THE THIRD, FOURTH, AND SIXTH CRANIAL NERVES. 219 



millimetres from the commencement of the nerve. I have 

 attempted to trace the fibres beyond this point, but always 

 unsuccessfully. 



In addition to the meningeal branch, two other exceedingly 

 fine twigs (fig. 4, h, c) join the main trunk at about forty milli- 

 metres from its superficial origin. They accompany the main 

 trunk for twelve or fourteen millimetres, lying in the layers of 

 the nerve-sheath, and then separate, without at any time 

 blending with the proper fibres of the 4th nerve, or losing 

 their own identity.^ 



The fibres making up the 4th nerve are mostly large 

 medullated, 12^ to 19/x in diameter, together with a fewer 

 number of small medullated fibres, 4/^ in diameter, the former 

 being about three times as numerous as the latter. A section 

 of the 4th nerve resembles so closely the section represented 

 in fig. 3, c, as regards the appearance of its fibres, that it has 

 been considered unnecessary to make any special sketch of this 

 nerve in transverse section. No non-medullated fibres are 

 found in the main trunk. The meningeal branch consists 

 chiefly of exceedingly small medullated fibres, 3/x to 4/>t in 

 diameter, together with a smaller number of large medullated 

 fibres, 7/x to 8^^ in diameter. This nerve resembles the short 

 root of the ciliary ganglion very closely as regards the aspect of 

 its nerve-fibres. The branches h and c, fig. 4, are similarly con- 

 stituted to the meningeal branch. 



A vestigial structure has been described in the 4th nerve 

 at its origin.- I have met with some well defined granular or 

 slightly fibrillar material, similar to that seen in the 3rd nerve, 

 in all of the 4th nerves I have examined, at their superficial 

 origin, but not in any other part of their extra-cranial course. 



The Sixth Cranial Nerve. 



The 6th cranial nerve, like the two preceding nerves, con- 

 sists at its origin of numerous rootlets (fig. 5, sections 5 and 10). 



1 In text-books on anatomy the fourth nerve is described as conimnnicating 

 with the cavernous plexus in the cavernous sinus, and as giving oft" in tlie 

 spheroidal fissure a branch which blends with the lachrymal nerve. Perhaps the 

 nerves above described represent these. From tlie extreme smallness of these 

 twigs it is impossible te dissect them to their destination during removal. 



'■^ Loc. cit. 



