Two pyramid-shaped swellings are located on the medulla oblongata, on either side of its ventral (front) midline. More commonly referred to together as the brain stem, the pyramids are specifically located between the anterolateral sulcus and the anterior median fissure of the medulla.
The pyramids appear as two ridges that travel down the entire length of the medulla. They are distinctly visible, and often used as anatomical landmarks for the brain stem.
Since each pyramid is a fibrous bundle, made up of corticospinal fibers, they are able to contain corticospinal axons (long, threadlike parts of nerve cells). Inside each pyramid there are approximately 1,000,000 of these. These play a role in motor (voluntary movement) functions. 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 back-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 far off-center. Instead of simply swapping places, each pyramid moves into the white matter of the spinal cord, where they become the lateral corticospinal tract.