Oculomotor Nerve in Scyllium canicula. 563 



course of development. This ventral dislocation may easily have heen 

 carried to a greater length in the case of a greatly reduced oculomotor 

 dorsal root and have resulted in a continence of both dorsal and ventral roots 

 into a single structure. 



The Structure of the Oculomotor unlilce that of Ventral Spinal Boots. — The 

 existence in the oculomotor of sensory fibres (related to muscle-spindles) 

 derived in part, if not entirely, from ganglion cells upon the root of the 

 nerve is absolutely incompatible with the interpretation of the oculomotor 

 as homologous with a ventral spinal nerve root. That some of the afferent 

 fibres may have (as Tozer implies) an intracerebral origin does not affect the 

 case. Sherrington ('97a, p. 210) states that, in mammals, none of the fibres 

 of dorsal (posterior) spinal roots have an intraspinal origin. For many 

 vertebrates, other than mammals, however, it appears to be established that 

 afferent fibres with an intraspinal origin do exist. Thus the possible 

 occurrence of afferent fibres of intracerebral origin in the oculomotor does 

 not lessen the resemblance of this nerve to a true segmental nerve. What 

 is of the utmost importance is the indisputable fact that afferent fibres 

 enter the brain by the root of the third nerve. Of the spinal or typical 

 segmental nerves Tooth concludes ('92, p. 783) : " The posterior roots are the 

 only points of entrance of sensory, or, more broadly, centripetal impressions." 



The explanation which appears to be most in agreement with the facts is 

 that the oculomotor is not correctly viewed as the equivalent of a ventral 

 root only. Rather we must accept it as the homologue of a complete 

 segmental nerve, containing elements of both dorsal and ventral roots, 

 although some of these components have apparently become obsolete, and 

 the distinction of the originally separate dorsal and ventral roots has 

 disappeared. 



Against this hypothesis, which, it should be noted, is not quite identical 

 with that put forward by Gaskell and earlier observers, three objections 

 may be raised. Equally with that earlier view it is opposed to the 

 generally accepted interpretation of the ramus ophthalmicus profundus as 

 the dorsal nerve root of the oculomotor neuromere. 



In the first place, then, it may be urged that, in ontogeny, the oculomotor 

 and profundus nerves are very intimately related, and that the connection 

 of the latter nerve and the neuromere of the oculomotor (v, according to 

 Johnston) is only lost relatively late in development, when the profundus 

 acquires a new connection with the brain (in neuromere vii) through the 

 mediation of the trigeminal root. Indeed, the ramus profundus retains 

 its relation to the third nerve, throughout life, by the ciliary nerve (radix 

 ciliaris longus) and ganglion. 



vol. lxxxviii. — B. 2 x 



