has disappeared, but other 

 cochlear nuclei (superior olivary and trapezoid) are usually present. Often 

 fibres can be seen passing from the superior olive to the nucleus VI. 



Fibres of the secondary cochlear tract (corpus trapezoideum) are still traversing 

 the medial lemniscus, and decussating. The tract they are forming (lateral 

 lemniscus) is not yet very distinct. 



The spinal V is in the same position, but it and its terminal nucleus tend to 

 separate into groups of fibres and cells, and to change their relative positions. 

 The medial lemniscus is more flattened in cross section, extending transversely 

 instead of dorso-ventrally. The ventral spino-cerebellar tract and spino- 

 thalamic tract are in the same positions in the external part of the tegmentum 

 ventral to the spinal V and external to the superior olive. 



Other Afferent Cerebellar Connections. — The restiform body has now 

 merged with the white matter of the cerebellum. The nuclei and transverse 

 fibres of the pons (ponto-cerebellar neurones) have increased. The longitudinal 

 fibres in the pons at this level are principally the pyramids, but some are pallio- 

 pontile fibres which terminate in the nuclei pontis. Perpendicular fibres are 

 present. 



Intersegmental Neurones. — The reticular formation is practically unchanged. 

 One of its nuclei (nucleus reticularis tegmenti) can be seen as a lighter area 

 (Weigert) in the medial part, dorsal to the medial lemniscus. The rubro-spinal 

 tract is in the same position near or mingled with the spino-thalamic and ventral 

 spino-cerebellar tracts. These fibres are not easily distinguished among the 

 various fibres of the cochlear tract which cross them. The medial longitudinal 

 fasciculus is now a well-marked tract occupying the same position. From now 

 on, it contains ascending fibres from Deiters' nucleus and perhaps other reticular 

 nuclei besides the descending fibres from the nucleus of the medial longitudinal 

 fasciculus. 



Efferent Suprasegmental Neurones. — The pyramids and coUiculo-spinal 

 tract (predorsal fasciculus) occupy the same positions. The fastigio-bulbar 

 fibres have been mentioned. The superior cerebellar peduncle is now more 

 distinct as it is being formed by fibres from the dentate nucleus. It lies near the 

 dorso-lateral part of the ventricle. 



8. Transverse Section of the Hindbrain Through the Roots of Nerve V (Trigem- 

 inus) (Figs. 332 and 343) 



Efferent Peripheral Neurones. — Motor nucleus oj V. This is mesial to the 

 terminal nucleus of the \ and its coarse efferent root fibres may be seen, in favor- 

 able levels, passing out just internal and somewhat cephalad to the entering 

 afferent fibres. It is probable some of the fibres cross and pass out in the 

 opposite motor joot. Some of the finer terminal fibres present in the nucleus 

 are afferent root fibres of the V (two-neurone arc) and collaterals of secondary 



