CAT VISION

Tavishi If you've ever seen a stray cat running in the dark, their eyes have appeared mostly a shiny, glowing green due to the presence of a structure known as the tapetum lucidum.

The tapetum lucidum is a structure present in a lot of animals, including but not limited to cows, sharks, sea lions, kiwis, and goats. As usual, humans are boring and don't have a tapetum lucidum. But humans are missing a lot of cool things cats have, so this isn't particularly surprising. Case in point: cats have more vertebrae, letting them do silly things like the cat above. Of course, we are boring and can't do anything fun. But in the eye alone, humans and cats have worlds of difference. In both animals, the basic structure of the eye remains the same: eyelids, cornea, iris, pupil, the lens, and more. Cats differ from humans with the presence of a nictitating membrane, a tapetum lucidum, and differences in retinal structure. Cats have a third eyelid called the nictitating membrane which is common among many other animals, including rabbits, crocodiles, and more. The nictitating membrane has regressed into a vestigial state within humans, and now is the little squelchy pink bit in the corner of your eye. The proper scientific name for this is, of course, not little squelchy pink bit, but rather the plica semilunaris. In cats, the nictitating membrane is mostly fully functional. The nictitating membrane has fewer muscles attached to it than those of crocodiles or similar. However, cats do have a full nictitating membrane, which acts to prevent debris from entering the eye and keep the eye nice and moist. The nictating membrane extends from the medial canthus (fancy word for the inner corner of your eye), which is why vestigial nictating membranes are just little squelchy bits in your inner corner. Below, you may be able to see it appears like a little film is covering the kitty's eye; this is the nictating membrane!

Furthermore, the tapetum lucidum, which is actually quite pretty. Unlike the nictitating membrane, the tapetum lucidum doesn't go hide away at any point, and isn't controllable. The tapetum lucidum is a shiny structure located just behind the retina, and essentially makes night vision better for cats. The tapetum lucidum functions to improve night vision, by increasing the amount of light available to rod cells. The tapetum lucidum varies in structure from species to species; within cats, the tapetum lucidum is composed of layers upon layers of zinc and riboflavin (otherwise known as vitamin B2). The tapetum lucidum is considered to increase visual acuity by around 44%, which is a huge help to carnivorous predators like cats. The retina is composed of two primary types of photoreceptive cells: cones and rods. Rods are responsible for monochromatic vision in low light conditions (cats have lots of these). Cones are responsible for color vision (cats have fewer amounts of these). In humans, the fovea is a depression in the retina in which the most cone cells are present, and thus, the center of the greatest visual acuity. In cats, rather than one circular area of visual acuity, the visual streak is a line shaped area of the highest concentration of cone cells. Because of the fewer cone cells in cat eyes, all cats, are, by our standards, myopic. In dark conditions, cat vision is wonderful. But when it comes to distinguishing far away objects during the day, kitty useless. But because of the nature of how kitty evolved, this isn't particularly an issue, and is easily made up for with the tapetum lucidum and rod cells. But also, the tapetum lucidum can kind of be an issue during the day; the reflective nature of the tissue intensifies the light during the day far more than is necessary. And thus, near the top bit of the eye, where sunlight from above would strike the eye, the opposite of the tapetum lucidum is present: a tapetum nigrum, which instead absorbs light. And also, cat pupils are funky. The pupil is often a frequently misunderstood part of the eye. Instead of being understood as an opening, the pupil of the eye is frequently believed to be a structure on its own. However, the pupil itself is actually a little opening in the iris of our eye. In humans, the pupil is always spherical. But in cats, the pupil is shaped similar to a little slit. This allows less light to go through when wanted (see: few cone cells!)

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