that fills the anterior chamber. in the iris contract; so that the pupil size increases and more light enters the eye. Smooth muscle present in the ciliary body is called the ciliary muscle which alters the convexity of the lens for near and far vision. The ability of the eyes to focus objects at varying distances is called accommodation which is achieved by Figure .
L.S. of the eye 6XVSHQVRU\ OLJDPHQW RIOHQV &LOLDU\ERG\ DQGFLOLDU\ PXVFOH &LOLDU\SURFHVV 0HGLDOUHFWXV PXVFOH 9LWUHRXVERG\ 2SWLFQHUYH &HQWUDOUHWLQDO DUWHU\DQGYHLQ )RYHD 6FOHUD &KRURLG 5HWLQD /DWHUDOUHFWXV PXVFOH $QWHULRU FKDPEHU 3XSLO &RUQHD ,ULV 3RVWHULRU FKDPEHU /HQV Figure . Rod and Cone cells Retina forms the inner most layer of the eye and it contains two regions: A sheet of pigmented epithelium (non visual part) and neural visual regions. The neural retina layer contains three types of cells: photoreceptor cells – cones and rods (Figure .
and Table . ), bipolar cells and ganglion cells. The yellow flat spot at the centre of the posterior region of the retina is called macula lutea which is responsible for sharp detailed vision. A small depression present in the centre of the yellow spot is called fovea centralis which contains only cones.
The optic nerves and the retinal blood vessels enter the eye slightly Table: . Differences between rod and cone cells Rod cells Cone cells Rods are responsible for vision in dim light The cones are responsible for colour vision and works best in the bright light. The pigment present in the rods is rhodopsin, formed of a protein scotopsin and retinal (an aldehyde of vitamin A ) The pigment present in the cones is photopsin, formed of opsin protein and retinal. There are about millions rod cells There may be - millions cone cells Rods are predominant in the extra fovea region Cones are concentrated in the fovea region XI Std Biology-Zoology Chapter- thin lens; so the image of closest object is converged behind the retina.
This defect can be overcome by using convex lens that converge the entering light rays on the retina. Presbyopia: Due to