2 28 NERVE COMPONENTS OF BONY FISHES. 



substance and in which are the origins of these four 

 muscles, two on each side, the canal being filled with the 

 muscular fibres. The two obliqui superiores arise farther 

 cephalad, the obliqui inferiores a little farther back from 

 the dorsal wall of the canal. The latter lie nearer the 

 median line and pass down between the superiores di- 

 rectly to their insertions on the ventral sides of the eye 

 balls, each receiving a second slip of fibres in transitu 

 from the ventral lip of the canal. The latter fibres are 

 smaller than the others and run down the ventral edge of 

 the muscle. 



The obliqui superiores at the lips of the canal diverge 

 and each receives a slip of fibres from the ventral lip 

 which are, as in the former case, smaller, and which run 

 along the dorsal edge of the muscle. 



II. — The N. Abducens. 



The sixth nerve arises by two fine roots, one .3 mm. 

 caudad of the other. They arise mainly from a common 

 nucleus of large cells which lies between them some dis- 

 tance from the median line and about two-thirds of the 

 distance between the floor of the fourth ventricle and the 

 ventral surface of the brain. There is a strong tract run- 

 ning transversely between the sixth nuclei of the two 

 sides, but whether any of the root fibres have a crossed 

 origin was not definitely determined. Another strong 

 tract runs from each sixth nucleus dorsally through the 

 overlying motor nucleus of the facialis and into the fasci- 

 culus longitudinalis dorsalis. Here again it is impossible 

 to tell whether all or any of these fibres are root fibres, or 

 whether this is a secondary tract. There are, however, 

 two bundles of fibres, one on each side, which run up, 

 one from each of the VI roots, laterally of the VI nucleus 



