The "M" in "MAC" stands for "Mean" - in the sense of "average" - not "Medium".
The MAC is actually a theoretical concept; it doesn't really exist on the aircraft, it's a way of averaging (hence the "mean" part) the overall wing shape to a single notional wing chord. Consequently, it doesn't really have a "leading edge" or "trailing edge" - it's just a reference concept.
Just for aerodynamic pedantry...