A Look into a Cat's Eye - Basic Feline Ocular Anatomy for the Pet Owner

A cat's eyes are complex organs which use a variety of tissues and fluids to enable a cat to see. While feline eyes are in some ways similar to those of humans, there are some radical differences which are designed to improve a cat's night vision. Below is a basic overview of feline ocular anatomy. To find the best, most scientific description, interested readers should consult a good veterinary text on feline anatomy, such as Hill's Atlas of Veterinary Clinical Anatomy.

A cat's eyeball, like a humans, is enclosed within the orbit, the part of the cranium surrounding the eye. Inside the orbit the eye sits, its movements controlled by extraocular muscles which enable the cat to gaze in various directions.

Upper and lower eyelids cover the eyes when the cat sleeps or chooses to blink. Cats do not blink automatically, as humans do. In addition, cats have a third eyelid, called the nictitating membrane, which provides additional protection from bright light. A cat's eyelids protect the eyes from the intrusion of foreign matter, such as dust, cat litter, or sand, and they lubricate the cornea with oily secretions from the Melbomian glands.

Two muscles enable eyelid movement. The orbicularis oculi closes the eyelid, while the levator palpebrae superioris opens the eyelid.

The eyeball itself is comprised of the sclera ("SKLEH-ra"), the white membrane that gives the eyeball its shape. Set into the sclera is the cornea, through which light passes into the rest of the cat's eye. The cornea enables objects to be focused upon. Though the cornea is usually clear, it can become scarred from abrasions, and abrasions can lead to the formation of fibrous scar tissue.

Behind the cornea is the lens, which is responsible for fine focusing of seen objects. It too is clear but can become clouded by cataracts. The lens' position in the eye is maintained by peripheral lens zonules. If all of the zonules rupture, the lens can slip out of place in the eyeball.

Between the cornea and lens of a cat's eye is the anterior or front chamber, which contains clear aquaeous humor.

Behind the lens is the uvea, the pigmented middle part of the eye. It is composed of three structures: the iris, the ciliary body, and the choroid. Vitreous humor, a clear, gelid liquid, fills the space between the lens and retina, also known as the posterior, or rear, chamber. The vitreous' function is to keep the retina in place against the back of the eye.

The iris expands or contracts depending on the amount of light shining into it. The ciliary body produces aquaeous humor. Glaucoma can result if the ciliary body produces too much aquaeous humor or if any part of the eye's drainage path becomes blocked. Therefore, glaucoma medications for cats, such as Trusopt and methazolamide, are used to depress fluid production in the ciliary body.

The choroid is located between the retina and sclera. It contains the tapetum lucidum or "bright tapestry," which is a layer of reflective cells found in nocturnal animals and in deepwater fish. These cells reflect light back onto the retina, enabling a cat to see extremely well in dim light. In fact, a cat requires six times less light to see in the dark than a human does.

In addition to being the home of the tapetum, the choroid also supplies blood and oxygen to the retina. In some species, it is the retina's only supply of nourishment.

The retina's function is to collect light energy and to convert it into chemical signals which then travel up the optic nerve and into the brain. It contains nine layers of photoreceptors, the rods and cones. Rods collect light based on brightness, whereas cones collect light based on color wavelengths. Domestic cats are somewhat colorblind, able to see shades of blue and green better than more reddish hues. Thus, while domestic cats have better night vision than humans, their overall visual acuity is poorer than in humans. In much the same way, humans suffering from achromatopsia, or purely black and white vision, see very poorly in daylight.

Last of the eye's structures, the optic nerve passes through a point in the sclera called the lamina cribrosa. The optic nerve is surrounded by a protective padding of fat as it extends from the eye to the occipital (rear) area of the brain. A cat's eyes can be completely normal but unable to function if the vision center of the brain is damaged. Such a condition is called cortical blindness. Cortical blindness can result from many causes, most notably trauma or stroke.