The Eyes
The business of seeing is a complicated process made possible by the eye. Housed in a socket called the bony orbit, and protected by the upper and lower eyelids as well as the third eyelid (called the haw or sometimes the nictitating membrane), the eyeball is a delicate yet powerful organ of sense. Your dog needs his eyes to chase after and fetch balls or birds, find his way in darkness, and look lovingly at you.
The eye is made up of three layers. The fibrous outer layer of connective tissue contains the cornea (which bends incoming light rays and focuses them onto the retina) and the white of the eye, called the sclera. A middle layer called the uvea contains blood vessels and nerves as well as the light-regulating iris and a muscle called the ciliary body located behind the iris. The job of the iris is to control the amount of light that enters the eye. It does this by adjusting the size of the opening in its center, the little black dot known as the pupil. And it's the iris that gives your dog's eyes their color. The inner layer of the eye contains the light-sensitive layer of cells known as the retina, which contains specialized light receptors known as rods and cones. Their job is to convert incoming light into nerve impulses, which are then relayed via the optic nerve to the brain to be processed into an image. Amazingly, all of this occurs in a split second.
Knowing the anatomy of the eye will help you understand diseases of the eye that may affect your dog.
Lubricating the eyes are the lacrimal glands, which produce tears. Besides keeping the eyes from drying out, tears contain immune substances to help fight infections. Each eye has two lacrimal glands.
The Sense of Sight
Vision involves the perception of light as well as the intensity of that light. Different wavelengths of light appear as different colors. Vision also requires perception of forms, or images. All of these perceptions are integrated by the brain's visual center, connected to the eyes via the optic nerves, to produce sight.
Although a dog's senses tend to be more sensitive than those of a human, they have relatively poor eyesight. Because dogs evolved as hunters, their eyes are located in the front of their head, which allows them to focus on their prey. This eye placement is good for coordination and accuracy, but it's not so good for peripheral vision. Dogs can focus well on what's in front of them, but they have to look around more than a person would to see the rest of the picture. A dog's eye anatomy makes him good at detecting and following movement, as well as seeing in poor light, but he's not so good at recognizing details or differentiating colors. Dogs can see color, but not to the same extent as humans.
Eye Diseases
Dogs can suffer from a number of eye diseases. Eye problems can result from anatomical anomalies — such as too many eyelashes — irritation or injury, infections, or congenital or hereditary conditions. Common signs of eye problems are pain, discharge, redness, and filminess or cloudiness of the eye.
There are ways to tell if your dog's eyes are hurting him. Dogs indicate eye pain in many ways. You may notice that the eye is tearing more than usual or that your dog is squinting or seems sensitive to light. He may paw at his eye or whine. The eye may also look red or irritated. Take your dog to the veterinarian if his eye appears to be painful.
Common eye problems include extra eyelashes, which irritate the eye; eyelid defects such as entropion or ectropion; cherry eye, a congenital defect in which a tear gland bulges out from beneath the eyelid; conjunctivitis, or pink eye; dry eye; corneal injuries; cataracts; glaucoma; and retinal diseases. Depending on the condition, your veterinarian can address the problem with surgery or medication. Unfortunately, some eye diseases, such as progressive retinal atrophy, have no treatment and lead to eventual blindness. On the up side, dogs can get around very well without sight by making more use of their senses of smell and hearing.
