The eyes explained

Horses’ eyes are complicated. They’re made up of many different parts (see diagram, left), which are useful to know about when it comes to spotting potential issues:

The bony cavity or socket that contains and protects the eyeball is called the orbit. It contains muscles, nerves, blood vessels and the structures that produce and drain tears.

The white of the eye is called the sclera and is covered by a thin pink membrane, the conjunctiva, which runs to the edge of the eyeball and covers the inside of the eyelid.

The cornea is a clear dome on the front surface of the eye that lets light in, protects the front of the eye and helps focus light on the back of the eye.

The iris is the circular, coloured area of the eye. It controls the amount of light that enters the eye by making the pupil larger or smaller.

The lens, which sits behind the iris, changes its shape to focus light onto the retina.

Small muscles called ciliary muscles contract to thicken the lens, which allows your horse to focus on nearby objects. The ciliary muscles relax, causing it to become thinner when focusing on distant objects.

The retina contains photoreceptors, each attached to a nerve fibre. All the nerve fibres are bundled together to form the optic nerve. The photoreceptors in the retina convert an image into electrical impulses, which are carried to the brain by the optic nerve.

The eyelids are thin folds of skin that can cover the eye and blink to protect it. Blinking also helps spread tears over the surface of the eye, keeping it moist and clearing away small particles.

The nictitating membrane (third eyelid), found under the other eyelids in the inside corner of the eye, extends upwards when needed to protect the eyeball from scratches or in response to inflammation.

Lacrimal glands produce the watery portion of tears. Mucus glands produce mucus that mixes with this, creating a more protective tear that is slower to evaporate. Nasolacrimal ducts allow the tears to drain from each eye into the nostrils.