Sunday, December 5, 2010

Anatomy of Eyes



Eye anatomy details the arrangement of various eye structures within the orbit. Here's an overview of eye anatomy, eye structure and parts of the eye.

Eye anatomy consists of studying the various eye structures and their arrangement in layers and segments within the orbit.

The eyes are highly specialized and complex structures that confer on human beings (and other animals) the most important of the five special senses – the gift of vision. 




Eye anatomy basically consists of transparent structures like the cornea and the lens, which focus light rays onto the retina, a neural tissue that helps capture images and transmit them to the brain. In addition, there are structures required for protection, support,  nutrition and various other functions. These eye structures are arranged optimally for eye function.

Eye Anatomy – Eyeball, Orbit and Appendages of the Eye

Each eyeball is a globe-like structure, although not strictly spherical. Each globe is nestled safely within deep, cavernous bony sockets in the skull called the orbits. The orbits are quadrilateral, pyramid shaped cavities and are actually lined by seven of the different bones making up the skull.
Within the orbit, the eyeball is suspended by six muscles, the extraocular muscles that are responsible for all its movements. The eyes are protected by skin folds that form the eyelids. The inside of the eyelids and most of the visible surface of the eyeball except the pupil, are covered by a thin, transparent membrane called the conjunctiva. Also, within the orbit alongside the eyeball, are the lacrimal glands or tear glands. These structures form the accessory organs or appendages of the eye.

Eye Structure – Layers, Segments and Chambers of the Eye

The eyeball is basically made up of three layers that have different functions. They are:
  • Outer fibrous layer – This forms the strong outer capsule that holds all the finer elements of the eye. It is transparent in and around the region of the pupil for light transmission and is called the cornea. Over the rest of the eyeball it is opaque and is called the sclera, part of which is seen as the whites of the eyes.
  • Middle vascular coat – This layer consists of parts called iris, ciliary body and choroid that contain pigment and blood vessels and provide nutrition to the eye. Towards the front of the eyeball, this layer becomes the the iris, seen as the pigmented portion of the pupil, giving the eye its colour. The iris is also the structure that helps regulate the size of the pupil’s aperture.
  • Inner nervous layer or the retina – It contains the neural elements like rods and cones necessary for vision.
Within the eyeball, located just behind the iris, is a transparent, crystalline lens whose main function is to focus light rays for vision. The eyeball can be further divided into two parts:
  • Anterior segment
  • Posterior segment
The anterior segment consists of the crystalline lens and the parts in front of it, which are the iris and cornea. It also contains an anterior and a posterior chamber filled with a fluid called aqueous humour. The anterior and posterior chambers together contain about 0.3ml of aqueous humour. The posterior segment lies behind the lens and includes the retina, choroid, optic disc and vitreous humour, the gel-like material filling the space behind the lens.

Nerve fibres arise from the retina and join the optic nerve to carry visual impulses to the brain. The optic nerve, the second cranial nerve, arises from the retina at the point called optic disc and runs from the eyeball like a stalk.

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