● `color{violet}("Our paired eyes")` are located in sockets of the `color{violet}("skull")` called `color{brown}("orbits.")`

● The `color{violet}("adult human eye ball")` is nearly a `color{brown}("spherical")` structure.

● The `color{violet}("wall of the eye ball")` is composed of three layers.

● The `color{violet}("external layer")` is composed of a `color{violet}("dense connective tissue")` and is called the `color{brown}("sclera. ")`

● The `color{violet}("anterior portion")` of this layer is called the `color{brown}("cornea. ")`

● The middle layer, `color{brown}("choroid,")` contains many `color{violet}("blood vessels")` and looks `color{violet}("bluish in colour. ")`

● The `color{violet}("choroid layer")` is thin over the posterior two-thirds of the `color{violet}("eye ball,")` but it becomes thick in the anterior part to form the `color{brown}("ciliary body.")`

● The `color{violet}("ciliary body")` itself continues forward to form a pigmented and opaque structure called `color{violet}("the iris")` which is the visible coloured portion of the `color{violet}("eye.")`

● The `color{violet}("eye ball")` contains a transparent `color{brown}("crystalline lens")` which is held in place by ligaments attached to the `color{violet}("ciliary body.")`

● In front of the `color{violet}("lens")`, the aperture surrounded by the iris is called the `color{brown}("pupil.")`

● The diameter of the `color{violet}("pupil")` is regulated by the `color{violet}("muscle fibres")` of iris.

● The inner layer is the `color{brown}("retina")` and it contains three layers of cells – from inside to outside – `color{violet}("ganglion cells,")`

● There are two types of `color{violet}("photoreceptor cells")`, namely, `color{brown}("rods")` and `color{brown}("cones.")`

● These `color{violet}("cells")` contain the `color{violet}("light-sensitive proteins")` called the`color{brown}(" photopigments.")`

● The daylight (`color{brown}("photopic")`) vision and colour vision are functions of cones and the twilight (`color{brown}("scotopic")`) vision is the `color{violet}("function of the rods.")`

● The `color{violet}("rods")` contain a `color{violet}("purplish-red protein")` called the `color{brown}("rhodopsin or visual purple,")` which contains a derivative of `color{brown}("Vitamin A.")`

● In the `color{violet}("human eye,")` there are three types of cones which possess their own characteristic `color{violet}("photopigments")` that respond to red, green and blue lights.

● The sensations of different colours are produced by various combinations of these `color{violet}("cones")` and their `color{violet}("photopigments.")`

● When these `color{violet}("cones")` are stimulated equally, a `color{violet}("sensation of white light")` is produced.

● The `color{brown}("optic nerves")` leave the `color{violet}("eye")` and the `color{violet}("retinal blood vessels")` enter it at a point medial to and slightly above the `color{violet}("posterior pole")` of the `color{violet}("eye ball.")`

● `color{violet}("Photoreceptor cells")` are not present in that region and hence it is called the `color{brown}("blind spot.")`

● At the `color{violet}("posterior pole")` of the `color{violet}("eye lateral")` to the `color{violet}("blind spot,")` there is a yellowish pigmented spot called `color{violet}("macula lutea")` with a central pit called the `color{brown}("fovea.")`

● The `color{violet}("fovea")` is a thinned-out portion of the `color{violet}("retina")` where only the `color{violet}("cones")` are densely packed.

● It is the point where the `color{brown}("visual acuit (resolution)")` is the greatest.

● The space between the `color{violet}("cornea")` and the `color{violet}("lens")` is called the `color{brown}("aqueous chamber")` and contains a thin watery fluid called `color{violet}("aqueous humor.")`

● The space between the `color{violet}("lens")` and the `color{violet}("retina")` is called the `color{brown}("vitreous chamber")` and is filled with a transparent gel called `color{violet}("vitreous humor.")`

● The light rays in `color{brown}("visible wavelength")` focussed on the `color{violet}("retina")` through the cornea and lens generate `color{violet}("potentials (impulses)")` in rods and cones.

● As mentioned earlier, the `color{brown}("photosensitive compounds (photopigments)")` in the human eyes is composed of `color{brown}("opsin")` (a protein) and `color{brown}("retinal")` (an aldehyde of vitamin A).

● `color{violet}("Light induces")` dissociation of the `color{brown}("retinal from opsin")` resulting in changes in the structure of the `color{violet}("opsin.")`

● This `color{violet}("causes membrane")` permeability changes.

● As a result, `color{violet}("potential differences")` are generated in the `color{violet}("photoreceptor cells.")`

● This produces a `color{violet}("signal")` that generates action `color{violet}("potentials")` in the `color{brown}("ganglion")` cells through the `color{violet}("bipolar cells.")`

● These `color{violet}("action potentials")` (impulses) are transmitted by the `color{violet}("optic nerves")` to the `color{brown}("visual cortex")` area of the `color{violet}("brain,")` where the `color{violet}("neural impulses")` are analysed and the image formed on the `color{violet}("retina")` is recognised based on earlier memory and experience.