If the crack resists closing by squeezing and the split side surfaces are relatively flat, you could get a piece of similar wood and slice off some long shavings with a plane. Work (carpenter's) glue into the crack and brush it on the sides of the shaving. Slide the shaving in to fill the crack. Trim, smooth and blend after the glue dries. If you can't get at the inside, you could still do this if you avoid sliding the shaving in deeper than the thickness of the wood.

(Patience is a virtue!)