Hyderabad Ophthalmologists' Association

Parts of the Eye

 

eye-crosssection 1. Vitreous humour

This fluid preserves the spherical shape of our eyeball, as well as helping to support the retina. Despite having the consistency of egg white it is mainly made of water (99%).

2. Ciliary body

The ciliary body comprises two parts – the ciliary process and the ciliary muscle. It is the latter which causes the lens to change shape. If the eye is focusing on a distant object the muscles relax, causing the ligaments to tighten and the lens to lengthen. When we focus on an object nearby the muscles tighten, the ligaments slacken, and the lens shortens.

The lining of the ciliary body also secretes aqueous humour, the fluid which fills the front of the eye.

3. Lens

The lens is responsible for refracting light. Held in place by ligaments, which connect to the ciliary body, the lens can also change shape to focus on objects at different distances – a reflex known as accommodation. It does this approximately 100,000 times a day.

4. Cornea

Although normally only half a millimetre thick, the cornea is responsible for seventy percent of the total focusing of the eye. It is the most important layer in the refractive procedure and, together with the lens, forms a clear image on the back of the retina.

5. Conjunctiva

The conjunctiva is merely a thin, transparent membrane covering the cornea, and yet its function is vital – it protects the eye from airborne debris. This is actually only one of the protective features of the human eye. Others include the orbit (or eye socket), the eyelashes and, quite surprisingly, the eyebrows – their function being to stop sweat from running into the eye.

Tears, which constantly bathe the surface of the eye, also remove dust and dirt – as well as killing bacteria.

6. Aqueous humour

The watery liquid at the front of the eye, secreted mainly by the ciliary body.

7. Iris

The iris is a thin diaphragm that lies behind - and is visible through - the cornea. The iris contains the pupil, which dilates and constricts to regulate the light that reaches the retina.

8. Choroid

The choroid runs behind the retina and, at the front of the eye, forms the ciliary body. As it contains many blood cells, the choroid supplies the eye with nutrients and oxygen – as well as removing waste.

The choroid also has a high concentration of a pigment called melanin in its cells. It is this pigment that prevents internal reflection within the eye – stopping us from receiving a blurred image.

9. Retina

The back of the retina contains a deep layer of light sensitive cells called rods and cones. This is where the image is projected. Rods are responsible for night vision, while cones are responsible for daytime vision. The cones also allow us to see in colour and detail.

The retina also contains a layer of ganglion cells on the surface. These cells connect the nerve fibres to the optic nerve. This means that the nerve fibres actually pass over the sensitive part of the eye, but, odd as it may seem, no disturbance in vision is caused.

10. Sclera

Basically, this is the white of the eye. Attached to the sclera are six exterior muscles, which enable us to look left, right, up and down. At the front of the eye, the sclera forms the cornea.

11. Optic nerve

The optic nerve is responsible for carrying information about the image to the brain. It contains no sensory receptors itself, and therefore therefore the head of the optic nerve (otherwise known as the Optic Disc) corresponds to the normal blind spot of the eye.

12. Fovea

This area of the retina is packed with cone cells. Because of this, it gives the most acute vision. For example, when we are reading, the word on which we are focusing is being projected onto the fovea.

 

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