You make it seems as if only dogs have this fluid.
Nearly every animal has this fluid, including humans, and it is not the reason for night vision.
This deals with cones, rods, carotene, and the tapetum.
Vitamin A starts as beta-carotene, one of the families of plant chemicals known as carote noids. These exist in green-leaf plants, carrots, and some other types.
When we eat these types of plants, we convert it into other types of vitamin A called retinols.
In the eye, beta-carotene is converted to the aldehyde form of vitamin A called retinaldehyde and bounds to a protein called opsin, which resides in rods and cones.
These line the back of the eye and are what trigger the transient excitation of electrical energy that our brains use to interpret visually the world we see.
As long as there is enough beta-carotene in the retina, rods and cones can do their respective jobs (offering both color vision and night vision).
Rods are more sensitive to light than cones, and in the time of need they will absorb more carotene to help a person/animal see better at night.
This process involves the dilation of the retina to allow more light in and the gradual absorption of beta-carotene, which is why night vision is gradual; when the lights go out, you are immediately blinded until your rods can adjust.
Likewise, when the lights are turned on your rods must disperse of more beta-carotene, until which time your eyes will burn from over stimulation.
Although functionally the same, animals still have the edge over humans at seeing in the dark. This has to do with an extra reflective layer animals have called a tapetum.
And this is also related to the devil eyes you see in animal pictures, and red eyes you see in pictures of humans.
In humans, after the eye has been dilated, and a photo is taken, more light will enter the eye and reflect off a human’s choroid. The effect is always red because the anatomy of our eyes is different from those of animals. The red comes from blood vessels.
Some cameras uses two flashes to reduce the effect. The first flash forces the pupil to constrict so that the second flash will not catch so much reflectivity.
Pupil restriction is faster than rods absorbing beta-carotene, so you will still be blinded by both flashes.
However, in animals, the red-eye effect is not always red, and is much more intense.
The green/blue/yellow eye in animals was correctly related to night vision previously in this topic, but incorrectly related to vitreous fluid.
Both night vision and the “pet eye†effect are caused by light bouncing off the animal’s tapetum, which is a reflective layer in the back of the animal’s eye intended to improve vision at night.
Tapetal color can vary to some extent with coat color. Some animals, and a few dog and cat breeds (for instance, blue point siamese cats), have no tapetal pigmentation. These animals show a red reflex as humans.
The color of the eyes in photographs also depends on the angle of the eye relative to the camera.
It is common to take a picture “across†a dog’s face and see one colored eye and one normal eye.
L. Spiro
[Edit]
I forgot to mention that rods are dispersed in higher numbers just outside the center of the eye, decreasing towards the outside of the eye, while cones are entirely focused towards the center of the eye.
This is why, at night, you can’t see anything by looking directly at it.
You must look a bit to the side where the focus on the item will encounter the highest number of rods.
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