lunes, 4 de abril de 2011


Sight or vision describes the ability of the brain and eye to detect certain wavelengths of electromagnetic radiation (light), and interpret the image as "sight." Different receptors are responsible for the perception of color (the frequency of photons of light) and perception of brightness (number of photons of light). Photoreceptors are found in the retina. Just in case you don't remember or you don't know the external parts of the eye you can see in this link
Eye structure
The eye has a structure to be able to focus light onto the retina. In the drawing above you can study the different parts of the eye. Each eye is in the socket of the skull. They have a spherical structure (eye ball) with a three-layered wall. This layers are:

  • Sclera: It is the outermost white layer that is tough and opaque. The anterior part is transparent and is called cornea. It is covered by a thin transparent membrane called the conjunctiva The sclera provides protection and gives shape to the eye. It provides surface for the attachment of eye muscles.The anterior transparent cornea allows the light to pass through the lens. The bulging shape of the cornea also refracts the light that results in the light focussing properly on the retina.

  • Choroid: It is darkly pigmented and highly vascular. In its front is the iris, a circular, muscular diaphram with a hole in the centre. The hole is called pupil. The iris give colour to the eyes. The muscles in the iris contract and expand to increase and decrease the size of the pupil, respectively.

  • Retina It is the innermost layer of the eye wall. It is also pigmented layer. It has the photoreceptor cells.There are two forms of photosensitive cells: Rod cells are highly sensitive to light which allows them to respond in dim light and dark conditions, but, they cannot detect color. Cone cells respond to different wavelengths of bright light (different colors). They do not respond well in poor light conditions. Humans have three different types of cone cells. Depending on the distribution of rods and cones, there are two spots on the retina. They are the blind spot and the yellow spot. The blind spot is the point on the retina where the optic nerve leaves the eye. At this region, there are no rods or cones and therefore, no image is formed on this spot. (you can discover your blind spot in this link). Yellow spot (Macula lutea or fovea) . It is the point where most light rays are focussed. It contains only cone cells and is, thus, most sensitive in bright light. The image formed at this point is the sharpest.

  • Lens. It is a biconvex, circular, transparent object located just behind the pupil. It serves to focus the light on the retina. It is an elastic structure and its shape can be varied to adjust to objects at varying distances. The adjustment is made with the help of muscles. The lens is held in place with the help of ligament called the suspensory ligament. The chamber between the lens and the cornea is filled with aqueous humour. The posterior, larger chamber between the lens and retina is filled with a semi-solid substance called the vitreous humour.
This video is a good explanation of eye structure

and if you prefer something more real, this is a good option: cow's eye disection

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