Oculomotor Nerve in Scyllmm canicula. 



561 



(iii) that, in such forms it is probable that the afferent fibres are wholly, or 

 in part, derived from these ganglion ceils ; 



(iv) that, in development, ganglion cells from the neural crest enter into' 

 association with the anlage of the oculomotor, apparently establishing 

 a transient dorsal root ; 



(v) that, in development, certain ganglion cells migrate peripherally along 

 tbe fibres of this nerve to the (sympathetic) ciliary ganglion, such 

 cells having probably arisen primarily from the neural crest. 



Now in a typical segmental nerve (in the general acceptation of the term) 

 we find a dorsal root bearing a ganglion and a ventral root without ganglion 

 cells. These roots unite at, or just peripheral to, the ganglion. From the 

 ganglion, or from the common nerve distal to the ganglion, there arises a 

 branch which enters a ganglion of the autonomic nervous system. The cells 

 of this latter ganglion are derived predominantly, if not exclusively, in 

 embryonic life, from cells which migrate from the spinal ganglion situated 

 upon the dorsal root of the related segmental nerve. The nerve fibres of the 

 dorsal root are, in general, derived from the cells of its dorsal ganglion and 

 are described as afferent (somatic and visceral sensory) fibres, transmitting 

 centripetal impulses. The nerve fibres of the ventral root are of intraspinal 

 origin (arising from the ventral and lateral cornua of the spinal cord), and are 

 efferent (motor), transmitting only centrifugal impulses. 



In a comparison of the condition observed in the oculomotor with that 

 defined as typical for a segmental nerve we cannot but be struck with the 

 fact that the oculomotor appears to combine most of the features of dorsal 

 and ventral roots of the typical segmental nerve. That it may lack certain 

 nerve components, normally present in a typical segmental nerve, is not 

 denied, nor is a distinct dorsal root recognisable. On the other hand it 

 presents features altogether unknown in any typical segmental ventral 

 nerve root. 



The Absence of Certain Components from the Oculomotor. — The suggestion 

 that there has occurred in this nerve a cenogenetic atrophy of certain com- 

 ponents is neither new nor improbable. Indeed, Johnston has pointed out 

 ('07, pp. 151-153) that, in the nerves of that region of the head occupied by 

 the lateral eyes, the absence of the general cutaneous component of the 

 sensory system might reasonably be expected. Similar reasoning might 

 connect the absence of visceral sensory components with a possible dis- 

 appearance of visceral tissue in the region of the more anterior somites. 

 With the disappearance of these nerve elements from the nerves of the 

 segments occupied (or encroached upon) by the lateral eyes and the serially 



