The World According to Cat - Part II


Human eyes and cat eyes share some similarities we are both mammals, after all, but cat eyes have evolved as superefficient aids to hunting prey.

The wild ancestors of the modern cat needed to maximize the time they could spend hunting, so their eyes enabled them to see the merest glimmer of light. This has affected the structure of cats’ eyes in several ways.

First, they are huge in comparison with the size of their heads; indeed, their eyes are almost the same actual size as our own. In the darkness, their pupils expand to three times the area ours do. A reflective layer behind the retina, known as the tapetum, further enhances the efficiency with which their eyes capture light.

Any incoming light that misses the receptor cells in retina bounces off the tapetum and back through the retina again, where some will happen to strike a receptor cell from behind, enhancing the sensitivity of the eye by up to 40 %.

Any light that misses the second time around will pass back out through the pupil, giving the cat its characteristic green eye shine whenever a light is shone into its eyes in the dark. The receptor cells in the retina are also arranged differently from ours.

They fall into the same two basic types rods for black and white vision in dim light, and cones for color vision when it is bright but cats’ eyes have mainly rods, whereas ours have mainly cones.
 Instead of each rod connecting to a single nerve, cats’ rods are first connected together in bundles; as a result, cats’ eyes have ten times fewer nerves traveling between their eyes and their brains than ours.

The advantage of this arrangement is that the cat can see in the near dark when our eyes are nearly useless. The disadvantage is that in brighter light, cats miss out on finer details; their brains are not being told precisely which rod is firing, only a general area on the retina onto which the light is falling.

The result of this disadvantage is that in full daylight, cats cannot see as well as we can. The rods become overloaded, as ours do under the same conditions, and have to be switched off.

The small number of cones that cats do have are spread all over their retinas, rather than concentrated in the center of the retina, in the fovea, as ours are, so they get a general and not very detailed picture of their surroundings during the day.

Because their pupils are so large when wide open, they cannot be shrunk to a pinprick in bright sunlight, as ours can. Instead, cats have evolved the ability to contract their pupils to narrow vertical slits, less than 1⁄32 of an inch wide, which protects their sensitive retinas from being overwhelmed with light.


They can further reduce the amount of light entering by half shutting their eyes, thereby covering the top and bottom of the slit and leaving only the center exposed.

Popular Posts

The World According to Cat - Part I

Image
We may easily overlook the simple fact that cats live in a world that is subjectively speaking quite distinct from ours. We share just enough overlap between our different perceptions of the world to be able to relate to one another, each species having evolved its senses to fit its lifestyle. It is unreasonable to consider cats’ abilities as inferior or superior to our own. Biologists abandoned the idea of one species being “superior” to another decade ago, though cat owners may suspect that their cats feel otherwise. We now consider each species as having evolved to fit a particular way of life; cats are good at being cats, because their ancestors evolved sense organs and brains to suit that role. Because the domestication of the cat is still incomplete, that role is still in a state of flux, for example, cats are still adapting to urban living while their sense organs have remained more or less unchanged. One major difference between cats and people...
Keep reading

Artificial Respiration and Heart Massage

Image
Artificial respiration is an emergency procedure used to exchange air in an unconscious cat who is not breathing. Heart massage is used when no heartbeat can be heard or felt. When heart massage is combined with artificial respiration, it is called cardiopulmonary resuscitation (CPR). When a cat stops breathing, heart function soon also stops, and vice versa. It is therefore important to know both aspects of CPR. One person can perform CPR, but it is easier if two people are available. One does the breathing and the other does the heart massage. The following emergencies may require artificial respiration or CPR: .     Coma ·          Electric shock ·          Head injury ·          Metabolic problems ·          Obstructed airway (choking) ·        ...
Keep reading

What If Your Cat Has an Eye Problem ? Signs

Image
Your cat has an eye problem if there is matter in the eye; the eye waters; the cat blinks, squints, paws at the eye or gives evidence that the eye is painful; or the nictitating membrane is visible. The first thing to do is examine the eye and try to determine the cause. Eye problems can go from minor to serious very quickly. Be prepared to visit your veterinarian if you can’t resolve the problem right away. SIGNS OF EYE AILMENTS Eye problems are accompanied by a number of signs and symptoms. Pain is one of the most serious. A cat with a painful eye needs prompt veterinary attention. • Eye discharge: The type of discharge helps define the cause. A clear discharge without redness and pain indicates a problem in the tear drainage system. A clear discharge with a reddened eye could be conjunctivitis, including from a viral infection. A thick, sticky, mucus or puslike discharge, along with a red (inflamed) eye suggests possible conjunctivitis as well, including from chlamydp...
Keep reading