MORPHOLOGY OF EYE MUSCLE NERVES 107 



as proof of the derivation of the neurilemma from the neural 

 crest. Dohrn, however, thinks that the evidence of cellular 

 migration into the trochlear anlage from the ramus superficialis 

 indicates that genetic relations between the trochlear anlage and 

 rudiments of ganglia still persist today, even though they are 

 merely transient. 



Dohrn's thorough investigation of the histogenesis of the troch- 

 lear in elasmobranchs, however, indicate that the so-called gan- 

 glia of the trochlear are irregular fragments of the neural crest 

 lying in the region through which the trochlear grows; further, 

 that the trochlear anlage in such forms as Pristiurus attains 

 connection with the myotome without any relation whatever 

 with these fragments, which develop in inverse ratio with the 

 development of the ramus superficialis V, of which therefore they 

 appear to be the equivalent. In other words, the relations of 

 the trochlear with these 'ganglia' of the Torpedinidae in which 

 they appear seem similar to its relations with the ramus super- 

 ficialis V in the Squalidae. This equivalency is recognized by 

 Gast ('09) who says that the sensory elements of the trochlear 

 which appear as ganglia in the Torpedinidae are represented by 

 the ramus ophthalmicus superficialis V in the Squalidae. 



Whether or not this equivalency be admitted, the relations 

 of the trochlear to the 'ganglia' of the nerve resemble those of 

 a spinal somatic motor nerve to the ganglionic or nervous deriva- 

 tives of the neural crest. Such relations disprove the somatic 

 motor character of the nerve in question quite as little in one 

 case as in the other. 



While in Squalus the trochlear anlage has no such relations 

 to irregular fragments of the neural crest as in the Torpedinidae, 

 the nerve does have relations to cell masses which precisely 

 resemble those of the oculomotor to the anlage of the ciliary 

 ganglion, or those of a spinal somatic motor nerve to sympathetic 

 anlagen. Such relations of the trochlear anlage are represented 

 in figures 54 and 55. With a strong presumption in favor of 

 the view that the trochlear is a somatic motor nerve as evi- 

 denced by its histogenesis and by its central and peripheral con- 

 nections the most reasonable interpretation of the mass of cells 



