Oswald in the doorway: the blog of the Oswald Innocence Campaign, by Ralph Cinque

It is well known in Medicine that the index finger is the most dexterous, the most sensitive, the most precise, and the best for fine motor skills, of the four fingers. And you can prove it to yourself right now in a simple experiment. Bend and extend the index finger of your dominant hand. Think about the amount of control you have over it and how connected it feels to your brain. Now, do it with your middle finger. You may notice that when you start moving your middle finger that the ring finger and pinky want to join in. You don't have as much control of your middle finger as you do of your index finger.  The middle finger is not as independent as the index finger. You can feel that it doesn't have as strong a connection to your brain as your index finger. Your index finger is more "you" than your middle finger. When you move your index finger, you definitely feel that it is "you" moving. But, it's different with your middle finger; when you try to move it separately, it feels a little weird, a little awkward,  

There is an anatomical and physiological reason for it, and I will let Dr. Ajeesh Sankaran, orthopedist, explain it:

In movements of the fingers, you will notice that all fingers move together in a well coordinated action when you make a fist or relax it. It is when you try to move them separately that differences in our ability to control them become apparent.  Try the following: make a fist and try to straighten each finger separately. Also straighten all fingers and try to bend each finger separately. You should notice that the greatest difficulty arises in the ring and middle fingers. Why this is so is somewhat straightforward with a detailed understanding of the musculature of the hand and forearm.

Thumb- the thumb can practically be moved all over the place, as it were, irrespective of the position of the other fingers. That is because it has independent muscles for all movements.

The above picture shows clearly how the thumb has many little muscles around its base to perform its movements. Another set of muscles in the back of the hand and forearm helps to straighten it from a bent position. Unsurprisingly, they are also independent.
Little finger- if you have tried out the little test above to see independent finger movements, you would notice that the little finger actually moves better than the ring and middle fingers. The reason is also apparent in the above diagram, as the little finger also has separate muscles to perform some movements. However it also relies on muscles it shares with the other three fingers, limiting control. As far as extension is concerned, the little finger may have a separate muscle, as a variation in some people, to power it and this improves control.
Index- the four fingers are flexed by mainly by two muscles in the forearm ('long flexors') and one set of muscles in the hand (lumbricals). The independence of the index stems from the fact that one of the forearm muscles (flexor digitorum profundus) has a functionally separate tendon slip to the index. Since the same muscle is bending all fingers, when it contracts all of them tend to bend together. However, this separate slip to the index allows it to bend independently or remain straight while the others bendNotice the deep groove all along the muscle which separates the index finger part from the rest. This seems to be an evolutionary trend in humans, and we can foresee that Human 2.0 might have a completely different muscle for index finger flexion.
The index finger also has a separate extensor (extensor indicis proprius) to straighten, along with another another forearm muscle (extensor digitorum communis) that it shares with the others. This obviously allows us to point with it, while maintaining a tight fist with the rest of the fingers and the thumb.
The independent extensors of the thumb, index and little fingers allows us to make the Devil's horn sign

Ring and Middle finger- these fingers have no flexors or extensors independently. They move with the muscles common to all fingers. When you try to bend or straighten them the others will tend to do so too.
However, why the ring finger is more difficult to control than the middle is not so straightforward. The reason here seems to lie in the brain and not the hand. It seems that cortical neurons that move the ring finger coupled to those of the little finger. So the brain does not distinguish properly its movements from those of the little. But, this is a theory that needs more research at the moment.

Ralph Cinque: So, the fact that, traditionally, the index finger is the trigger finger is not an arbitrary thing. And, I don't think that anyone with an intact hand would have any tendency, either consciously or subconsciously, to use the middle finger. Jack Ruby had an intact right hand, and therefore, there no reason to think that he would have operated the trigger with his middle finger, as seen in the Jackson photo.

So, that man with the gun was NOT Jack Ruby. I've given you plenty of other reasons to reject the claim that he was Jack Ruby, but this one is just as important. Jack Ruby would not have fired the gun with his middle finger, as seen there.