9. Organs of Special Senses – iii

Iris in humans, is the colored (typically blue or brown) area, with the pupil (the circular black spot) in its center, and surrounded by the white sclera.

Overlying cornea is completely transparent so is not visible, except the high-gloss luster it gives the eye. Also pictured are the red blood vessels within the sclera.

Eye colour is the colour of the iris. In humans, the iris may look green, blue, brown.

In response to the amount of light exiting the eye, muscles attached to the iris expand or contract the pupil.

Inner Layer

The inner layer of the eye is formed by the retina; its light detecting component.

The retina is composed of two layers:

Pigmented (outer) layer – formed by a single layer of cells. It is attached to the choroid, and supports the choroid in absorbing light

Within the eyeball, there are structures that are not located in the three layers. These are the lens and the chambers of the eye.


The lens of the eye is located anteriorly, between the vitreous humor and the pupil. The shape of the lens is altered by the ciliary body, altering its refractive power.

Anterior and Posterior Chambers

There are two fluid filled areas in the eye – known as the anterior and posterior chambers. The anterior chamber is located between the cornea and the iris, and the posterior chamber between the iris and ciliary processes.

The eye works much the same as a camera. The shutter of a camera can close or open depending upon the amount of light needed to expose the film in the back of the camera. The eye, like the camera shutter, operates in the same way. The iris and the pupil control how much light to let into the back of the eye. When it is very dark, our pupils are very large, letting in more light. The lens of a camera is able to focus on objects far away and up close with the help of mirrors and other mechanical devices. The lens of the eye helps us to focus but sometimes needs some additional help in order to focus clearly.

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