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Anatomy 101: The equine eye – from the outside in

  • The bone cavity in the skull that contains the eye is called the orbit. The orbit is actually formed by several bones and also contains muscles, nerves, blood supply and glands and ducts for producing and draining tears respectively.
  • The white of the eye is called the sclera. It is very tough.
  • The sclera is covered by a thin membrane called the conjunctiva.
  • The conjunctiva runs to the edge of the cornea and then lines the inside of the eyelid.
  • The cornea is a clear domed structure on the front of the eye. The cornea is very thin (approximately half a millimetre thick) and transparent to allow light to enter the eye. The cornea both protects the front of the eye and focuses light on the back of the eye (the retina) for vision.
  • Behind the cornea is a space know as the anterior (front) chamber. This is the space between the inner surface of the cornea and the surface of the iris and is filled with a fluid called aqueous humour.
  • The iris is the circular, coloured tissue surrounding the black pupil of the eye. This is the tissue that constricts (shrinks) or dilates (widens) the pupil depending on the light intensity. The iris therefore dictates how much light is able to enter the eye.
  • The pupil is the horizontal oval shaped, black area seen in the middle of the eye. When the environment outside is dark the pupil gets bigger to let more light in, and when the environment is light it gets smaller to reduce the amount of light that can enter.
  • The lens is a soft structure that sits behind the iris. It is clear. It changes its shape to focus light correctly onto the retina for good, clear vision.
  • From the back surface of the lens to the retina at the back of the eye there is a clear, jelly like substance called the vitreous humour.
  • The retina forms the back part of the eye. The retina is a thin, translucent layer that contains photoreceptors (light sensitive cells). It is this layer that therefore transmits light information to the brain via the optic nerve.
  • The optic nerve enters the back of the eye and runs back in the brain. It can be seen on the retina using an ophthalmoscope, as a small white dot (called the optic disc).