Two pyramid-shaped swellings are located on the medulla oblongata, on either side of its ventral midline. More commonly referred to together as the brainstem, the pyramids are specifically located between the antero-lateral sulcus and the anterior median fissure of the medulla. These large axon tracts appear as two ridges that travel down the entire length of the medulla. They are so visible that they have been named as brainstem landmarks. Since each pyramid is a fibrous bundle, made up of corticospinal fibers, they are able to contain corticospinal axons. Inside each pyramid there are approximately 1,000,000 of these. They arise out of the brain's cerebral cortex and then descend through the internal capsule, the brain's cerebral peduncle, and the ventral pons before finally reaching the medulla. At the medulla's caudal-most point, directly above the beginning of the cervical spinal cord, the fibers of each pyramid cross in a process referred to as the decussation of the pyramids. This process is easily identified because the medulla's midline will suddenly be way of center. Instead of simply swapping places, each pyramid moves into the white matter of the spinal cord, where they become the lateral corticospinal tract.
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In Depth: Pyramid