THE N. TROCHLEARIS. 229 



and through the motor VII nucleus into the fasciculus 

 longitudinalis dorsalis. These appear to be root fibres, 

 thus putting the VI nerve directly into relation with the 

 fasciculus. 



Johnston ('98, p. 581) describes all of the VI fibres as 

 arising from the fasciculus in Acipenser. 



The sixth nerve at its exit from the brain contains some 

 fine fibres mingled with the coarse ones. It has a short 

 intra-cranial course mesially of the VIII nerve and dor- 

 sally of the lateral edge of the sub-cranial canal, which is 

 here very wide. Under the lobi inferiores it passes ven- 

 trally through a foramen in the cranial floor into the 

 dorso-lateral angle of the sub-cranial canal. Here it is 

 crowded close against the lateral face of the m. rectus 

 externus, which muscle a part of the fibres soon enter. 

 Other fibres run to the ventral side of the muscle to enter 

 the fine-fibred slip of the muscle which arises in the 

 median line (Fig. 2). Of the latter nerve fibres, some of 

 finer calibre run farther cephalad than any of the coarse 

 fibres to supply these fine muscle fibres near their insertion 

 upon the eye. 



III. — The N. Trochlearis. 



The nucleus of the fourth nerve is merely a caudal 

 extension of that portion of the third nucleus {q. v.) which 

 lies dorsally of the fasciculus longitudinalis dorsalis. The 

 root fibres (mostly very large with a few smaller ones) pass 

 up at once and cross over the mesocoele to emerge in the 

 usual manner behind the optic lobes, after which they 

 form a close round bundle closely applied to the inner 

 face of the sensory V root (this part of their course not 

 being shown on the plots). 



Here the trochlearis root receives and carries for a 



