MORPHOLOGY OF EYE MUSCLE NERVES 101 



neous nerves; and third by their common classification as so- 

 matic afferent nerves. 



Three objections, however, may be raised against this homol- 

 ogy; first, that while spinal somatic sensory nerves grow median 

 to the myotome, the ophthalmicus profundus, like other typical 

 cranial nerves, grows lateral to the somites; second, that cellular 

 elements enter the ganglion of the ophthalmicus profundus from 

 the skin, while the cellular elements of spinal sensory nerves are 

 exclusively^ derived from the neural crest; and third, that the 

 profundus is not an independent segmental nerve but a sensory 

 branch of another nerve. 



The first of these objections may be met by calling attention 

 to the fact that in some cases, for example in the post-otic region 

 of Ammocoetes, typical cranial nerves lie partly lateral and 

 partly median to the myotomes, indicating that no sharp line 

 of demarkation can be drawn on this basis between cranial and 

 spinal somatic sensory nerves. Such differences appear to be 

 correlated with differences in the relative size of the somite. 

 Furthermore, there are comparative anatomical grounds for 

 thinking that somatic sensory nerves were primarily inter-myo- 

 tomic in position. From such primitive relations the present 

 modified relations of cranial and spinal somatic sensory nerves 

 may readily have been derived. 



The second objection is more serious, but if the comparability 

 of the somatic sensory nerves of Amphioxus and Squalus be 

 granted, it will be seen that the cranial somatic sensory nerves 

 have retained the primitive relations of the former, while the 

 absence of direct contact with the skin in the case of spinal 

 somatic sensory nerve ganglia, may be regarded as a secondary 

 modification. In this respect, as in respect to the retention of 

 a mixed function, typical cranial nerves appear more conserva- 

 tive than spinal nerves. Therefore, while it must be admitted 

 that the cranial somatic sensory ganglia are more complex in 

 their derivation than are spinal ganglia and that the serial 

 homology of the two is incomplete, nevertheless their partial 

 homology appears demonstrable. 



