The eye is an organ with several parts, each uniquely adapted to meet the special needs of the dog. The eyeball is seated in a bony socket and protected by a cushion of fat. Muscles surrounding the eyeball close the eyelids tightly in response to pain, irritation, and objects approaching the eye. This makes it difficult to inspect the surface of the eye for injuries and foreign bodies.
The large, clear window at the front of the eye is the cornea. Bordering it is a narrow rim of white connective tissue called the sclera, which is much less conspicuous in dogs than it is in humans. The sclera surrounds and supports the entire eyeball. In certain breeds the sclera may be pigmented or spotted.
The round opening at the center of the eye is the pupil. Around the pupil is a sphincterlike muscle called the iris. Like a shutter on a camera, the iris opens and closes to regulate the amount of light that enters the eye. The iris contains the pigment that gives the eye its color. While most dogs have eyes that are a shade of brown, blue eyes are normal for some breeds and colors and have normal vision. In some Northern breeds and dogs with the merle pattern, odd eyes (one brown and one blue) are not uncommon.
A pinkish membrane called the conjunctiva covers the white of the eye and doubles back to cover the inner surface of the eyelid. This membrane contains blood vessels and nerve endings. When inflamed, the conjunctiva appears red and swollen. The eyelids are tight folds of skin that support the front of the globe. Eyelashes are always present on the upper eyelids, but not on the lower eyelids. There are small hairs on the edge of the lower lids.
The dog has an important third eyelid, the nictitating membrane, located at the inner corner of the eye. This third eyelid is normally inconspicuous, but when it extends across the surface of the eye, it looks as if the eyeball has rolled back into its socket.
Tears are secreted by the lacrimalglands. Each eye has two lacrimalglands: one beneath the bony ridge at the top of the eye socket and the other incorporated into the third eyelid. Tears are conveyed to the surface of the eye by small ducts that empty behind the lids. Tears prevent the cornea from drying out. They also contain substances that fight infections. Tears help flush out any irritating substances or foreign bodies that come in contact with the eye. Tears gather at the inner corner of the eye and are carried by the nasolacrimal ducts into the nasal cavity near the front of the nose. The inner eye has three fluid-filled chambers. The anterior chamber is found between the cornea and the iris. The small posterior chamber lies between the iris and front of the lens. The large vitreous chamber, containing a clear jelly, fills the cavity behind the lens and in front of the retina.
The lens is held in place by a series of strands called the suspensory ligaments. These attach to the ciliary body, a structure composed of muscle, connective tissue, and blood vessels. The ciliary body secretes the fluid that fills the anterior and posterior chambers. Contraction of the ciliary muscles changes the curvature of the lens, which enables images of objects at different distances to be focused onto the retina.
Light enters the eye by passing through the cornea and anterior chamber and then through the pupil and lens. It then travels through the vitreous and is received by the retina. The retina is a layer of photoreceptor cells that convert light into electrical impulses. These impulses are then carried to the brain via the optic nerves.
How Dogs See
Dogs have relatively poor vision in some areas when compared with people. Dogs are nearsighted and accommodate poorly. Most dogs see at about 20/75 – remember that for people normal eyesight is 20/20. Accommodation is the process during which the lens changes shape to focus light on the retina. Dogs accommodate poorly because the ciliary muscles that change the shape of the lens are relatively weak.
The dog’s retina contains a small number of cone cells that distinguish between blue, yellow, and gray. However, the canine retina lacks photoreceptors for red and green, and thus is similar to the retina of people who are red-green colorblind. While dogs do perceive some colors, it is believed that the ability to perceive subtle shades of gray is the most important function of the cone cells. Dogs can detect degrees of brightness.
On the positive side, dogs have large pupils and a wide field of vision, making them adept at following moving objects. Dogs also have an abundance of rods in the retina, which are the cells that detect light. Along with the cone cells that distinguish shades of gray, the rods enable dogs to see very well in relative darkness. Dogs also have a fair degree of binocular vision and depth perception. Furthermore, any shortcoming dogs may have in eyesight is more than made up for by their superior senses of hearing and smell.
Visual abilities vary somewhat with breed, head shape, and eye shape. A dog with a long muzzle will have “visual streaks,” which are areas in the retina with extra cells for vision over a wide area and for detecting up motion. Many of the sighthounds fit that description. A dog with a short muzzle and prominent eyes will have an “area centralis.” This is a central spot on the retina with extra cells to pick up details. These are the dogs who tend to watch television more closely.