Eye
A pair of eyes are located in sockets of the skull known as orbits. The human eyeball is nearly a spherical structure. The wall is formed by three layers. The outer layer of dense connective tissue forms the sclera. The transparent anterior portion of this layer is termed as a cornea. The middle layer is called choroid. Which is bluish in colour and contains many blood vessels. The posterior two third region of this layer is thin, while the anterior part is thick and forms the ciliary body. Forward segment of the ciliary body is pigmented and opaque known as iris. This is the visible coloured portion of the eye. A transparent crystalline lens is present anteriorly which is held in position by the ligaments of ciliary body. The aperture surrounded by the iris in front of the lens is known as pupil. The movement of the pupil is regulated by the muscle fibres of iris.
The retina is the innermost layer of the eye. It consists of three sub-layers ganglion cells, bipolar cells and photo-receptor cells, sensitive to light. The photo-receptor cells are of two types called rods and cones. They contains light sensitive proteins termed as photo-pigments. The cones are responsible for daylight (photopic) vision and colour vision. While the rods function in dim light (Scotopic) vision. The purple red protein called rhodopsin is present in the rods which is vitamin A derivative. The cones are of three types, which contain their own characteristic photo-pigments that respond to red, green and blue lights. Various combinations of these cones and their photo- pigments produce sensation of different colours. The sensation of white light is produced due to the simultaneous equal stimulation of these three types of cones.
The optic nerve leaves the eye at a point slightly away from the median posterior pole of the eyeball. The rods and cones are absent in this region and hence it is known as blind spot. A yellowish pigmented spot called macula lutea is present lateral to the blind spot with a central pit termed as fovea. It is a thinned out portion of the retina where only the cones are densely packed and therefore have greatest visual acuity (resolution). The aqueous chamber is a space between the cornea and the lens. It contains a thin watery fluid known as aqueous humor. The larger space between the lens and the retina is termed as vitreous chamber. It is filled with the transparent gelatinous fluid known as vitreous humor. The aqueous humor and the vitreous humor maintain the shape of the eyeball.
Mechanism of vision
The light rays (visible wavelengths) from the objects pass through the cornea and the lend. The rods and the cones contain photo-pigments, which are conjugated proteins. They are composed of opsin (a Protein) and retinol (Vitamin A derivative) The light induces dissociation of retinol from the opsin, resulting in changes in the structure of the opsin. This causes the change in the permeability of the retinal cells. It generates action potential which is carried via bipolar neurons and ganglion cells and further conducted by the optic nerves to the visual cortex (Vision centre) of the brain. The neural impulses are analyzed and the image formed on the retina is recognized. Based on earlier memory and experience, the neutral impulse are analysed and image formed on retina is recognised..