492 THE BKAIN OF MAMMALS, BY TH. MEYNERT. 



of fig. 254, which represents a higher level than the left, roots 

 of the sixth are already visible without any part of its nucleus 

 being apparent. 



Whether additional root fasciculi of the sixth proceed, as 

 Schroder v. d. Kolk maintains, from some unknown centre of 

 origin situated above and external to this nucleus must remain 

 at present undecided. The same writer made the plausible 

 remark that the course of the root of the sixth, directed as 

 it is from the raphe', indicated (in opposition to the apparent 

 tendency of the oculomotorius to effect a decussation in 

 the raphe) a non-decussating central origin of the sixth, by 

 which means its synergetic action with the opposite rectus 

 internus would be explained. If, however, the origin of the 

 sixth is as I have stated, this observation is no longer applicable 

 to it, however correct the principle may be in reference to 

 anatomically preformed co-ordinations of movement, as indeed 

 I have recognized in the partly decussating, partly non- 

 decussating origin of the tegmentum of the crus cerebri. That 

 this principle, however, is not applicable to the innervation 

 through the crusta of the crus cerebri, I have already stated in 

 general terms on p. 453 et seq., and no exceptional conditions 

 would obtain in the case of the nucleus of the sixth, merely 

 on account of its connection with the raphe. 



But whether the abducens receives its nerves decussatingly 

 or directly from determinate centres of co-ordination (corpora 

 quadrigemina) may be decided by the investigation of the rela- 

 tions of descending tracts to it even at a totally different level. 

 The interesting fact discovered by Gudden is perhaps of impor- 

 tance in regard to the control exercised over the abducens by a 

 centre far removed from it at the level of the origin of the oculo- 

 motorius, that a flat fasciculus proceeding from the nates enters 

 transversely into the crus cerebri (tractus transversus pedun- 

 culi), which only became very feebly developed when Gudden 

 destroyed the function of the retina in newly born animals, 

 and certainly therefore stands in close functional relation with 

 it. The fasciculus itself had already been observed and cor- 

 rectly delineated by Inzani and Lemoigne. 



Clarke also considers that root-fasciculi of the nervus abducens 

 curve round the transverse section of the nucleus of the facial, and 



