The Anatomy of the Cat's Eye

A phrase such as 'the cat's eye' can conjure images of the mysterious and exotic for some people while the anatomy of the cat's eye can inspire others to invent useful objects. These include life saving ideas such as road reflectors or even the sonar reflectors used to warn whales from nets and underwater structures.

The cat's eye is similar in make up to the very eyes that are reading these words. A spherical layer of tissue made up of collagen and elastic fibers containing specialized structures to enhance and focus light and turn it into electro-chemical signals to be transmitted to the visual cortex of the brain.

Internal Structure

Like the human eye, the cat's eye has an outer layer called the sclera that is opaque except for the anterior (front) 1/3 that is transparent to allow light to enter. Once light passes through this clear spot, the cornea, it travels in a watery fluid called the aqueous humor and reaches the diaphragm like mechanism, the iris.

This colored portion of the cat's eye opens or closes to varying degrees to increase or decrease the amount of light that passes through the dark center (the pupil) to reach the lens.

Muscles attached to the lens can stretch or relax the lens to allow it to focus on objects either near or far. The light that is focused through the lens then traverses the interior of the cat's eye in a gelatinous material called the vitreous humor.

The aqueous and vitreous humors help to focus the light and maintain the spherical shape of the cat's eye as well as aiding in the circulation of nutrients and removal of waste products.

The light rays then cross and converge on the posterior surface, the retina, made up of two types of cells, some shaped like cones and most shaped like rods. This area is also where the tapetum lucedum is located. It is the reflective surface that causes the eerie 'night shine' of the cat's eye.

Imaging

Being a nocturnal animal the cat's eye has rods, which are light sensitive, far out numbering the color sensitive cones. These light receptors are connected to the bundle of nerve cells named the optic nerve and change the images focused through the lens into electro-chemical signals that are transmitted to the brain by way of this optic nerve.

In cats the visual cortex has many more nerve cells than found in other mammals to allow the processing of more information (light images) passing through the larger cornea and an iris that dilates to a greater degree.

External Protection

As in humans, the anatomy of the cat's eye allows for protection by upper and lower eyelids with lashes to keep out dust and smaller particulate matter as well as antibacterial tears that cleanse and lubricate the surface. Where the cat's eye differs is the nictating membrane, a third eyelid interior to the others and that closes from the inner corner outwards.

Predatory Vision

The cat's eye generates its own image and each image is combined in the brain and the overlapping visual fields fill in the blind spot in the cat's eye. The blind spot is created in the area where the optic nerve joins the cat's eye and this binocular vision allows for depth perception, an important function for the hunting animal.

As in all predators the cat's eye is oriented forward to aid in the hunt, unlike prey animals that have greater peripheral vision both to the side and upward to help them avoid the predator that rarely attacks from the front.