The External Structure of the Eye
This includes the
Eyelids
The Eyelids or thin folds of skin that coverand protect the cornea and conjunctiva from chemical or physical activity. Two activities happen to protect the eye. Squinting which is closing the eye partially, sheilds the eye from excessive light, that can damage the internal structure such as the retina. Blinking on the other hand, which is closing and opening the eye rapidly, spreads tears across and and removes irritants from the cornea and conjuctiva.
Eyelashes
The Eyelashes are short, stiff hairs that shield the eye from irritants such as dust.
Sclera
This is known as the "white of the eye." It is a tough and opaque outer layer. The Sclera protects the inner structure of the eye
Conjunctiva
Also known as mucus membrane. This is a transparent membrane that covers the sclera and lines the inner surface of the eyelids. It lso secretes mucous and a small volume of tears for the eye to be moist. This outermost layer covers the Retina (see The Inner Eye)
Cornea
This is the see-through skin that covers the eye. It is clear like glass (transparent) and has no blood vessels in it. This five-layered membrane is continuous with the Sclera.
It's function is to refract light rays that reach the eye, providing most of the refractive or focusing power. Unlike that of the Lens, the focusing power is fixed.
It's function is to refract light rays that reach the eye, providing most of the refractive or focusing power. Unlike that of the Lens, the focusing power is fixed.
Tear Gland
This secretes tears (tears contain salts and a bacterial enzyme) to
- Wash out foreign particles or chemical irritants
- Lubricate the conjunctiva which reduces friction between the the eyelids and eyeball
- Prevent the eye from drying out. The tear film allows atmospheric oxygen to dissolve and diffuse into the cornea.
Iris
This is a pigmented layer of muscular tissue which gives the eye its colour. That is why people have different eye colours such as brown, green, blue etc. It all depends on the Melanin Pigment. If a person has less melanin, it means the eye colour appears light. If a person has more Melanin, this means the eye colour appears darker. If you want to find more click here.
The Iris has two sets of involuntary muscles: the Circular and the Radial Muscles which control the diameter of the Pupil and hence, the amount of light that enters the eye. If the size of the pupil is small, then it means that it is restricting the amount of light entering the inner eye. If the pupil is large, then it means that it is allowing light to enter the inner eye.
Pupil
The Pupil is an opening at the centre of the Iris. It permits light to enter the eye.