Anatomy and Function of Your Pet’s Eyes
Dogs’ and cats' eyes work similarly to our eyes and can develop similar eye problems like cataracts, glaucoma, and more. Read on to learn some fun and interesting facts about the anatomy of the eye, changes to watch for in your pet’s eyes, and when to see the vet.
This article was written by a FirstVet vet
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Eyes work by adjusting to different light conditions, focusing on objects near, far, and in between. This constantly produces images that are rapidly sent to the brain to be processed. The anatomy of dogs’ and cat’s eyes work by adjusting to different light conditions essential for hunting or tracking prey.
Important Structures of the Eye
The orbit is the bony cavity or socket which holds the eyeball as well as muscles, nerves, blood vessels, and structures that produce and drain tears.
The sclera is the white area of the eye covered by the conjunctiva which extends to the edge of the cornea covering the inside of the eyelid.
The cornea is the clear rounded part on the front surface of the eye which functions as protection and allows light in to focus onto the retinaat the back of the eye.
The iris is the rounded, colored part of the eye that works to make the pupil smaller or larger, adjusting the amount of light entering the eye.
The pupil is the black area in the center of the eye controlled by a circular muscle that becomes smaller in bright light to allow in less light, and larger in decreased light to allow in more light.
Behind the iris is the lens which works to bring objects into focus. The area called the retina has light-sensing cells called photoreceptors. Of the 2 types of photoreceptors called cones and rods, conesgive cats an increased ability to hunt with more accurate and binocular vision. Cats have many rods which give them 6 times better vision in dim light than people.
Rumor has it that cats can see in the dark. While they can’t see if it’s totally dark, their vision is dramatically better than ours because their specialized eye functions allow them to see even when there is very little light.
You may be familiar with the blue or greenish reflection of a cat’s eyes when shining a flashlight or caught by a car’s headlights. This reflective layer of the retina in the eye is called the tapetum lucidum, a layer that humans do not have. Fun Fact: Road engineers mimic this effect for highway reflective markings on roads and signs, making it easier to see by motorists at night or in dim lighting!
The most sensitive part of the retina, called the area centralis, holds thousands of photoreceptors, each attached to a nerve fiber. Together, this bundle of nerve fibers forms the optic nerve which transmits the image seen by the eye to the brain.
Dogs and cats have both upper and lower eyelids that blink reflexively, protecting the eye. Blinking keeps the eye moist by spreading tears over the surface of the eye and clearing away small pieces of dirt and dust.
Cats and dogs get additional eye protection from the third eyelid or nictitating membrane. Usually not noticeable, the third eyelid is located in the inside corner of the eye (closest to the nose) and is usually whitish-pink in color. The third eyelid extends over the eye to protect it from scratches (like when running through a bush or brush) or is often seen in sick dogs and cats in response to illness or inflammation. Additionally, it helps keep the eye moist with tear production.
Eyes require moisture which comes from tears produced by several glands located around the eye. Tearsare made of water, oil, and mucus produced by the lacrimal glands, glands in the third eyelid, mucus glands, and meibomian glands. A special tube called the nasolacrimal duct permits tears to drain from the eye into the nose.
Physical Exam of the Eye
Examining your pet’s eyes is part of their regular wellness check-up. Your vet will use an instrument called an ophthalmoscope, which shines a bright light as well as magnifies the eye and internal parts of the eye to help assess eyesight or any problems associated with the eye.
The following tests may also be necessary, with some requiring sedation or anesthesia:
- Schirmer Tear Test: measures the tear production in the eye by placing a small paper strip under the eyelid. A condition called dry eye can cause corneal scarring and pigmentation which can lead to blindness if left untreated.
- Corneal stain: allows visualization of corneal scratches or injuries to be seen using a drop of fluorescein stain placed onto the eye. Corneal scratches can become infected can lead to serious damage if not treated quickly and appropriately.
- Intraocular pressure (IOP): determines the pressure within the eye using a special instrument called a tonometer. Elevated IOP may mean your pet has glaucoma, which can lead to pain and blindness if left untreated.
- Dilating the pupils: eye drops that cause temporary dilation of the pupils help your vet examine the inner part of the eye using an ophthalmoscope. You’ll notice that their pupils look larger than normal. Your pet will be more sensitive to light until the effects wear off after a few hours, so you may want to keep them indoors.
When to Contact a Veterinarian
If you notice anything abnormal about your pet’s eye(s) contact your vet or the closest pet emergency hospital right away. Eye problems such as trauma happen quickly and can worsen quickly without proper care. Sometimes severe eye disease or trauma makes it necessary to surgically remove the eye.
Infections and other illnesses in the body can also cause eye problems. These include chronic kidney disease, diabetes, fungal infections, cancer, and more. Additional tests including bloodwork, urinalysis, x-rays, and ultrasound help determine the cause and diagnosis so that appropriate treatment can begin.
Symptoms of Eye Problems
- Swelling in or around the eye
- Growths or tumors of the eyelid
- Green or yellow discharge from the eye(s)
- Unequal pupil size (one pupil larger than the other)
- Squinting, unable to open the eyelids
- Rubbing at the eye
- Change in color of the eye or iris
- Proptosed eye (eye is protruding out of the eye socket)
- Bleeding from the eye or blood in the eye
- Cloudy appearance to the eye
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