Longest pa of optic nerve ?
. Ans. is 'c' i.e., Intraorbital Optic nerve Each optic nerve (second cranial nerve) stas from the optic disc and extends up to optic chiasma, where the two nerves meet. It is the backward continuation of the nerve fibre layer of the retina, which consists of the second order neurons i.e., axon originating from the ganglion cells. It also contains the afferent fibres of the papillary light reflex. Morphologically and embryologically, the optic nerve is comparable to a sensory tract. Unlike peripheral nerves it is not covered by neurilemma (so it does not regenerate when cut). The fibres of optic nerve, numbering about a million, are very fine (2-10 m in diameter as compared to 20 mm of sensory nerves). Optic nerve is about 47-50 mm in length and can be divided into 4 pas : ? Intraocular pa (1 mm) : - It begins at optic disc (optic nerve head) and exits the nerve through a hole in sclera that is occupied by a mesh like structure called the lamina cribrosa. The nerve fibres from the retina leave the eye through pores (holes) in lamina cribrosa, a sieve-like structure made up of collagen meshwork. Intraorbital pa (30 mm) : - Extends from back of the eyeball (at lamina cribrosa) to the optic foramina. Intracanalicular pa (6-9 mm) : - It lies within the optic canal and closely related to ophthalmic aery which crosses obliquely over it. Intracranial pa (10 mm) : - It lies above the cavernous sinus and converges with its fellow from contralateral side to form optic chiasma. Like other pas of CNS, the optic nerve is covered by meningeal sheaths (Pia, archnoid and dura mater) as soon as the nerve leaves the eyeball.
Intraocular
Intracranial
Intraorbital
Intracranial