MORPHOLOGY OF EYE MUSCLE NERVES 115 



gram) the result would be essentially the conditions that obtain 

 today in those forms of selachian embryos in which a peripheral 

 chiasma persists (fig. B, p. 25) , and are shown in the right hand 

 side of the diagram. It is necessary, however, to assume that phy- 

 logenetically the chiasma came to be more and more central and 

 that gradually the fibers of the trochlear nerve became exclusively 

 crossed fibers. What factors determine the survival of the 

 crossed fibers and the elimination of the direct is no less myste- 

 rious than those which have produced the ventral chiasmae of 

 the eye and the pons. Possibly purely mechanical conditions 

 of the sort suggested by the writer ('98) and by Johnston ('05) 

 are responsible. All who have discussed the chiasma agree in 

 one essential point — namely, that the chiasma of the trochlear 

 is secondary and that it constitutes a coenogenetic modification 

 of a somatic motor nerve. Therefore, its existence does not 

 affect our views of its morphology. All other questions are 

 subordinate to this, and it does not appear greatly to matter 

 whether or not the true histor}^ of the origin of the chiasma has 

 been or ever will be told. 



d. Conclusions regarding the morphology of the trochlearis. The 

 trochlear has been regarded as a somatic motor nerve on the 

 basis of anatomical evidence by Stannius ('49), Huxley ('74, '75), 

 Schneider ('79), Gaskell ('86, '89), Osborn ('88), Strong ('90), 

 Ftirbringer ('97), Wiedersheim ('98), and Gaupp ('99) and on 

 the basis of embryological evidence by Van Wijhe ('82), His f^'88), 

 Martin ('90), Dohrn ('90, '91), Zimmermann ('91), Hoffmann 

 ('94), von Kolliker ('96), Neal ('96, '98, '12), Koltzoff ('01), 

 Filatoff ('07), Belogolowy ("08, '10). 



On the other hand it has been regarded as a splanchnic motor 

 nerve on the basis of anatomical evidence by Bell ('30), Hatschek 

 ('92), Haller ('98), Furbringer ('02). Stannius ('51) and Lan- 

 gerhans ('73) reached the same conclusion on the basis of the 

 histological structure of the superior oblique muscle; while Bal- 

 four ('78), Marshall ('81, '82), Dohrn ('85, '90, '04, '07), Beraneck 

 ('87), Houssay ('90), Hatschek ('92), von Kupffer ('94) and 

 Sewertzoff ('98) supported this view on the basis of ontogenetic 

 evidence. 



