THE EYE AND ITS APPENDAGES. 



359 



FIG. 380. 



stitute the subdural and the subarachnoidean lymph-spaces of 

 the optic nerve, which communicate with the corresponding inter- 

 cranial cavities. 



On reaching the eye- 

 ball the tissue of the 

 dural sheath passes 

 uninterruptedly into the 

 outer two-thirds of the 

 sclera ; the greater part 

 of the pial sheath 

 blends with the inner 

 third of the sclera, some 

 of its fibres, however, 

 joining the choroid. 

 The arachnoidean 

 sheath unites with the 

 dural, in consequence 

 of which arrangement 

 the subdural and sub- 

 arachnoidal spaces be- 

 c o m e continuous at 

 their ocular extremities. 



The trunk of the 

 optic nerve, about 3 mm. in diameter, consists of a great number 

 (almost 800) of bundles of medullated nerve-fibres separated by 

 intervening fibrous partitions, 

 offshoots from the pial sheath. 

 Each bundle is composed 

 of small medullated fibres 

 (2 /i), which are without neu- 

 rilemma. 



On reaching a level corre- 

 sponding with the confluence 

 of the sheaths of the nerve 

 with the sclera, the optic fibres 

 pass through the sieve - like 

 lamina cribrosa and lose 

 their medullary coat, contin- 

 uing to their retinal distribution 

 as naked axis - cylinders. 

 Occasionally the medullated 

 fibres retain their medullary 



substance after their passage through the lamina cribrosa, such 

 conditions presenting very striking ophthalmoscopic appearances. 



Transverse section of human optic nerve : d, dural sheath : 

 a, arachnoidean sheath ; /, pial sheath; n, bundles of nerve- 

 fibres separated by fibrous septa (e). 



FIG. 381. 



Section of human optic nerve under higher magnU 

 fication : b, bundles of nerve-fibres enveloped in con- 

 nective-tissue sheaths (jc) ; n, neuroglia nuclei; x t 

 nuclei of interfascicular connective tissue (/); v, blood- 

 vessels. 



