Eyes are "windows to the soul". I don't need to say much about the exquisiteness and complexity of this organ.
However, in this article, I want to complain: the human eye is actually not well designed. The lens is easily deformed, which makes us nearsighted or farsighted, let alone. Even eyes with perfect vision have one major defect - the blind spot.
What are blind spots?
A blind spot is an area of vision that is almost in front of us, but which we "blind" to see.
There is a simple way to know where your blind spot is: close one eye and look at a word on a piece of paper with the other. Move the word left and right, when it disappears, it means it has moved into your blind spot; if you move it again, it reappears, it means it has moved out of your blind spot.
Why do we leave such a "dead angle" in our field of vision? To understand this, one needs to know how the eye works. Think of our eyes as a room with only one small window. In the room a group of children stood in a row against the wall opposite the window. Each child can only see a small piece of scenery outside through the window hole, and because of different perspectives, the scenery they see is also different.
In this metaphor, each child is a photoreceptor, capturing the light it perceives. The human eye contains approximately 126 million photoreceptor cells. That means 126 million kids in the room, each dutifully looking out and taking note of what he sees!
But integrating all these individual observations into a single picture is what the brain does. Each photoreceptor must transmit a signal to the brain via a neuron. These neurons come together to form the optic tract, which is like a bundle of data cables that pass through a hole in the back of the eye and connect to the brain.
Still using the metaphor above. It's like having a phone line attached to each child to report back to the center what they see. It is a pity that these telephone wires are not drilled out of the wall behind the child individually, but are combined and merged into a bundle leading to the back. This requires a hole in the center of the wall; for this, the middle child will have to make room for the hole.
This brings up a problem: there is no child transmitting information at the position of this hole, that is to say, there are no photoreceptor cells. This means that there is an area in the outside scenery whose light reaches your eyes but is not recorded by any of your photoreceptor cells, and you are "blind" to it.
The blind spot is the part of the retina where there are no photoreceptor cells, because that's where the optic nerve bundle leads to the brain.
Do other creatures have blind spots, too?
It turns out that this design of the eye exists in most animals. All vertebrates—including mammals, birds, fish, reptiles, and amphibians—have blind spots in their eyes.
For most animals with eyes facing forward, the presence of a blind spot is not a problem. The fields of vision of our left and right eyes partially overlap, and the left eye can compensate for the blind spot of the right eye, and vice versa. But for animals whose eyes are on the sides of the head, where the fields of view of the left and right eyes don't overlap, the blind spot poses a risk.
If you watch animals in the wild, say a deer, a fish, they are always on the move. In fact, they are always on the lookout for potential threats. They change the field of view by moving to ensure that there are no dead spots in the previous field of view that are obscured by blind spots.
All vertebrates have blind spots in their eyes. What about invertebrates?
Here's where things get interesting. Many invertebrates (like bees, flies and mantis shrimps) have only compound eyes. Compound eyes are made up of a large group of "small eyes", but usually don't need to integrate a complete image. Therefore, there is no blind spot in compound eyes.
blind spot area on the eyeball
But three animals have eyes similar to ours: spiders, box jellyfish and cephalopods such as squid and octopuses.
Spiders have many pairs of eyes, but most spiders have poor eyesight. They mainly rely on touch, smell and vibration to catch prey. A few hunt mainly by vision, such as jumping beads and tarantulas. They usually have two pairs of centrally located eyes with large lenses that gather light. Some of them even have reflective coatings for better night vision, while others have excellent color vision for daytime hunting.
The box jellyfish has a retina, cornea and lens in its eyes, but the box jellyfish does not have a brain.
No matter what kind of spider's eyes, they can only perceive changes in the external light (thus, they know the prey), but not a complete image. This also means that there are no nerve bundles in their eyes that need to come out, so there are no blind spots.
Let's look at the eyes of the box jellyfish. Box jellyfish have no brains, just a single neural ring to coordinate their tentacles. But they have retinas, corneas and lenses in their eyes. In other words, their eye structure is basically the same as ours. They don't have a brain, though, and they don't have optic nerve bundles in their eyes, so they don't have blind spots.
Finally, we come to the coolest eyes: cephalopod eyes. Take an octopus, for example. Octopus eyes actually evolved independently of vertebrate eyes. And their eyes are better designed than ours. Their neurons grow behind the photoreceptor cells. If you still use children as an analogy, it is like a telephone line connected behind each child, directly coming out of the wall behind them, without having to make a big hole in the center of the wall after summarizing. Therefore, their eyes have no blind spots.
Why hasn't evolution gotten rid of its blind spots?
Having a blind spot seems to be a big risk for humans and other vertebrates. It means you might miss a potential threat. Why hasn't evolution solved this problem?
First, remember that evolution is a gradual accumulation of random mutations over time, not giant leaps, and that evolution doesn't go in any particular direction.
In the case of our eyes, changing the entire structure of the eye would be a huge change, and more than just mutations in a few genes can do.
Second, our eyes, despite their imperfections, are currently functioning very well. If a new mutation occurs, most of the mutations may not only fail to improve, but also destroy the function of the eye, so they will not be inherited.
Finally, especially when it comes to humans, natural vision is no longer the difference between our life and death these days. People with poor eyesight, who in their natural state might die from not being able to find food or seeing wild animals clearly, can now wear glasses.
So, even if we see a better design in an octopus, we'll just have to make do with our crappy eyes.
The blind spot is the part of the retina where there are no photoreceptor cells, because that's where the optic nerve bundle leads to the brain.
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