What is it about the process that makes you want to *take responsibility* for "your code" ?  Do you have the sole responsibility to keep certain features working?  Did the lead say "Michael, I want you to take responsibility for ..." ?  Or is your responsibility implicit, in that the lead and the rest of the team look to you every time certain features are broken?

Either way, if you have the responsibility, then you need then you need authority over the code.  The next time the other fellow makes unilateral changes and the lead comes back to you to fix them, you should sit down with the lead and ask to have your authority and responsibility aligned.