The nerve fibers of the retina extend in bundles to the papilla of the optic nerve (D3) where they unite to form the optic nerve before exiting the eyeball. Sclera and choroidea are very thin at the site of pene-tration, and the sclera is perforated (laminacribrosa) (D4). Once the extremely delicatenerve fibers have passed the sclera, they be-come enveloped by myelin sheaths. The optic nerve is actually a fiber tract of the CNS and contains astrocytes and oligoden-drocytes; hence, its nerve fibers do not have Schwann cell sheaths. As part of the brain, the optic nerve is surrounded by mengines. The dural sheath (D5) and the arachnoidsheath (D6) merge with the sclera (D7). Be-tween arachnoid sheath and pial sheath (D8) lies a CSF-filled space (D9) which makes a shift between nerve and sheath possible. A number of septae from the pia mater extend between the nerve bundles.
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