Guide to the Human Eye
The human eye is not actually a sphere; rather, it is a two-piece fused unit. The smaller, more curved part is the cornea, which is attached to the larger sclera.
The cornea is what light first passes through as it enters the eye, and it helps to focus the light. The sclera - or the white part of the eye, is composed of tough, fibrous tissue that protects the inner part of the eye. The two are connected by a ring called the limbus.
Despite their presence at the front of the eye, the cornea and aqueous humour are not seen because of their transparency. Instead the iris, and its black centre, the pupil, are visible. The varied dilation of the pupil in light conditions allows more or less light to enter the eye.
Light must travel through the lens after it travels through the pupil. The lens focuses light, and can change its shape to focus on near and distant objects.
Between the lens and the retina, light passes through the centre of the eye. The centre of the eyes is filled with a clear, jelly-like substance called the vitreous.
The retina is a light-sensitive tissue on the back lining of the eye, containing photoreceptor cells primarily composed of rods and cones. The earlier refractions from the pupil and lens create an inverted image on the retina.
The centre of the retina is called the macula, which contains a high concentration of photoreceptor cells which convert light into nerve signals. This high concentration enables us to see fine details.
There is a layer of blood vessels behind the retina that supply oxygen and nutrients to the outer layers of the retina. These are called the choroid.
After visual information has been processed by the retina, it is sent via the optic nerve to the rest of the brain. The optic nerve is composed of axons of retinal ganglion cells.