Missed first base

[blueump]

BR running to first on a ground ball. The first baseman stretches to catch the throw, with just his toes on the edge of the bag.

The BR (maybe even in an attempt to spike the first baseman) runs inside the line and trips over the heal of the first baseman, never touching the base. They both fall to the ground, although the first baseman keeps his foot on the bag and catches the ball.

The runner arrives first, but never touches the base. Is the foot part of the base? do you call him safe waiting for an appeal? or do you have an out?