32 Anatomy of the Nervous System 



VI I. -X.). In man, the chief nucleus appears as an enlarge- 

 ment at the anterior end of the column formed by the spinal 

 nucleus. Many fibres run through the nuclei, the lower part 

 of the spinal nucleus enclosing conspicuous bundles. 



About the level of the anterior end of the hypoglossal 

 nucleus (p. 45) a small area of rather clear gray matter, 

 similar to the gelatinous substance of Rolando of the spinal 

 cord, appears superficial to the dorsal part of the spinal V 

 nucleus, and about the lower extremity of the fourth ventricle 

 this spreads out and forms a layer over the outer surface of 

 the nucleus, exactly as the gelatinous substance covers the 

 dorsal column in the cord (PI. VII.). Traced downwards, 

 this is found to be continuous with the similar substance in 

 the cord, the nucleus as a whole being continuous with the 

 dorsal column. In connection with the cells of this nucleus, 

 the fibres of the spinal V root end, and from these cells arise 

 the secondary fibres which are to convey the impulse onwards. 

 Fasciculi concomitantes of the spinal V root lying in and 

 medial to the medial side of the spinal V nucleus, and largely 

 of heterolateral origin, are described for the rat by Fuse. 



The secondary fibre-tracts arising from the chief sensory 

 and the spinal nuclei of the trigeminus cannot be followed in 

 normal material, coursing through the reticular substance 

 in a diffuse condition. They comprise both short reflex con- 

 nections, largely, if not entirely crossed, and ascending fibres 

 to the midbrain and to the thalamus [trigeminal lemniscus), 

 of which the majority are crossed. It would appear that the 

 fibres arising in the chief and spinal nuclei follow separate 

 routes, and that the trigeminal lemniscus arises only from 

 the chief nucleus. It comprises two portions, one in the 

 dorsal part of the reticular formation, the other, which 

 decussates slightly more posteriorly in the rat, running close 

 to the dorsal portion of the medial lemniscus and to the 

 raphe. Fuse states that in the rat the ventral trigeminal 

 lemniscus ends largely (he believes entirely) in the ventral 



