RETINA ::
It is a very delicate transparent and the inner most nervous coat of the eye In the transverse axis of centre of pupil there is very sensitive part of retina called macula or yellow spot it contains the photoreceptors called cones Rest of the retina contains photoreceptors called rods Rods give a purpulish tint due to the presence of a coloring matter known as rhodopsin In the centre of retina there is a depression known as Fovea centralis This part is full of cones and surrounding this is macula or yellow spot The cones contain the pigment iodopsin The point in retina from where the optic nerve leaves the eye ball is called blind spot This spot is about 3.5 mm medial to macula
Histologically retina is composed of nerve cells rods and cones arranged in various layers outer most layer is pigment layer containing the pigment melanin This is followed by layer of rods and cones Inner to rods and cones there is a layer of nuclei of rods and cones and a layer of bipolar neurons They make synapses with horizontal cells and nuclei of rods and cones Lastly there is an internal layer of cells (ganglion cells ) the axons of which form optic nerve fibres
Rods and cones are the photoreceptor cells of retina Each retina contains approximately 6 million cones and 120 millions rods 
The rods are meant for twilight and night vision i.e.dim vision They are ansent in the fovea but are found scattered in the remaining part of the retina and are concentrated on the periphery Cones concentrate in the centre of the retina The spot is called fovea centralis through which one has critical vision They visualise the objects in their natural colour and concentrate in the centre
Rods contain the light sensitive pigment called rhodopsin whereas cones contain the photochemical called iodopsin Rhodopsin is made up of protein scotopsin and the carotenoid pigment retinal (derived from retinal or vit A) Iodopsin is made up of the protein photopsin Structurally rods are elongated cylindrical structure about 40-60 u long and 2u in diameter whereas cones are pyramid shaped 28-84u long and 2.5u in diameter Each cone is connected with its own nerve cell whereas 10-14 rods are connected to one nerve cell
Cones are also concerned with color vision There are three different types of photochemicals present in different cones thus making these cones selectively sensitive to colors of blue green and red The color vision is thus fusion of red blue and green Cones are for the critical vision and rods view objects in dim vision
The field of vision is variable in person to person and not equal on all sides It can be measured by an instrument called perimeter The process of measuring the field of vision is called perimetry Retina can be viewed by an instrument called ophthalmoscope Fundus can be seen by fundoscopy which aids in the diagnosis of various conditions
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