Sleepies, eye gunk, eyeboogers… Whatever you call them, the proper name for that gunk that collects in the corners of your eyes is rheum. It’s exuded from your eyes while you sleep (as you know) but also your nose and mouth. When it comes from your eyes it’s primarily made of mucus discharged from your cornea or your conjunctiva. It doesn’t only happen when you’re asleep though! During the day our blinking flushes the mucus away into the nasolacrimal duct along with your tears over the Habs losing again.
If you have more rheum than most, you may be suffering from one of a few different conditions. Conjunctivitis (pink eye), chlamydia, infection of the eyelid (blepharitis) and more can all cause an excess of rheum, especially in your eyes.
If you’ve ever woken up unable to open your eye for the amount of eye goop present, you’re not alone. Not only because it’s happened to me several times, but also because it is a very common side effect of pink eye.
The familiar red and glassy-eyed stare of someone who’s high was thought to be due to the irritation of eyes by pot smoke. Now we know that weed makes your eyes red for the same reason it makes you dizzy- vasodilation.
Cannabinoids bind to cannabinoid receptors and induce the dilation, or widening, of the blood vessels. This increases the blood flow to these areas,and causes an overall decrease in blood pressure. The increased blood flow to your eyeball causes the red appearance, and the lowered blood pressure causes the dizziness.
You can test it yourself, by consuming marijuana through a non-smoked method,and looking for reddening of your eyes.
Dogs are dichromates whereas humans are trichromates. That means that our eyes contain 3 different types of cone cells, each of which are responsible for detecting a unique colour (for a description of both rods and cones, click here). That’s why every colour you can think of is a combination of the 3 primary colours.
But dogs only have 2 types of colour-sensing cone cells, and instead of them sensing red, blue or yellow, they’re tuned to violet and yellow-green. This means that dogs have less sensitivity in their green, yellow and red detection than humans. But conversely, humans have less sensitivity in their blue and purple detection than dogs.
And cats? Well they are a bit of a mystery. We know that they have at least two types of cone cells, one tuned to violet and one to green. But there have been studies the showed evidence of a third type of cone cell, one sensitive to light at 500 nm (greenish-blue to us). Other studies have rejected this finding, and yet others have found evidence of a cone cell sensitive at 610 nm(red to us). Currently, it’s believed they have vision similar to rhesus monkeys, called photopic trichromatic vision. In essence, they likely see similar colours to us, but not quite in the same clarity or saturation.
Cats’ and dogs’ vision systems evolved to help them hunt. They’re better at seeing movement than still objects; they see best in low light rather than bright; and they have larger visual fields (up to 270° compared to the 180° of humans!)
Cats and dogs also have evidence of their previous nictitating membranes, or third translucent eyelids, in the corners of their eyes. These would once have allowed them to maintain their sight on prey when hunting without their eyes drying out.
So when you throw a red ball into a green field, Rover sees a yellow ball being thrown into a white or gray field. Those colours just aren’t that different, so don’t be too surprised when he can’t find the ball that’s obvious to you!
You know how we (humans) have eyeballs? Well, owls don’t. They have eye tubes or cylinders, rod-shaped eyes that do not move in their sockets as eyeballs do. Instead, owls have to move their bodies or heads in order to look around. Since moving their torsos would likely make noise that would alert their prey to their presence, owls have evolved to have necks that can spin up to 270° essentially silently.
But why favour neck-spinning over the seemingly simple eye ball-spinning method of looking around? Well, night vision requires large corneas that allow for light to be collected effectively even in the dark, which is why most nocturnal animals (like the slow loris or tarsier) have huge eyes. But owls have small skulls, so their big eyes couldn’t expand out. They instead developed into the rod shape of today’s owls. They aren’t alone though: some deep-sea fish (like the anglerfish) also have rod-shaped eyes for seeing in the dark.