The eyes are an essential sensory organ. Fish, birds, insects and reptiles have them and just about every mammal. Finding food, defense, offense, mating, or just having fun requires the brain to see what is around us.
It is interesting to see the embryonic development of the eyes. They start as stalks of neural tissue sprouting from the front part of the brain. Reach straight out in front of you with both hands and pretend you are holding a ball. The arms are these stalks called the optic nerves, and the cupped hands are the back of the eyeball, called the retina. Your body represents the brain. The eyes are a direct extension of the brain visible to all who look at them.
The nerves for each eye originate in separate brain halves and then mix and cross over before reaching the eyeball. Most of the visual information from the left eye goes to the right half of the brain, while the opposite is true for the right eye. Only a small amount of data from an eyeball remains on the same side of the brain. The optic nerves form a unique layer of light receptors called the retina within the eye. The shape of this is special to the horse. Called a ramped retina, some theories state the ramp allows for a different focal point depending if the head is lowered for grazing (the flat of the face looking towards the horizon for predators) or raised to look down the nose at something close (you).
Forward of the retina is a lens that forms uniform light rays. It is suspended in fluids and covered by a diaphragm (the uvea, which contains the iris giving the eye color) to control the amount of light coming into the eye (the pupil). Unlike most animals, the horse adds a glob of tissue to this diaphragm (usually only on the upper edge but can also be on the bottom edge)to help shade the sun called the corpora nigra.
Finally, the front of the eye has a transparent membrane called the cornea. This membrane seals the inside of the eye from the outside and is fully transparent unless damaged. The upper and lower eyelids protect the eyeballs for further protection, and a special 3rd eyelid adds to this protection and is called the nictitating membrane. The outside of the eyeball is bathed in fluid (tears) to lubricate, preserve corneal transparency, initiate the light rays’ bending for focusing, and distribute nutrition and immunoglobulins to maintain the health of the cornea. The tears drain through small holes on the inside corner of the eye, then travel through a tunnel within the skull, finally emptying them at the nostrils (the nasolacrimal ducts).
With so many parts to the eye, there are several areas where problems can occur. Trauma is the leading problem, but infections, immune inflammation and neoplasia can also affect these structures. Loss of clear vision (corneal ulcer), loss of all vision (cataracts, blind), loss of the eyeball (enucleation), and tear overproduction or poor draining all can occur. Most of these can be seen with our own eyes, especially if we look for them.