Intraorbital length of the optic nerve -

Ans. is 'b' i.e., 30 mmo Optic nerve is about 47-50 mm in length and can be divided into 4 parts : -i)Intraocular part (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.ii)Intraorbital part (30 mm):#Extends from back of the eyeball (at lamina cribrosa) to the optic foramina.iii)Intracanalicular part (6-9 mm):#It lies within the optic canal and closely related to ophthalmic artery which crosses obliquely over it.iv)Intracranial part (10 mm):#It lies above the cavernous sinus and converges with its fellow from contralateral side to form optic chiasma.