Midbrain 65 



Other correlation centres are the dorsal nucleus of the 

 raphe, which lies in the median plane just dorsal to the 

 trochlear nuclei, some groups of cells in the raphe posterior 

 to the decussation of the brachia conjunctiva, and a number 

 of nuclei in various parts of the reticular formation which 

 are not usually distinct in Weigert sections. Among the last 

 of these, there must be mentioned particularly the nucleus 

 of Darkschewitsch (PI. X\'.). This mass of small cells is 

 situated at about the level of the anterior end of the medial 

 longitudinal bundle, to which it may contribute some fibres. 

 It lies just dorsal to the interstititial nucleus of Cajal and is 

 probably related particularly to the fibres of the posterior 

 commissure (p. 71). 



Situated in a median position at the ventral surface of 

 the tegmentum, lies the interpeduncular nucleus (ganglion 

 inter peduncular e), a mass of gray matter occupying the 

 posterior perforated space of human anatomy, w^hich lies 

 at the bottom of the interpeduncular fossa (Pis. XIII., XIV., 

 XX\T.). This centre receives a large fibre-bundle, the 

 fascicidus retroflexus of Meynert (Pis. XI\\-X\TIL, XXM.), 

 from each of the habenular nuclei of the diencephalon, and 

 gives off fibres, as already mentioned, which run to the 

 dorsal tegmental nucleus as the tegmental tract of the inter- 

 peduncidar nucleus (PI. XIII.). This is a group of very fine 

 fibres which may be observed to leave the dorsal surface of 

 the interpeduncular nucleus and to run dorso-posteriorly 

 on each side of the raphe. These structures, being concerned 

 with the correlation of olfactory with other, chiefly somatic, 

 impulses, are largest in animals having a highly developed 

 sense of smell. They are considerably larger in the rat than 

 in man, whose olfactory sense is rather poorly developed. 



A bundle which is rather distinct in the ventral part of 

 its course, but of which the origin is still uncertain, becomes 

 visible just behind the mamillary body. This is the transverse 

 peduncular tract (PI. X\\). It probably contains some optic 



