EYE-MUSCLE NERVES IN NECTURUS 159 



have here, just after its decussation, the reception by the troch- 

 lear nerve of these sensory fibers; but further investigation is 

 required to establish these relations in Necturus with certainty. 



Figure 8 (right side) shows a case in which the trunk of the 

 trochlear nerve runs at first directly caudad and then laterad, 

 splitting into two bundles, one of which runs through the choroid 

 plexus of the fourth ventricle. One or two fibers are sometimes 

 given off the main trunk of the nerve in its passage laterad, to 

 the lateral processes of the choroid plexus. 



Passing directly caudad over the cerebellar commissure and into 

 the choroid plexus of the fourth ventricle there are quite con- 

 stantly present three to six large medullated fibers which seem 

 to leave the main trunk of the trochlear nerve. These fibers, 

 previously noted by Kingsbury ('95), are lost among the blood 

 vessels of the choroid plexus. The connections of these fibers 

 have not been demonstrated. 



There have been observed in the dura mater in many speci- 

 mens fibers similar in size to those just described. Such fibers 

 have been traced from the choroid plexus of the fourth ventricle 

 cephalad to the pineal region. Their origin has not been deter- 

 mined. 



Having reached the dura mater, at this point quite closely 

 adherent to the skull, the trochlear nerve turns, and passing 

 usually about 1 mm. dorsal to the oculomotor nerve runs cepha- 

 lad in the dura of the lateral wall of the cranial cavity (fig. 3). 

 Passing now dorsal to the optic nerve, the trochlear nerve runs in 

 the dura to a point usually about 0.5 mm. cephalad of the olfac- 

 tory bulb and, forming a loop, returns to its foramen which in 

 the majority of cases lies in the parietal bone about 2.5 mm. 

 dorsal to the optic foramen. Quite frequently the foramen for 

 the trochlear nerve may lie 0.2 mm. cephalad to that for the 

 optic nerve or it may lie as much as 0.3 mm. behind it. In sev- 

 eral cases the two foramina were in the same transverse plane 

 but they are always separated by almost the entire depth of the 

 cranial cavity. 



One animal was examined in which, on one side, the foramen 

 for the trochlear nerve in the parietal bone, was 0.6 mm. caudal 

 to the lateral angle of the olfactory foramen. 



