Medullary pyramids:

fiber bundles in the medulla that appear triangular in cross-section and contain motor fibers, the majority of which are part of the corticospinal tract. These motor fibers decussate at the base of the pyramids in the pyramidal decussation.

Learn more:

2-Minute Neuroscience: Medulla Oblongata

Know your brain: Medulla oblongata

Know your brain: Corticospinal tract