THE SEGMENTAL VALUE OF THE CRANIAL NERVES, 155 



3. The course of the main stem of the nerve is (fag. 17, IV) at right 

 angles to the axis of the head at the point of origin of the nerve. 



4. Morphologists are very far from agreeing as to the existence of a 

 visceral cleft in front of the mouth, so that it would be premature to 

 discuss the relations of the third nerve to this " lachrymal cleft," for 

 whose existence there is, however, much to be said. Concerning the 

 head cavities, however, the evidence yielded by the third nerve is of a 

 perfectly definite and convincing character. The nerve in Elasmo- 

 branchs passes downwards and backwards from its root of origin to 

 the interval between the dorsal ends of the first and second head- 

 cavities, where it expands into a ganglionic swelling — the ciliary 

 ganglion. Beyond this point the main trunk of the nerve passes 

 down between the two cavities, the relations of the third nerve to the 

 first and second cavities being precisely the same as those of the fifth nerve 

 to the second and third cavities. 1 



5. As just noticed, there is a very evident ganglionic swelling at 

 the point of division of the third nerve into its two main branches. 



These considerations are, I think, when taken in conjunction with 

 its previously established constancy throughout the vertebrate series, 

 sufficient to establish the proposition that the third nerve is of seg- 

 mental nature. The further question, whether the third represents 

 aD entire segmental nerve, or only a portion of one, will be best 

 answered by considering the fourth nerve. 



IY. The Fourth, or Trochlear Nerve. — Having established the 

 constancy of this nerve, we have now to consider its morphological 

 import. Concerning its development we know very little, but that 

 little is of importance. In the dog-fish it has been shown 2 that the 

 fourth nerve, at the earliest period at which it has been recognised, 

 arises from the brain at the same spot as in the adult, i.e. the dorsal 

 surface of the hinder end of the mid-brain ; further, that its course is 

 from the first that of a segmental nerve. 



Now, if the visceral clefts and arches, and the head-cavities give us, 

 as they most certainly do, reliable clues as to the segmentation of the 

 head, then it is seen at once that there is no room for a segmented nerve 



1 Marshall, "Head Cavities and Associated Nerves of Elasmobranchs," Quart. Journ. of 

 Micros. Science, Jan. 1881, pp. 78 seq. 



a Marshall and Spencer, "Cranial Nerves of Scy Ilium," Quart. Journ. of Micros. Science, 

 July 1881, pp. 672—674. 



