Describe in detail the structure of the eye,its various parts,and their functions.

(N/A) Paired eyes are located in sockets of the skull called orbits.
Parts of an eye: Each eyeball is nearly spherical in structure.
Its diameter is approximately $2.5 \ cm$ and it weighs around $6$ to $8 \ g$.
The wall of the eyeball is composed of three layers:
$(a)$ Sclera $(b)$ Choroid $(c)$ Retina
$(a)$ Sclera: It is a fibrous,white layer. The posterior $5/6$ part is a white membrane formed of collagen fibers,whereas the anterior $1/6$ part is formed of fibrous tissue devoid of blood vessels and is called the cornea.
Conjunctiva: Conjunctiva is a thin,transparent membrane made up of stratified epithelium. It externally surrounds the transparent cornea and the exposed part of the white layer. It is the thinnest epithelium of the animal body.
$(b)$ Choroid: It is the middle layer,contains many blood vessels,and looks bluish in color.
The choroid layer is thin over the posterior two-thirds of the eyeball,but it becomes thick in the anterior part to form the ciliary body.
The ciliary body continues forward to form a pigmented and opaque structure called the iris. The iris is the visible colored portion of the eye.
The eyeball contains a transparent crystalline lens which is held in place by ligaments attached to the ciliary body.
In front of the lens,the aperture surrounded by the iris is called the pupil. The diameter of the pupil is regulated by the muscle fibers of the iris.
$(c)$ Retina: It is the innermost layer and it contains three layers of neural cells from inside to outside: Ganglion cells,Bipolar cells,and Photoreceptor cells.
Photoreceptor cells are of two types: rod cells and cone cells.
These cells contain light-sensitive proteins called photopigments.
Daylight (photopic) vision and color vision are functions of cones,and twilight (scotopic) vision is the function of the rods.
The rod cells contain a purplish-red protein called rhodopsin,which contains a derivative of Vitamin $A$.
In the human eye,there are three types of cones which possess their own photopigments that respond to red,green,and blue lights.
The sensations of different colors are produced by various combinations of these cones and their photopigments.
When these cones are stimulated equally,a sensation of white light is produced.
The optic nerves leave the eye and the retinal blood vessels enter it at a point medial to and slightly above the posterior pole of the eyeball.
Photoreceptor cells are not present in that region,and hence it is called the blind spot.
At the posterior pole of the eye,lateral to the blind spot,there is a yellowish pigmented spot called the macula lutea with a central pit called the fovea.
The fovea is a thinned-out portion of the retina where only the cones are densely packed. It is the point where the visual resolution is the greatest.
The space between the cornea and the lens is called the aqueous chamber and contains a thin,watery fluid called aqueous humor.
The space between the lens and the retina is called the vitreous chamber and is filled with a transparent gel called vitreous humor.