However, the actual prevalence of the disease might be higher since many cases may go undiagnosed due to the often subtle nature of the associated clinical signs. Thankfully, Glaucoma in cats is relatively uncommon compared to glaucoma in dogs. Eye pressure above 25 mm or a significant difference in pressure between eyes - generally greater than 7 to 8 mm - is considered abnormal. Pressure may vary based on the time of day, the cat's level of excitement or anxiety and their positioning. Normal eye pressure in cats generally ranges from 10 to 20 millimeters. The increase in eye pressure is caused by reduced drainage of the aqueous humor - a clear fluid that circulates between the cornea and pupil. It can cause pain and potentially blindness. Thus, it shows us how having a reflective layer-like a tapetum lucidum-increases the amount of light information available.Feline glaucoma is an eye disease characterized by high eye pressure (aka intraocular pressure). This experiment shows how the amount of light from a singular light source is doubled when a reflective layer is present. What happened to the “retina” when the mirror was used instead of the wall? Imagine that the cardboard is a retina and the mirror is a reflective layer like the tapetum lucidum. Again, avoid looking directly into the light or its reflection in the mirror! Note the difference in light on the side of the cardboard facing the mirror. Repeat steps 2 and 3 while using your mirror instead of a wall.Without looking directly into the light, glance at the side of the cardboard facing the wall.Hold the cardboard about 6 inches away from a blank wall and shine the flashlight through the hole toward the wall.It does not have to be perfect! If using a thinner paper, try folding it a few times before making the hole (the harder it is to see light through, the better!). Make about a hole in your paper or cardboard using a pencil or pen.In this two-part activity, you will be able to see how the tapetum lucidum works and then simulate how this reflective layer helps cats see well at night. What you see is the red color from the blood vessels nourishing the eye. This is the infamous “red-eye” in photographs. The flash on a camera is bright enough, however, to cause a reflection off of the retina itself. If you shine a flashlight in a person’s eyes at night, you don’t see any sort of reflection. Though our eyes have much in common with cats’ eyes, humans do not have this tapetum lucidum layer. This reflected light, or eyeshine, is what we see when a cat’s eyes appear to be glowing. In the last route, some of the light that bounces off the tapetum lucidum, misses the retina, and bounces back out of the cat’s eyes. This allows cats to see better in the dark than humans. The tapetum lucidum reflects visible light back through the retina, increasing the light available to the photoreceptors. Some of the light passes through or around the retina and hits the tapetum lucidum. These photoreceptor cells trigger nerve impulses that pass via the optic nerve to the brain, where a visual image is formed. Some of the light directly hits the retina, a layer at the back of the eyeball containing cells that are sensitive to light. When light enters a cat’s eye, it can take a few routes. It is also what causes the glowing eye phenomenon known as “eyeshine.” How Does It Work? It basically helps these animals see super-well at night. The tapetum lucidum (Latin for “shining layer”) is essentially a tiny mirror in the back of many types of nocturnal animals’ eyeballs. We now know that cat’s eyes appear to glow because they, along with the eyes of many other nocturnal animals, reflect light.Īll eyes reflect light, but some eyes have a special reflective structure called a tapetum lucidum that create the appearance of glowing at night. Ancient Greeks believed there was a light source inside the eyes that was like a gleaming fire. Ancient Egyptians believed cats captured the glow of the setting sun in their eyes and kept it safe until morning. Have you ever walked around a dark corner only to be surprised by glowing eyes staring back at you? The glowing eyes of a cat at night can sometimes be shocking and even a little scary if unexpected.
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