PupilThe opening in the middle of the iris. In bright light a cat's pupil narrows to an upright slit; in dim light it opens wide.
IrisThe colored ring round the pupil. It makes the pupil larger or smaller to control how much light enters.
CorneaThe clear dome on the front of the eye. It lets light in and helps to focus it on the retina.
LensSits just behind the iris and focuses light onto the retina.
RetinaThe lining at the rear of the eye. It holds the cells that sense light. A cat's has many rod cells, which collect dim light.
Optic nerveCarries the picture from the retina to the brain. A bundle of nerve fibers.
ScleraThe white of the eye: the tough outer layer of the eyeball.
Third eyelidAn extra eyelid, the nictitating membrane. It lies under the other two in the inner corner of the eye. It slides across to protect the eye.
Tapetum lucidumA mirror layer behind the retina. It sends light back through the retina, which helps in dim light and makes a cat's eyes shine at night.
The bird 7
CorneaThe clear front of the eye, which lets the light in.
LensA clear body behind the iris that focuses light on the retina. A bird changes its shape quickly to see near and far.
RetinaThe light-sensing lining at the rear of the eye.
Optic nerveThe nerve that carries the picture from the retina to the brain.
PectenA pleated, comb-like fold full of blood vessels. It stands into the eye from where the optic nerve enters, and only birds have it.
Scleral ringA ring of small thin bone plates in the sclera. It lies close to where the sclera meets the cornea. It is cut through here, so it shows above and below.
Nictitating membraneA thin third eyelid. It lies in the inner corner of the eye and can sweep across the whole front of the eyeball very quickly.
How this list was compiled: Every name checked on 2026-09-30 in two sources. Cat: the Merck Veterinary Manual's page for cat owners, Eye Structure and Function in Cats, https://www.merckvetmanual.com/cat-owners/eye-disorders-of-cats/eye-structure-and-function-in-cats; Wikipedia, Cat senses, Tapetum lucidum and Nictitating membrane. Bird: O. C. Bradley, The Structure of the Fowl (1915, public domain), https://archive.org/details/structureoffowl00brad; Wikipedia, Bird vision and Pecten oculi, https://en.wikipedia.org/wiki/Bird_vision; R. W. Hegner, College Zoology (1912, public domain). Licence: own work
The drawing: Drawn for this site on 2026-09-30 from plain geometry; nothing traced. Licence: own work
How it works
This cat eye diagram shows the eye from the front, then cut through from front to back. A second view does the same for a bird. Each animal eye diagram has the usual parts and a few of its own.
Read the full guide
The cat
From the front you see the iris, the pupil narrowed to an upright slit and the third eyelid in the inner corner. Cut through, light passes the cornea and the lens to reach the retina. Behind the retina lies the tapetum lucidum, the layer that makes a cat's eyes shine.
The bird
The bird eye diagram has a cornea, lens and retina too. It adds the nictitating membrane, a third eyelid that sweeps right across the eye, the scleral ring of small bones and the pleated pecten. Press Quiz me to name each part yourself.
Third eyelid and nictitating membrane are two names for the same part. Cats, birds and sharks all have one.
Questions and answers
Why do a cat's eyes shine in the dark?
Because of the tapetum lucidum, a mirror layer behind the retina. It sends light back through the retina, and some of it comes out again as a glint.
Why is a cat's pupil a slit?
A cat's pupil opens wide in dim light and closes to a narrow upright slit in very bright light. The slit cuts down the light that reaches the sensitive retina.
What is a cat's third eyelid?
The nictitating membrane: an extra eyelid that lies under the other two in the inner corner of the eye, near the nose. It slides across to protect the eyeball.
What is the pecten in a bird's eye?
A pleated, comb-like fold full of blood vessels that stands into the eye from where the optic nerve enters. Only birds have one; its use is not settled.