The Anatomy of the Equine Eyeball
Aqueous humor – This clear, watery fluid is held in chambers behind the cornea. It delivers oxygen and nutrients while removing waste. Choroid – The pigmented membrane located between the retina and sclera. Its network of blood vessels supplies oxygen and nutrients to the retina. Ciliary body – Located behind the iris and attached to the lens, it helps change the shape of the lens. Conjunctiva – A thin, clear membrane that covers the sclera and lines the inside of the eyelid. It creates a barrier that helps prevent objects from getting into the back of the eye. Cornea – The transparent layer that forms a dome-like cover over the eyeball. It’s critical to the eye’s ability to focus. Iris – The colored part of the eye. It works to make the pupil larger or smaller, regulating how much light enters. Lens – Located behind the iris and pupil, this disc-like tissue is flexible and elastic, allowing it to change shape to assist with focusing. Optic nerve – A cable-like nerve that sends light signals from the retina to the brain, which converts those signals to an image. Pupil – The hole in the center of the iris that lets light pass through. It can enlarge (dilate) to let in more light and shrink to let in less light. Retina – An extremely thin layer lining the back of the eye. It absorbs light and transmits it through the optic nerve to the brain. Sclera – The outer, white layer of the eyeball, made of a dense, fibrous membrane. It helps protect the eye and control eye movement. Third eyelid – Also known as the nictitating membrane, this is essentially a clear eyelid that will automatically move across the eyeball as another layer of protection that still allows the horse to see. Uvea – The layer under the sclera, made up of the iris, the choroid and the ciliary body (not pictured above) Vitreous humor – This clear, gel-like fluid fills a space at the back part of the eye. It helps hold the retina in place and acts as a shock absorber for the inside of the eye.