Functions of the eye parts

Functions of the eye parts
Aqueous Humour: Between the iris and the cornea is a space filled with a watery fluid called the
aqueous humour. This has a refractive index of 1.337. The aqueous humour baths both the cornea and
the lens. It is constantly replenished by the ciliary body. It drains from the eye through the canal of
schlem.
Choroid: The choroid lies between the sclera and the retina it provides the blood supply to the eye. The blood
supply gives nutrition to the various parts of the eye, just like any other portion of the body.
Ciliary body: Just in front of, and continuous with, the choroid is the ciliary body. This is a muscular
tissue in the form of a ring, and is roughly triangular in cross−section. The ciliary body has ligaments
attached to it. The ligaments are attached to the crystalline lens. The ciliary body controls the tension
on these ligaments which in turn alters the power of the lens. The ciliary body also produces aqueous
humour.
Cornea: The front portion of the sclera has a section that is transparent. This transparent window known as
the cornea is attached to the sclera and is the major refractive power of the eye. In other words it is the part of
the eye that provides most of its focussing power.
Fovea: The center of the macula; gives the sharpest vision. When we fixate or look directly at an object it is
imaged on the fovea.
Iris: The iris is in front of the ciliary body. The iris is a muscular and pigmented tissue forming a
circular curtain with a hole in the centre. The hole in the centre of the iris is called the pupil. The iris
controls the amount of light which enters the eye by changing the pupil's diameter.
Macula: The small sensitive area of the retina that gives central vision; contains the fovea.
Optic Nerve: The optic nerve connects the retina to the lateral geniculate nucleus, which is in the middle of
the brain. This is the first connection made by the visual system in the brain.
Retina: The retina is made up of transparent, sensory and nervous tissue carrying blood vessels, nerve
cells and nerve fibres. At the back of the retina, the nerve fibres all come together and emerge as the
optic nerve. The retina lies very close to the choroid (middle coat) but is attached to it only at the optic
nerve and the ora serrata (where the retina ends and the ciliary body begins).
The nerve endings on the inside of the wall of the retina terminate in light−sensitive cells of two types ñ
rods and cones. Rods are used for peripheral vision and night vision. Cones require bright light and
provide fine detail and colour vision. The point on the retina where the nerve fibres leave to form the
optic nerve is called the optic disc or blind spot.
Sclera: The outside covering of the eye is a protective envelope of leathery connective tissue known as the
sclera. This is the white coating on the outside of the eyeball, commonly known as the white of the eye. It
completely envelops the globe except at the front of the eye and maintains the shape of the globe. It also
provides a firm anchorage for the extra ocular muscles that control the eye's movement.
The crystalline lens has several functions:
• Transparency−−to provide a clear medium through which light rays from an object can reach the retina.
• Optical−−to focus a sharp image of an object on the retina.
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• Accommodation−−to vary its refractive power, thus providing clear images of objects over a wide range of
distances. The dynamic accommodation process maintains a sharp retinal image via a continuous feedback
mechanism. Loss of the accommodative function, presbyopia, is considered ``normal'' or ``natural'' in the
fifth decade.
• Anatomic−−to create a functional barrier between the anterior and posterior segments of the eye.
Vitreous Humour: A jelly−like transparent fluid fills the inner chamber of the eye. This fluid is called
the vitreous humour and it is contained in a thin membranous sac called the hyaloid membrane (not
shown). The fluid of the vitreous humour has a refractive index of 1.337.
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