The visual acuity of the average dog has been compared to that of a human at sunset. Most see only generalized forms rather than distinct images or features. Exceptions to this rule include the sight hounds (greyhounds, afghans), which indeed have keen eyesight.
Contrary to popular belief, dogs might not be as colorblind as people think. In fact, the canine eye possesses all of those structures necessary to perceive their world in color. Now, whether they take full advantage of this is still a matter of speculation. It seems, however, that since the sense of sight is not as vital to most dogs as, let’s say, the sense of smell, there might be no real need for color perception.
The sense of vision is important to nocturnal hunters such as the cat, more so than their canine counterparts. Unique adaptations of the feline eye allow for this greater visual acuity. For instance, the unique slit-shaped design of the feline pupil allows it to dilate exceptionally wide in dimly lit surroundings. In addition, the ability to focus in on objects and to detect even the slightest of movements is highly refined.
Certainly such visual characteristics account for the effectiveness of the feline as a hunter. As do dogs’ eyes, cats’ eyes also posses those structures necessary to perceive the world in color. As far as visual capabilities are concerned, one interesting breed to take note of is the Siamese cat. Although anatomically their eyes differ little if at all from those of their brethren, research has revealed that they might perceive their world a little bit differently. While binocular vision (visualizing one scene with both eyes) is the standard for humans and most animals, Siamese cats might actually visualize two different presentations for the same scene one for each eye. That is what a person would see, a set of keys. A Siamese cat, on the other hand, might visualize two sets of keys because each eye is focusing in on the set separately. Because of this apparent lack of binocular vision, depth perception is not as refined in this breed as with others. Could all this explain the unique behavior exhibited by this fanciful breed.
Anatomy and Physiology
Each eye is housed within a bony socket of the skull, and is surrounded by an upper eyelid and a lower eyelid. In addition, a nictitating membrane, or third eyelid, is located on the inside corner of each eye. Serving a protective function similar to that of the conventional lids, this third eyelid will passively extrude over the eye in the event of injury or illness. Special glands lining the inside portion of this lid also bear significance in the disease condition in dogs known as “cherry eye.”
The conjunctiva is the delicate membrane seen lining the pink inner portion of the eyelid and a substantial portion of the eyeball itself. Conjunctivitis is the term applied to inflammation involving this membrane. It often results in red, weeping eyes.
The white portion of the eyeball is properly termed the sclera. Changes in the color of the sclera can be indicative of underlying disease. For instance, a sclera that is yellow-tinged could reflect a serious underlying liver or bleeding disorder.
The cornea is the clear, transparent structure at the front of the eye through which the colored iris and black pupil can be seen. When light passes through the cornea, it enters into the fluid-filled anterior chamber of the eye, which is located between the cornea and the iris.
Monitoring the pressure maintained within the eyes by this fluid is a valuable diagnostic tool for the veterinarian trying to diagnose eye disorders in pets. For instance, glaucoma, or increased pressure within the eye, is a serious disease that can lead to blindness if not treated promptly. It can originate as a result of increased amounts of this anterior chamber fluid. On the contrary, a decreased pressure reading signifies active inflammation within the eye itself (uveitis), prompting appropriate treatment measures.
The iris is the structure that contains the pigment that gives the eye its characteristic color. In dogs, brown is by far the dominant eye color seen, with a few blue eyes interspersed here and there.
The pupil is a hole formed by the iris. The size of the pupil is determined by the contraction and expansion of the iris in response to varying degrees of light. Thus, the iris serves to regulate the amount of light that is actually allowed into the eye. Injury or illness can affect pupil size. For example, poisonings caused by organophosphate insecticides can cause the pupils to be pin point in size. Furthermore, pupils that are unequal in size can be indicators of a primary neurological disease, including disorders of the middle ear.
Once through the pupil, light enters into the posterior chamber of the eye, containing the lens and the retina. The lens serves to gather incoming light and then focus it in on the retina, which lines the back surface of the eye. Special fibers attaching to the lens allow it to change sizes to accommodate for distances.
The retina contains a multitude of nerve endings that, when stimulated by light, send nervous impulses which feed into the optic disk and then into the brain. The end result is a visualized, perceived image. The appearance of the retina can be altered by a variety of disease states, offering valuable diagnostic insight to the veterinarian attempting to pinpoint the source of an illness.
The tapetum is a specially pigmented structure that lines the back surface of the eye along with the retina. The tapetum serves as a light-gathering, reflective device that improves night vision in the dog and cat. It is responsible for the characteristic green color seen when light from approaching automobile headlights or other sources reflect off of it.
Contrary to popular belief, dogs might not be as colorblind as people think. In fact, the canine eye possesses all of those structures necessary to perceive their world in color. Now, whether they take full advantage of this is still a matter of speculation. It seems, however, that since the sense of sight is not as vital to most dogs as, let’s say, the sense of smell, there might be no real need for color perception.
The sense of vision is important to nocturnal hunters such as the cat, more so than their canine counterparts. Unique adaptations of the feline eye allow for this greater visual acuity. For instance, the unique slit-shaped design of the feline pupil allows it to dilate exceptionally wide in dimly lit surroundings. In addition, the ability to focus in on objects and to detect even the slightest of movements is highly refined.
Certainly such visual characteristics account for the effectiveness of the feline as a hunter. As do dogs’ eyes, cats’ eyes also posses those structures necessary to perceive the world in color. As far as visual capabilities are concerned, one interesting breed to take note of is the Siamese cat. Although anatomically their eyes differ little if at all from those of their brethren, research has revealed that they might perceive their world a little bit differently. While binocular vision (visualizing one scene with both eyes) is the standard for humans and most animals, Siamese cats might actually visualize two different presentations for the same scene one for each eye. That is what a person would see, a set of keys. A Siamese cat, on the other hand, might visualize two sets of keys because each eye is focusing in on the set separately. Because of this apparent lack of binocular vision, depth perception is not as refined in this breed as with others. Could all this explain the unique behavior exhibited by this fanciful breed.
Anatomy and Physiology
Each eye is housed within a bony socket of the skull, and is surrounded by an upper eyelid and a lower eyelid. In addition, a nictitating membrane, or third eyelid, is located on the inside corner of each eye. Serving a protective function similar to that of the conventional lids, this third eyelid will passively extrude over the eye in the event of injury or illness. Special glands lining the inside portion of this lid also bear significance in the disease condition in dogs known as “cherry eye.”
The conjunctiva is the delicate membrane seen lining the pink inner portion of the eyelid and a substantial portion of the eyeball itself. Conjunctivitis is the term applied to inflammation involving this membrane. It often results in red, weeping eyes.
The white portion of the eyeball is properly termed the sclera. Changes in the color of the sclera can be indicative of underlying disease. For instance, a sclera that is yellow-tinged could reflect a serious underlying liver or bleeding disorder.
The cornea is the clear, transparent structure at the front of the eye through which the colored iris and black pupil can be seen. When light passes through the cornea, it enters into the fluid-filled anterior chamber of the eye, which is located between the cornea and the iris.
Monitoring the pressure maintained within the eyes by this fluid is a valuable diagnostic tool for the veterinarian trying to diagnose eye disorders in pets. For instance, glaucoma, or increased pressure within the eye, is a serious disease that can lead to blindness if not treated promptly. It can originate as a result of increased amounts of this anterior chamber fluid. On the contrary, a decreased pressure reading signifies active inflammation within the eye itself (uveitis), prompting appropriate treatment measures.
The iris is the structure that contains the pigment that gives the eye its characteristic color. In dogs, brown is by far the dominant eye color seen, with a few blue eyes interspersed here and there.
The pupil is a hole formed by the iris. The size of the pupil is determined by the contraction and expansion of the iris in response to varying degrees of light. Thus, the iris serves to regulate the amount of light that is actually allowed into the eye. Injury or illness can affect pupil size. For example, poisonings caused by organophosphate insecticides can cause the pupils to be pin point in size. Furthermore, pupils that are unequal in size can be indicators of a primary neurological disease, including disorders of the middle ear.
Once through the pupil, light enters into the posterior chamber of the eye, containing the lens and the retina. The lens serves to gather incoming light and then focus it in on the retina, which lines the back surface of the eye. Special fibers attaching to the lens allow it to change sizes to accommodate for distances.
The retina contains a multitude of nerve endings that, when stimulated by light, send nervous impulses which feed into the optic disk and then into the brain. The end result is a visualized, perceived image. The appearance of the retina can be altered by a variety of disease states, offering valuable diagnostic insight to the veterinarian attempting to pinpoint the source of an illness.
The tapetum is a specially pigmented structure that lines the back surface of the eye along with the retina. The tapetum serves as a light-gathering, reflective device that improves night vision in the dog and cat. It is responsible for the characteristic green color seen when light from approaching automobile headlights or other sources reflect off of it.
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