MORPHOLOGY OF EYE MUSCLE NERVES 79 



muscle is secondary, Dohrn ('07, p. 416) advances the following 

 argument : 



The development of the trochlearis forms a crux for the Heiisen- 

 Gegenbaur-Furbringer hj^jothesis of the primary connection of the 

 nerve and its terminal organ. So long as we have to do Avith the con- 

 nections of spinal somatic nerves with somites already formed, one 

 may believe in the existence of plasma bridges and be willing to rec- 

 ognize in them at least the paths that guide the growing nerve. The 

 separation in this case between tube and somite is the least possible 

 and few cells lie between them. But if one extend this view to the 

 eye muscles he at once meets insuperable difficulties. All these have 

 a considerable distance to traverse in reaching their end-organ, and 

 the abducens, oculomotorious and trochlearis solve the problem each 

 in its own way. I have already pointed out ('01) that this theory 

 must be tested, not on the spinal somatic nerves, but on the eye mus- 

 cle and splanchnic motor nerves, if it is to be regarded as tenable, 

 and I must now put the question to the defenders of the Hensen theory 

 how they conceive of the connection of the trochlearis through plasma 

 bridges "with its terminal organ in the other antimere. 



In embryos of 12 to 18 mm. the actual trochlearis fibers remain 

 either completely within the neuroblasts from which they are to arise 

 or they have at most made their way through the medullary wall to 

 the chiasma, but are not yet protruded from the wall. The supe- 

 rior oblique muscle, on the other hand, has developed from the man- 

 dibular somite as the most dorsal of the eye muscles and has already 

 begun to differentiate muscle fibers. What and how many plasmodes- 

 matous connections must be assumed in order to establish the most 

 complicated of all paths traversed by any motor nerve? How is one 

 to demonstrate that first plasmodesms of the neural tube, then others 

 of the mesenchyma, then — in case ' Kettenf asern ' are present before 

 the trochlearis fibers extend through their region of distal distribution — ■ 

 plasmodesms of the 'Kettenfasern,' and finally those of the muscle fibers 

 of the superior oblique muscle are likewise prepared to furnish neuro- 

 fibrils through further differentiation? Would this be in the least 

 more easily conceivable than the outgrowth of the nerve from its 

 neuroblasts to its terminal organ? It is true that the outgrowth 

 points to problematical forces, but shall we be mthout a riddle if we 

 put another in its stead — ^one that presents a number of unfounded 

 assumptions? 



Held ('06), who admits the insufficiency of the hypothesis of 

 plasmodesmatous paths for the trochlearis nerve, thinks that he 

 may assume as the direct path-determining impulse for the out- 

 growing neurofibrils the principle of the axial position of the neu- 

 roblast and that of the shortest distance. These obviously fail 



