558 



EXPANSION OF THE OPTIC NEPVE [sect. 226. 



p. 236) to the eye, this nerve has the same structure as an ordinary- 

 nerve, and its varicose dark-bordered fibres, 0*0005"' to 0*002'" thick, 

 form a polygonal bundle, 0-048'" to 0-064'" in diameter, surrounded 

 by an ordinary neurilemma. Arrived at the eye, the optic nerve 

 loses its sheath in the sclerotic coat, which possesses an opening 

 for the passage of the nerve, in the shape of a funnel with the 

 base outwards ; the inner neurilemma likewise terminates on the 

 inner surface of the sclerotic, and also on the choroid, where it is 

 connected with the above-mentioned lamina cribrosa ; so that the 

 tubules of the optic nerve, in their further course in the interior 

 of the eye, pass on independently without their envelopes of con- 

 nective tissue. Within the canal of the sclerotic, and up to the 

 slight elevation, or colliculus, which it forms upon the inner surface 

 of the retina in its passage forwards, the optic nerve is still white 

 and furnished with dark-bordered tubules ; but, from thence 

 onwards, its elements, in man and in many animals, become quite 

 Fig. 229. clear, yellowish or greyish, and transparent, like 



the finest tubes in the central organs ; they 

 measure, on an average, no more than o - ooo6'" to 

 o - ooo8'", while not a few only amount to 0-0002" 

 to 0*0004'" ; a few fibres, however, will be found 

 which measure as much as o*ooi'" to 0*0015" or 

 even 0*002"'. These tubules are distinguished 

 from other pale terminations of nerves by the 

 absence of nuclei in their course, by a some- 

 what stronger refracting power, and by the con- 

 stant occurrence, in the dead body, of varicosities ; 

 the two latter characters lead us to regard the 

 contents of the tubes as not being nervous me- 

 dulla of the ordinary kind, but rather as of a 

 partially semi-fluid nature, and perhaps somewhat 

 fatty ; so that we should class the nerve-fibres of 

 the retina with the most delicate elements of the 

 brain. I have not yet been able to demonstrate 

 axial-fibres among the fibres of the retina ; but, 

 on the other hand, I believe I have distinguished 

 an envelope on the larger varicosities, where they 

 are raptured, as frecp:iently happens. In any case, 

 the retinal fibres do not consist exclusively, indeed 

 not to a preponderating degree, of ordinary white substance of 

 Schwann, for even when they are treated very thoroughly with 

 ether, they remain behind, narrower indeed, but more distinct and 



Elements of the 

 expansion of the hu- 

 man optic nerve, 

 magnified 350 times. 

 a. evident coarser 

 nerve-tubes.with va- 

 ricosities ; b. a finer 

 one ; c. undulating 

 pale fibres without 

 varicosities, proba- 

 bly belonging to the 

 filaments of Mutter. 





