138 PROFESSOR MARSHALL. 



applies to all vertebrates except Amphioxus, and must therefore be 

 considered as primitive so far as vertebrates are concerned, differs so 

 fundamentally from, the development of the hinder cranial or spinal 

 nerves that no comparison whatever is possible between them. The 

 optic nerve must therefore be regarded as one sui generis, and 

 which can accordingly have no claim to be considered of segmental 

 value. 



The existence of this clearly non-segmental nerve between the 

 olfactory and the hinder nerves is undoubtedly an objection to the 

 view advocated above concerning the segmental value of the olfactory 

 nerve ; but until we otain a clearer light than we are at present able 

 to throw on the phylogenetic history of the vertebrate eye, and indeed 

 of the vertebrate race altogether, it is difficult to gauge properly the 

 weight of the objection. 



The Eye-Muscle Nerves. — Concerning the morphological value of 

 these three nerves — the third, fourth, and sixth pairs — opinions have 

 perhaps differed more than in the case of any of the other cranial 

 nerves. 



The nerves in question are small, with a singularly limited and 

 constant distribution to the muscles moving the eyeball, and to certain 

 other parts in connection with the eye, the third nerve supplying the 

 rectus superior, rectus interims, rectus inferior, and obliquus inferior 

 muscles of the eye ball, also the levator jmlpebrce superioris and the 

 circular muscle of the iris ; the fourth nerve supplying the obliquus 

 superior muscle, and in many vertebrates sending sensory branches to 

 the conjunctiva and the skin of the upper eyelid ; and the sixth nerve 

 supplying the rectus externus muscle, and in many cases the suspensory 

 muscle of the bulb of the eye and the muscles of the nictitating 

 membrane. In dealing with these it will be convenient to consider 

 them at first collectively, inasmuch as many points of importance 

 concern them all alike, and afterwards to consider briefly the several 

 points of individual interest which they present respectively. 



Until very recently it was the almost universal custom amongst 

 anatomists, when discussing the segmental value of the cranial nerves, 

 to "exclude the eye-muscle nerves altogether from consideration, on the 

 ground that they were not constant in their distribution, but that one or 

 more of the muscles normally supplied by them might lender special 

 circumstances be supplied by branches of the fifth nerve, the further 



