SEGMENTS OF TRIGEMINAL NERVE-GROUP 285 



degree, so that it becomes easy to see how the dorso-ventral muscles of the 

 prosomatic segments of the latter have become converted into the eye-niuscu- 

 lature of the former. The most powerful proof of all that such a conversion 

 has taken place is that a natural and simple explanation is at once given of 

 the extraordinary course taken by the IVth or trochlear nerve. Ever since 

 neurology began, the course of this nerve has arrested the attention of anato- 

 mists. Why should just this one pair of nerve-roots of all those in the whole 

 body be directed dorsalwards instead of ventralwards, and cross each other in the 

 valve of Vieussens, each to supply a simple eye-muscle (the superior oblique) 

 belong-ing to the other side ? For generations anatomists have wondered and 

 found no solution, and yet, without any straining of hypotheses, in consequence 

 simply of the investigation of the anatomy of the corresponding pair of muscles 

 in the scorpion group, the solution is immediately apparent. 



This pair of muscles alone, of all the musculature attached to the carapace, 

 crosses the mid- dorsal line to be attached to the other side, thus carrying its 

 nerve with it to the other side ; by a continuation of the same process the 

 relation of the trochlear to the superior oblique muscle can be explained. 



The comparison of the eye-muscles of the vertebrate with the dorso-ventral 

 segmented muscles of the invertebrate makes the number and nature of the 

 pro-otic segments much clearer. 



