Retina.

The retina is the part of the eye which reacts to light; it transforms beams of light into electric pulses which the brain interprets as images. The eye does not choose what to look at or what to let in. That's us; our minds that decide that, and it is always a decision. Sometimes the 'decision' is over so fast we never realise that we have an option.

Self control is a necessity in our Christian walk.
I work in a large office and it seems that no matter who you speak to someone knows someone who has had an affair with someone else in the office. The wierd thing is that it seems to be neither looked down on nor celebrated; it's just the malaise of the place. But it all begins little by little; these things don't suddenly spring out of nowhere, and invariably it begins with the eye:
[Jesus continued teaching his disciples] "You know the next commandment pretty well, too: 'Don't go to bed with another's spouse.' But don't think you've preserved your virtue simply by staying out of bed. Your heart can be corrupted by lust even quicker than your body. Those leering looks you think nobody notices - they also corrupt.
Let's not pretend this is easier than it really is. If you want to live a morally pure life, here's what you have to do: You have to blind your right eye the moment you catch it in a lustful leer. You have to choose to live one-eyed or else be dumped on a moral trash pile." Matt 5:27-29 (Msg)

The solution is not self mutilation [the eye is not to blame]!
But for each of us it may seem costly. There may be certain places or magazines to TV shows (technically 'innocent' in themselves) which you may need to start avoiding [for our own security], and each time we do it seems like a tough struggle. But it is worth it for the renewal we will experience.

Jesus chose to bring this subject up in the crowning pinnacle of his preaching career. He didn't stick to the comfortable or the inspiring, he went straight to the heart of the matter. If we are called to radiate the presence of God it is essential that we are filled by His light. Taking our eyes off Jesus [even for a moment] can cause us to easily sink, but his hand is always outstretched to us; we're in this together, that's what Immanuel means.

"The eye is the lamp of the body. If your eyes are good, your whole body will be full of light." Matt 6:22