Question

With the help of a neat and labelled diagram describe the anatomy of human eye. Explain the mechanism of vision.

Answer 1

Eyes are the paired sense organs located in the sockets of the skull which are called orbits. Eyes are supplied with the second cranial nerve called optic nerve.

The wall of eyeball consists of following three layers:

1. External layer: It is composed of a dense connective tissue called the sclera which maintains the shape of the eye and also protects it. The anterior portion of this layer is called cornea. Cornea absorbs oxygen from air and helps in focussing light waves as they enter the eye.

2. Middle layer: The middle layer is choroid which contains many blood vessels and it appears bluish in color. The choroid layer is thin over the posterior two-third of the eyeball and it becomes thick in the anterior portion to form the ciliary body. The ciliary body continues forward and forms a pigmented and opaque structure called the iris.

Eyeball possess a transparent crystalline lens held in place by the ligaments attached to the ciliary body. The aperture surrounded by iris, in front of the lens is called pupil.

3. Inner layer: This layer contains retina. The retina contains three layers of cells- ganglion cells, bipolar cells and photoreceptor cells. Photoreceptor cells are of two types- rods and cones that contain the light-sensitive proteins called the photopigments.

The optic nerve leaves the eye and retinal blood vessels enter it at a point medial to and slightly above the posterior pole of eyeball called blind spot because here, photoreceptors cells are absent.

A yellow pigmented spot called macula lutea having a central pit called fovea is present at the posterior pole of the eye and lateral to the blind spot. Visual resolution is maximum at the fovea.

The space between the cornea and the lens is called aqueous chamber which contains a thin watery fluid. The space between the lens and retina is called vitreous chamber filled with a transparent gel called vitreous humour.

Mechanism of vision:

Retina receives light rays through the cornea and lens generate impulses in the rods and cones. The photosensitive compounds in human eye consist of opsin (a protein) and retinal (an aldehyde of vitamin A).

The light received induces dissociation of the retina from opsin and this results in a change in the structure of the opsin. Due to this, the permeability of membrane changes which results in the potential differences in the photoreceptor cells and this produces a signal which generates an action potential in the ganglion cells through the bipolar cells. These impulses are transmitted by the optic nerves to the visual cortex area of the brain where neural impulses are analyzed and the image formed on the retina is recognized.