THE OPTIC NERVE. 



353 



sented chiefly by the perivascular lymph-spaces which surround the blood- 

 vessels. These spaces may be injected from the subpial lymph-space of the 

 optic nerve, and by the same method communications may be demonstrated 

 between (i) this space and the interstices between the nerve-bundles which 

 converge towards the optic papilla, (2) a space between the membrana 

 limitans interna and the hyaloid membrane of the vitreous, and (3) a narrow 

 cleft between the pigmented cells and the layer of rods and cones. 



The Optic Nerve. The optic nerve is surrounded by the three 

 sheaths the dural, the arachnoidal, and the pial which, with the subdural 

 and the subarachnoidal lymph-spaces, are continued over the nerve as pro- 

 longations of the corresponding brain-membranes. On reaching the eyeball, 

 the dural sheath bends directly outwards, its fibres commingling with those 

 of the outer third of .the sclera (Fig. 403); the arachnoid ends abruptly on 



Physiological excavation Lamina cribrosa 



Fibre layer 



Dural sheath ; ., Sulrarachnoidal 



space 

 Subdural space 



Pial sheath 



Ceiural retinal vessels within optic nerve 

 FIG. 403. Section of eyeball through entrance of optic nerve. X 20. 



the inner wall of the intervaginal space; whilst the pia arches outwards to 

 form part of the inner third of the sclera, sending longitudinal fibres as far 

 as the choroid. As the nerve-fibres enter the eyeball, for convenience 

 assuming that they are passing from the brain towards the retina, they trav- 

 erse a fenestrated membrane, the lamina cribrosa, which is formed by 

 interlacing bundles from the inner third of the sclera and from the pial 

 sheath. As the nerve-fibres penetrate the lamina cribrosa they lose their 

 medullary sheaths and, in consequence, the optic nerve is reduced one 

 third in diameter. The nerve projects slightly into the eyeball on account 

 of the thickness of the layer of arching nerve-fibres and forms, therefore, a 

 circular elevation, known as the optic papilla, about 1.5 mm. in diam- 

 eter, the centre of which is modelled by a funnel-shaped depression, the 

 so-called physiological excavation. The axis of the nerve is occupied by the 

 central artery of the retina, which gives off minute branches for the nutrition 

 of the nerve, that anastomose with the pial vessels, and, through the circulus 

 arteriosus Zinni, with branches of the posterior ciliary arteries. In trans- 

 verse sections (Fig. 404), the optic nerve appears as a mosaic of irregular 

 polygonal areas composed of bundles of medullated nerve-fibres surrounded 

 by connective tissue envelopes. Although provided with medullary sheaths, 

 the optic fibres are devoid of a neurilemma, in this respect agreeing 

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