Weil, Anomaly of the Trochlear Nerve. 37 



one step further back, since it is not yet clear, whether the 

 nerve is to be regarded as a " mesencephalic " segmental nerve 

 (Locy), in which case the anomaly would be imperfectly atavis- 

 tic, and so conservative, or "epigonic" as Cunningham 

 says; or, as belonging to the post-mesencephalic segment (van 

 Wijhe, etc.), in which case the nucleus of origin has been dis- 

 placed forwards. Finally, it is conceivable that the second 

 primary cerebral incisure has no real segmental value, and that 

 the fourth nerve, though crossing it, is still within the boun- 

 daries of a single cranial segment. Under this last supposition, 

 the anomaly is, of course, entirely without significance. 



For other theories of the original course of the Trochlear 

 nerve, which do not, however, deal with this phase of the ques- 

 tion, Fiirbringer's article may be consulted, in Gegenbaur's 

 Festschrift. 



DESCRIPTION OF PLATE III. 



Figure i is taken through the posterior quadrigemina ; figure j, through 

 the velum ; figure 2 occupies an intermediate level. IV. L, Trochlear Nerve of 

 left side ; IV. R, Trochlear of right side; V.M., Mesencephalic root of trigem- 

 inus; P.L.B., Posterior longitudinal bundle; I.F,, Inferior fillet; S.F., Su- 

 perior fillet; B.C., Brachium conjunctivum. 



In figures | and g, IV. R. is traveling back from its origin to the decussa- 

 tion; IV. L. is traveling forwards, from the decussation to its exit. 



