The organs of sight are a pair of eyes that are situated in deep protective bony cavities, called the orbits or eye sockets of the skull. Eyes help to provide a three dimensional, moving image, normally coloured in daylight.
Eyes consist of tissues present in three layers:
1. Outer Fibrous Layer
- The sclera, on opaque outermost covering, maintains the shape of the eyeball.
- The cornea is a thin transparent, front part of the sclera which lacks blood vessels but is rich in nerve endings.
- The cornea allows the light to pass into the eye.
- Conjunctiva protects are the cornea and also secretes oils and mucus that moisten and lubricate the eye.
2. Middle Vascular Layer
- The choroid is present beneath the sclera and contains numerous blood vessels and nourishes the retina.
- It becomes thick in the anterior part of the ciliary body which holds the lens in position.
- Iris forms a pigmented circle attached to the ciliary body in front of the lens.
- The movement of the iris controls the size of the pupil.
3. Inner Nervous Layer
- The inner layer is the retina and it contains different layers of cells, from inside to outside such as layers of ganglion cells, bipolar cells, photoreceptor cells, and pigment cells.
- The photoreceptors or visual cells are of two types: rod cells and cone cells.
- Rod cells contain a photosensitive pigment called rhodopsin.
- In the retina of human eyes, cone cells are of three types that possess characteristic photopigments (porpyrosin, iodopsin, and cyanopsin) that respond to red, green and blue lights.
- The daylight (photopic) vision and colour vision and functions of cones and the twilight (scotopic) vision is the function of the rods.
The space between the cornea and the lens is called the aqueous chamber which contains a thin watery fluid called aqueous humour. It helps to maintain the shape of the front part of the eye and provides nutrients to the lens and cornea. The space between the lens and retina is called the vitreous chamber which is filled with a transparent gel called the vitreous humour.
The optic nerve contains the fibres of the sensory neurons and leaves the eyeball from the backside. The point of departure of the optic nerve through the retina does not have any rods or cones and thus produces a blind spot or optic disc.
At the posterior pole of the eye lateral to the blind spot, there is a yellowish pigmented spot called macula lutea with a central pit called the fovea centralis.
The fovea centralis is a thinned-out portion of the retina where only the cones are densely packed. It is the point where the visual acuity (resolution) is the greatest.
Mechanism of Vision
- The rays of light coming from an object fall on the retina after passing through the cornea, aqueous humour, pupil, lens and vitreous humour.
- The stimulus perceived by rods and cones results in the dissociation of photosensitive pigment (e.g., rhodopsin) into opsin (a protein) and retinal (an aldehyde of vitamin A).
- This dissociation results in changes in the structure of the opsin.
- The stimulated photoreceptors initiate the nerve impulses, which are transmitted by the optic nerves to the visual cortex area of the brain, where the neural impulses are analyzed and the image formed on the retina is recognized.