MOEPHOLOGY OF EYE MUSCLE NERVES 109 



persistence of that portion of the second myotome in connection 

 with the myotome of the third somite. Dohrn has affirmed its 

 persistence correctly. There is no doubt whatever that it per- 

 sists in the superior obhque muscle of Squalus. Its relations 

 are diagrammatically shown in figure 81. Thus it comes about 

 that the trochlear and abducens nerves innervate portions of 

 the same somite. In this way a problem in nerve relations is 

 presented which will be fully discussed in connection with the 

 problem of the relations of the abducens nerve. Suffice it to 

 say here that this relationship of two somatic motor nerves to 

 a single somite does not in any way affect adversely our views 

 of their real morphology. As a matter of fact, emphasized by 

 Bardeen ('04), all typical somatic motor nerves have a bimeric 

 distribution to two adjacent myotomes. The most perplexing 

 problem raised in this connection is that there is reason to think 

 that the trochlear and abducens nerves do not belong to suc- 

 cessive metameres. This question will be taken up later. But, 

 even more difficult for one who attempts to demonstrate the 

 somatic motor character of the trochlear, is the problem of its 

 dorsal chiasma. In this feature the trochlear is the most pecul- 

 iar nerve in the vertebrate body. 



c. How may the dorsal chiasma of the trochlear be best explained? 

 Van Wijhe ('86), in order to explain the dorsal emergence of the 

 fibers of the trochlearis, assumed that, as a result of the exten- 

 sion of the anterior column to the olivary body and to the loop 

 extending behind the corpus quadrigeminum, the root of the 

 trochlear was drawn over the loop into its present position. 



According to His ('88) the peculiar relations of the trochlear 

 may possibly be explained as the result of the flexure of the 

 neural tube in the region of the isthmus, a condition which he 

 thinks is favorable to the sagittal growth of the neuraxon proc- 

 esses of the neuroblasts at the base of the cerebellum. 



Rabl ('89), however, on the basis of observations on the sela- 

 chii, birds and mammals, concluded that the roots of origin, 

 both of the oculomotor and of the trochlear were primitively 

 dorsal, but that gradually, through the enlargement of the pedun- 

 cular paths, the root of the oculomotor was shifted to the ven- 



