you could be lacking vitam E C and or B and that is what causes your nails to break quite easily. depending on how deep the split is you migth have to get it checked out beause it could hit a nerve, open and cause infection.
It is likely that at some point you had a knock to the finger which has damaged the nail bed so now when the nail grows it grows with a split.
I have a similar problem with one of my fingernails. I have been taking all sorts of minerals and vitamins, but nothing has helped.
I had a session with a manicurist the other week and she had no answer as to how to stop it splitting. She just applied some nail glue to it.
the roots at the bottom of the nail, were prob damaged in a small accident there for killing the roots causing the line down your nail. actually on my foot,on my big toe i have the samething, a line down the middle of my nail, i hope that helps