Labelled Diagram of the Eye

Labelled Diagram of the Eye

The pupil is located in the centre of each eye in the human body.

It generally appears to be the dark "centre" of the eye, but can be more accurately described as the *circular aperture in the centre of the iris through which light passes into the eye.

The size of the pupil, and therefore the amount of light that is admitted into the eye, is regulated by the pupillary reflex - which is also known as the 'light reflex'.

The processes involved in the pupillary reflex include:

  • Bright light reaches the retina,
  • nerves of the parasympathetic nervous system are stimulated,
  • a ring of muscle around the margin of the iris contracts,
  • the size of the pupil is reduced,
  • therefore ... less light is able to enter the eye.

Conversely, in dim lighting conditions the pupil opens due to stimulation of the sympathetic nervous system that contracts of radiating muscles, hence increases the size of the pupil.

*Although some animals' eyes are basically structured in a similar way to human eyes, they may appear to be very different. Compare, for example, the shapes of the pupils of cats' eyes (vertical slits) with the pupils of human eyes (circular pupils) - as shown in the pictures below.


More about Ophthalmology:

This section includes short definitions and descriptions of the parts of the eye.
For other descriptions in this category, choose from the list to the left (but note that this is not a complete / exhaustive list).

Other related sections include:

For further information see also our pages of books about ophthalmology.

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This is not medical, First Aid or other advice and is not to be used for diagnosis or treatment. Consult an expert in person. Care has been taken when compiling this page but accuracy cannot be guaranteed. This material is copyright.

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