The Iris

Time for another post about the human eye, this week we are looking at the iris, I’m sure we all know that the iris is part of our eyes but how many know its purpose or how it’s made up.

Light has to enter the eye in order for us to see, too much light can damage the eye, but too little light means we can’t see clearly. The eye has its own built-in mechanism for regulating this, the iris.

The iris is made up of two sets of muscle fibres; there is the radial muscles that run from the outside edge of the iris towards the centre. Then there are the circular muscles that form the inner rim of the iris and are arranged is a series of rings.

These muscles contract in response to signals from the brain, when one contracts the other relaxes, we cannot make them contract by thinking about it, they just do it when needed. Controlling the size of the iris controls how much light enters the eye.

The radial and circular muscles have opposite effects, when the circular muscles contract they pull the radial muscles inward, shrinking the size of the pupil, when the radical muscles contract they pull the circular muscles outwards the size of the pupil increases.

So, in normal light they are balanced, when in bright light they contract and in dim light they increase.

The drops that are put in your eyes when they are being checked cause the pupil to enlarge making it easier for them to be examined.