Don’t let Pablo out of his bag!

Don’t let the cat out of the bag! Pablo the cat is in his bag in the arms of his human and showing off his perfect cat eyes. What we can so clearly see in this picture is that he is facing the sun and his pupils have constricted as much as possible to block almost all the light and protect his retina.
What we think of as ‘Cat Eyes’ is the appearance of the vertical slit in the pupil of cats when viewed in daylight. In the presence of bright light, this slit can narrow down to admit only 1/300th the amount of light which it is able to admit in the fully open configuration at night. To put this in perspective, the round human pupil can only change it’s light gathering area by a factor of 15 vs the cat’s change by a factor of 200-300! This means that in full darkness a cat’s pupil is able to gather up to 20 times more light than the round pupil of a human. And that is not even considering the increased advantage of the yellow reflective tapetum which sits in the back of the cat’s eye behind the retina. That reflective layer increases the amount of light which strikes the rods and cones of the cat’s retina by bouncing light back through the retina a second time. This results in a 40% increase in sensitivity to incoming light so the net gain of visual ability at night may be in the range of 25 – 28 times that of a human. This huge advantage comes at a cost, though. The increased light gathering ability and the increased reflectiveness means that a cat has much less color perception than we do and the resolution of the images is not nearly as precise as is the resolution of the images we humans see. So it works out that we humans sleep at night and do our duties in the day with great visual acuity, while cats sleep in the day and do their hunting at night with great night vision but less visual acuity. And incidentally the horizontal slit of the grazing (prey) animals like cows, sheep and most clearly goats, limits overhead sunlight and affords a more panoramic view of the surroundings viewed from ground level. And the crazy umbrella inside the eye of the horse and attached to the iris is called the Corpora Nigra and provides horizontal shading over the horse’s horizontal pupil.

– Geoffrey Antipa

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