Describe the anatomy of the human eye.
A pair of eyes is 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 sclera. The transparent anterior portion of this layer is termed as the cornea. A thin transparent membrane called conjunctiva is present in front of the 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 suspensory ligaments. The aperture surrounded by the iris in front of the lens is known as the 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 photoreceptor cells sensitive to light. The photoreceptor cells contain light-sensitive proteins termed as photopigments. Photoreceptor cells are of two types, rods and cones. The cones are responsible for daylight vision and colour vision while rods function in dim light. The purple-red protein called rhodopsin is present in the rods which is a vitamin A derivative. The cones are of three types that respond to red, green and blue light. Various combinations of these cones and their photopigments produce the sensation of different color. The sensation of white light is produced due to simultaneous equal stimulation of all 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 these regions and hence it is known as the blind spot. A yellowish pigmented spot called macula lutea is present lateral to the blind spot with a central pit termed as the fovea. It is thinned out the portion of the retina where only the cones are densely packed and therefore have the greater visual acuity. The aqueous chamber is a space between the cornea and the lens. It contains a thin watery fluid known as aqueous humour. The larger space between the lens and the retina is termed as the viterous chamber. It is filled with the transparent gelatinous fluid known as vitreous humour. The aqueous humour and vitreous humor maintains the shape of the eyeball