Lamina cribrosa is a modification of :
(A) Sclera (B) Choroid (C) Optic nerve sheath (D) Retina
(A) The sclera (skleros=hard) is opaque and forms the posterior five-sixths of the eyeball. It is composed of dense fibrous tissue which is firm and maintains the shape of the eyeball. It is thickest behind, near the entrance of the optic nerve, and thinnest about 6 mm behind the sclerocorneal junction where the recti muscles are inserted. However, it is weakest at the entrance of the optic nerve. Here the sclera shows numerous perforations for passage of fibres of the optic nerve. Because of its sieve-like appearance this region is called the lamina cribrosa (crib=sieve).