MORPHOLOGY OF EYE MUSCLE NERVES 117 



splanchnic motor nerve. It is not a little surprising, however, 

 that Dohrn seems to have forgotten that the motor nidulus of the 

 trochlear has the relations of a somatic motor nerve and not 

 those of a splanchnic motor nerve. In the heat of the attempt 

 to prove the trochlear a dorsal nerve Dohrn seems also to have 

 forgotten that somatic motor nerves have relations with gan- 

 glia, both dorsal and sympathetic, comparable with those of the 

 trochlear. Of his special views regarding the segmental rela- 

 tions of the trochlear consideration will be given later. 



On the other hand it has been shown that in every essential 

 detail of central and peripheral relationship, of histogenesis and 

 of adult histological structure the trochlear must be regarded 

 as a somatic motor nerve. Its peculiarity consists in one spe- 

 cial feature — its chiasma, but in this respect it differs quite as 

 much from a typical dorsal nerve as from a ventral somatic 

 motor one. The interpretation of the mode of genesis of this 

 chiasma is quite as easy upon the assumption that it is mor- 

 phologically a somatic motor nerve as upon the assumption that 

 it is a dorsal nerve, the opinion of Dohrn ('07) to the contrary 

 notwithstanding. ^ 



If. Are the relations of the abducens comparable with those of a 

 somatic motor spinal nerve? 



As a purely motor nerve, with a ventro-lateral nidulus and 

 with a distribution to somatic musculature, there seems to be 

 no good reason for questioning the serial homology of the abdu- 

 cens with spinal somatic motor nerves. The absence of a sympa- 

 thetic ganglion is one striking point of difference, however, which 

 is correlated with the absence of any connection with a dorsal 

 nerve. In both these respects the abducens resembles the more 

 primitive somatic motor nerves of Amphioxus and Petromyzon, 

 and the absence of a sympathetic ganglion is to be regarded as 

 a case of the retention of an ancestral character. 



1 Dohrn ('07, p. 410) thinks that the writer's view of the trochlear as a somatic 

 motor nerve makes the solution of the problem of the chiasma more difficult. 



