106 H. V. NEAL 



S. Are the relations of the trochlearis comparable with those of a 

 somatic motor spinal nerve? 



There is practically a consensus of opinion that in its most essen- 

 tial relations, namely, in its relation to a nidulus in the somatic 

 motor column of the hindbrain, the relations of the trochlear 

 are comparable with those of spinal somatic motor nerves. There 

 is less general agreement, and yet a considerable majority of 

 morphologists agree, that the trochlear innervates somitic mus- 

 culature. The main objection which has been advanced against 

 this conclusion has been that raised by Stannius ('51) and 

 Langerhans (73) of the different histological structure of the 

 superior oblique muscle, which, according to these investigators, 

 resembles splanchnic rather than somitic musculature. This ob- 

 jection may be met by denying the truth of the assertion as a 

 generalization for all vertebrates. Differences in size and in 

 detail between fibers of the eye muscles and those of the lateral 

 trunk muscles may exist — especially in those forms in which the 

 eye muscles are differentiated from a loose mesenchyma, but 

 in forms like Squalus, there is no important histological differ- 

 ence between the muscles of the eye and those of the trunk. 

 Such slight differences as do obtain may be ascribed to differ- 

 ences in environment. That they are not due to difference in 

 genesis appears demonstrated by the evidence of the somitic 

 origin of the eye muscles. Therefore, in histological structure 

 as well as in its histogenesis and in relations, both central and 

 peripheral, the trochlearis conforms to the type of spinal somatic 

 motor nerves. But there are other relations of the trochlear 

 that may profoundly affect our views of its morphology. 



a. How may the 'ganglion' of the trochlear be interpreted/ 

 Gast ('09) thinks that the trochlear 'ganglia' described by Dohrn 

 ('85), Hoffman ('89), Martin ('90) and Froriep ('91), afford very 

 clear proof that the trochlear is a 'complete segmental nerve.' 

 Dohrn ('07, p. 396) regarded the evidence that in Torpedo em- 

 bryos rudimentary centripetal ganglionic nerve fibers unite with 

 the trochlearis as supporting the view that the trochlear is a 

 lateral-horn nerve. I ('98) had interpreted the same evidence 



